Minimization of Process and Wastewater Load in a Low-Pressure Environment through Vacuum Application for Fiber-Based Materials: Leather and Cotton Products
Categoría del artículo: Research Article
Publicado en línea: 28 may 2025
Páginas: 1 - 9
DOI: https://doi.org/10.2478/ftee-2025-0001
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© 2025 Ersin Onem et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.
Almost all wet processes in leather production are carried out in conventional wooden cabinet/drum systems (production tanks/reaction vessels). The purpose of these drums is to penetrate the chemical materials into the leather with the help of water and the mechanical effect caused by the rotational movement of the cabinet, and to carry out skin-related reactions. In traditional drums, there is no pressure control and no vacuuming of the ambient. Inside the drums, there are some parts placed to increase the mechanical impact. The rotational motion in conventional drum systems agitates the chemicals, helping to keep the chemical concentration in the aqueous solution homogeneous. In addition, with the help of agitating the drum, a pumping effect is created by the continuous movement of the leather fibers, and the diffusion of chemicals is aimed to be realized. However, in traditional drum systems, the desired effect cannot be achieved sufficiently, the processing time is long and the amount of wastewater containing chemicals is high. Because the distance between the fibers of the skin cannot be opened sufficiently, the penetration process is very slow and can only be achieved with a higher concentration of dyestuff in the dyeing bath [1].
In addition to the production of footwear, clothing and leather goods, natural leather is now in high demand in the aircraft, yacht, automotive and furniture sectors and its market volume has grown steadily. The turnover of leather and their articles reached approximately 600 billion US dollars [2]. On the other hand, the environmental problems of the industry is huge and brings more and more environmental challenges [3,4,5].
Looking at the recent activities of the Leather Working Group (LWG), which consists of member countries around the world, it is seen that an awareness is being raised for sustainable leather production [6]. Companies engaged in leather production are explicitly required to use less water and chemicals, use less energy, minimize waste volume and pollution load, and reuse the waste generated. Again, there are policies of various brands to make purchases from companies having eco-labels in the sale of leather products. As can be seen, it will be an inevitable fact that companies that do not act in accordance with sustainable environmental production criteria will have difficulties in exports in the future. It is also foreseen that this situation may become more serious in the coming years with the sanctions imposed.
Dyeing is an economically and ecologically important process step in leather and textile production. The dyeing process, in which the appearance characteristics of leather are improved and the leather is brought to the desired color, constitutes one of the highest budget items in leather production, and anionic acid dyestuffs are used in this process step [1]. One of the difficulties encountered in aqueous dyeing is that the dyestuffs cannot penetrate the leather cross-section and superficial dyeing occurs. This is due to the unique structure of leather and the insufficient mechanical effect of current technologies. The corium layer of the leather prevents the penetration of the dyestuff due to the tight structure formed by elastin and collagen fibers. The inability of the dyestuff to penetrate under the fibrous structure in the corium layer prevents the homogeneous and balanced distribution of the dyestuff and causes more water, dye and auxiliary chemicals to be used for dyeing. Again, the chemicals used in leather production remain as a layer on the surface, resulting in quality problems such as hardness and low fastness values in the finished leather [7]. There are some studies on increasing dyestuff exhaustion in conventional production systems [8,9].
When we look at the studies on water management in the leather and textile industry, it is seen that there are very few studies on reducing the proportion of water used in the processes [10,11,12], which generally remains at the point of preventing the hazard of wastewater after it is formed. In recent years, efforts have been made to ensure sustainability in the leather and textile industry [13,14,15,16] and waste management plans are enforced [17,18]. The need to develop technologies that reduce the use of water and chemicals and to save energy by enabling reactions to take place in a shorter time is emphasized [19]. Reducing the use of process water is recommended, but hypotheses on how to ensure the penetration of chemicals into the matrix are not developed. Efforts to save water have not been sufficient, and effective results have not been put into practice. In these studies, chemical consumption was low when the carrier medium was kept low, so the final leather quality could not be achieved at the desired levels [20]. However, the main objective should be to improve processes and reduce water use without compromising the performance characteristics of the finished leathers. So far, the most successful results in this direction have been obtained with studies carried out in supercritical fluid ambient. The system is based on the use of a supercritical fluid medium under high pressure as a carrier instead of water [2, 21,22,23,24,25].
In a study on the application of a vacuum in leather production, Gudro et al. (2014) examined the usability and preservation time of the vacuum effect in the conservation of raw hides [26]. In the study, it was determined that when a vacuum was applied, raw hides could be stored for up to 21 days at +4 °C storage conditions without deterioration and microorganism activity remained very low. It was emphasized that with the method raw hides could be preserved without the use of salt, and the pollution load caused by salt could be eliminated. However, this study, which was carried out for the protection (conservation) of raw hides until they are put into production in factories, could not be used in the leather industry due to the short duration of the protection and the impossibility of putting it into practice in international transportation operations. This study does not address the process steps of leather production in an aqueous environment, i.e. water and chemical savings.
In this study, in order to solve the problems of the traditional dyeing system, it was aimed to design special production recipes to reduce the water and dyestuff ratios used and shorten the process time under low atmospheric pressure by creating a controlled vacuum in the leather production drum, and to determine the properties of the finished leather and wastewater. By placing the system under a controlled vacuum; it is aimed to open collagen fiber bundles faster and more efficiently, thus using low volumes of water and dyestuff as a carrier medium, easier/faster penetration of dyestuff between the opened fiber bundles and significantly reducing the dye load in the waste. Vacuum conditions were also tested for cotton and metal complex dyeing, demonstrating the effectiveness of the system for fibre type products.
Commercial domestic pickled sheepskins with a thickness of 0.6 mm were used as leather material. After tanning and post-tanning applications, the leathers were ready for dyeing experiments with both the conventional and vacuum methods. A cotton sample woven with yarn number (68/1) was used, made for the experiment at the AZALA textile mill. The composition of the sample is given in Table 1 below.
Composition and identification of cotton sample
| 68/1 | 68/1 | - | 100%c | 5/2 |
After pre-treatments, leathers were dyed conventionally according to Table 2. Variations to be studied for vacuum experiments were marked in bold. These are water, dyestuff usage and time parameters. By reducing these ratios with the advantages offered by the vacuum system, more economical and environmentally friendly recipe designs were attempted to be created.
Conventional dyeing recipe
| Dyeing | 40 | ||||
| 2 | Dye auxiliary | 30 | |||
| 1 | Formic acid | 40 | pH: 4.0 | ||
| Drain-Washing-Horsing-Mechanical operations | |||||
Black colored acid dyestuff of anionic character was used as dyestuff. “Acid Black 210” dyestuff of Turkuaz Chemical Company (Turkey) (CAS: 99576-15-5), which is widely used in the industry, was preferred because it shows surface and cross-sectional color properties more clearly [27]. A metal complex dyeing agent was also used with the same recipe to make a comparison between dyestuffs.
Technical properties of the vacuum dyeing drum were as follows:
Stainless steel chrome nickel material for the drum, Cr304 chrome body, resistant to acidic chemicals, Height: 1631 mm, Width: 832 mm (with rotating heads), Inner chamber diameter of the vacuum drum: 600 mm, Inner chamber width of the vacuum drum: 300 mm, Vacuum drum rotation drive motor power: 0.37 kW Vacuum cabinet rotation drive reducer: NMRV050, Vacuuming pressure of the vacuum pump: Up to 600 mm Hg, A pump that draws the air inside the leather production cabinet and allows the fibers in the leather to open and/or close the gap between the opened leather fibers by providing air pressure into the cabinet at any time by exhausting the air pressure, Vacuum pump motor power: 3 kW Viewing window to observe the reactions inside the cabinet, Thickness of observation glass panes: 10 mm, A pressure sensor to continuously measure the air pressure inside the processing cabinet and monitor it from the outside, Protrusions on the inside of the cabinet that allow the leathers to be attached and lifted upwards during the rotation and to increase the mechanical movement that occurs when the lifted leather falls down due to gravity, Heating element for heating the fluid in the process ambient, A shut-off valve that allows the liquid flow to be interrupted at any time, Swivel head connection to shaft hubs: 2 pcs, Chemical dosing chamber from rotating head to vacuum cabinet, Control panel for monitoring production parameters, Chemical discharge drain valve under the cabinet, Bidirectional rotation of the cabinet.
A prototype vacuum cabinet was manufactured in line with the specifications specified without compromising the shape of traditional leather production drums. In order to test the vacuum leather production cabinet manufactured; 5 sheepskins and 100% water by weight were taken into the cabinet and a vacuum environment created inside, where the cabinet was rotated for 1 hour, material was given from the chemical dosing unit from time to time, and it was checked whether there was any vacuum leakage during the cabinet movement. According to the test result, during the opening and closing of the chemical dosing chamber, the vacuum pressure could not be kept constant during the time in the internal environment, and some pressure losses were observed. In order to eliminate this problem, it was aimed to install a separate section between the chemical dosing unit and the cabinet, to transfer the chemicals from the external unit to the intermediate unit first and then to the drum. Thanks to the double-stage transmission system, it is planned to prevent air leaks and pressure changes. The prototype produced is given in Figure 1.
Visuals of a vacuum leather production drum (pressure/vacuum tanks, vacuum pump, rotary drum/reaction medium, control panel) and schematic diagram of the experimental system
The arrangement between the cabinet and the chemical dosing unit is shown in Figure 2.
Chemical dosing unit, and the set-up between the drum and dosing unit
According to the tests of the new setup, the vacuum pressure remained constant in the internal environment and no increase or decrease in the internal pressure of the cabinet was observed.
In the dyeing processes carried out in the traditional system, 100% of the bath and 5% of the dyestuff were used and the process time was 60 minutes. In the production recipe in the vacuum environment, water usage rates were specified as variable A, dyestuff usage rates as variable B, and dyeing time as variable C. The parameters were as follows;
Variables by recipe are shown in Table 3.
Leather dyeing recipe under vacuum
| Dyeing | 40 | ||||
| 2 | Dye auxiliary | 30 | |||
| 1 | HCOOH | 40 | pH: 4.0 | ||
| Drain-Wash-Horsing-Mechanics | |||||
After application of the dyeing recipe, post processes such as retanning and fatliquoring of the leathers were completed and the finished products produced. Float uptake was measured immediately after dyeing according to the UV-spectrophotemeter method, and color measurements on leathers were made on finished products after all processes as in the conventional system. The float uptake and color measurements on leathers were considered as target parameters for optimization studies for comparison.
The cotton fibre samples were wiped with a stain remover at 40°C for 30 minutes, and then thoroughly rinsed and dried at 25°C temperature. The cleaned material was soaked in clean water for 30 minutes before dyeing. The dyeing of the samples was carried out with an initial temperature of 50°C, and the temperature was raised to 100°C. Then the samples were wrung out to a weight gain of 90%, dried at a temperature of 25°C, followed by washing with hot water at a temperature of 45°C and then with cold water [28]. The conventional method was applied by using 10 g of dye and 1000 mL of process water. The same process was also applied under vacuum conditions, using half the amount of process water and with the hypericum extract dissolved.
Spectrophotometric measurements were performed to determine the amount of dye remaining in the bath and float uptake after the dyeing process. A Shimadzu UV-1800 spectrophotometer was used for these measurements. Floats were examined before and after dyeing, and exhaustion values (%) were reached. The absorbance differences were compared with the absorbance value of the dyestuff itself before dyeings, and the necessary dilutions were made in order to operate in a healthy reading range in the device. Dyestuff exhaustion (E%) was calculated according to the following formula;
A Konica Minolta spectrophotometer CM-3600d device was used for color measurements of dyed leathers. After the dyeing process of the finished leathers, the leathers were kept in a special conditioning room at 50% relative humidity for 48 hours in order to minimize the color difference due to moisture. The prepared samples were read on the device and color intensity values determined.
Kubelka-Munk theory is based on the premise that the absorption and scattering of light by a sample can be described using two parameters, defined as the absorption coefficient (K) and scattering coefficient (S). The absorption coefficient describes the fraction of incident light absorbed by the sample, while the scattering coefficient describes the fraction of incident light scattered by the sample.
Using the reflectance value at the maximum absorbance (λmax) at the visible wavelength (360–700 nm) for each dyeing, the color intensity values of the leather samples were calculated with the Kubelka-Munk equation given below.
In this equation, K is the absorption coefficient of the sample, S the scattering coefficient of the sample, and R corresponds to the reflectance value of the sample at the maximum absorbance [29].
Determination of color fastness to-and-fro rubbing cycles of leathers dyed in the conventional system and vacuum ambient was carried out according to the TS EN ISO 11640 standard test method [30]. This method is based on the determination of the behavior of the leather surface against rubbing with felt.
In dyeing studies in vacuum system production ambient, it was aimed to reduce water usage ratios, dyestuff usage ratios and dyeing time, and as a target it was determined to reach at least the quality values of the leathers produced in the conventional system under the planned conditions. If the limit values in the conventional system were not met using vacuum applications, the process was deemed unsuccessful. However, if dyeing designs using less water, time, and chemicals matched the conventional system’s results, the vacuum process was considered successful. For this reason, obtaining better data than the leather properties obtained with the conventional method or reaching at least the values achieved in the conventional system with reduced economic burden recipe designs was accepted as a success criterion. Leather color intensity/strength (
For all trials 2 sheepskins were taken into the same drum and 3 replicate productions carried out. Thus, all analyses were carried out on 6 leathers and residual waters (3 pcs-3 replicates). Means and standard deviations of the results were calculated by using an SPSS 15.0 statistical software package.
In our study, since it was planned to carry out completely water-free production in a vacuum ambient, the vacuum environment and the leathers inside were subjected to trials in water-free applications, but in the absence of water, breaks in the leather fibers were detected with the rotational movement of the cabinet and the mechanical effect. The determinations made as a result of the preliminary trials were useful in shaping the experimental design, and completely waterless productions were abandoned. However, it was planned to investigate the minimum water usage rates so as not to damage the leather fibers.
The dyestuff float uptakes and color intensity/strength values of the leathers obtained as a result of the dyeing processes carried out in the conventional system are given in Table 4. A complete cross-sectional dyeing of the dyed leathers was obtained.
Leather and float analyses after conventional dyeing process
| Conventional dyeing process | 71.18 ± 3.28 | 12.36 ± 0.57 |
As a result of repetitive and parallel dyeing processes, full cross-sectional dyeing was obtained in each leather, and the average dyestuff bath exhaustion for the conventional system was determined as 71.18%. Leather color intensity values varied depending on the proportion of dyestuff consumed by the leather from the bath. As a result of 71.18% float exhaustion, the average color intensity value was determined to be 12.36 (
Optimization studies for dyeing studies in the vacuum cabinet were started primarily on water usage rates. The results obtained after the productions and analyses are given in Table 5.
Leather and float analyses after dyeing processes carried out in a vacuum cabinet in dependence on water usage ratios
| 80% water use | 92.15 ± 1.47 | 20.36 ± 1.61 |
| 50% water use | 91.45 ± 1.77 | 19.20 ± 1.13 |
| 25% water use | 90.32 ± 2.63 | 18.78 ± 2.05 |
| Free of water use (only dissolving water) | 86.56 ± 2.82 | 16.45 ± 1.16 |
As a result of dyeing studies in a vacuum cabinet; the float exhaustion was 92.15% with 80% water use, 91.45% with 50% water use, 90.32% with 25% water use, and 86.56% in water-free (only dye solution) production. Considering that the bath consumption rate in the traditional system is 71.18%, it is seen that a significant bath consumption is achieved in the vacuum environment. It is observed that the color intensity of the leathers increased with the float uptake. As a result of conventional production, the color intensity (
Leather and bath analyses after dyeing processes carried out in a vacuum cabinet in dependence on dyestuff usage ratios
| 4% dyestuff | 94.02 ± 2.42 | 15.49 ± 1.90 |
| 3.5% dyestuff | 95.25 ± 2.91 | 12.41 ± 1.29 |
| 3% dyestuff | 93.35 ± 4.30 | 11.32 ± 1.37 |
| 2.5% dyestuff | 92.86 ± 3.19 | 9.45 ± 0.76 |
As shown in Table 6, the float uptake was 94.02% with 4% dyestuff use, 95.25% with 3.5% dyestuff use, 93.35% with 3% dyestuff use, and 92.86% with 2.5% dyestuff use. Considering that the float exhaustion ratio in the conventional system was 71.18%, it was seen that higher bath consumptions were achieved even at decreasing dyestuff ratios in the vacuum environment. However, it was revealed that the color intensities of the leathers decreased with decreasing dyestuff ratios, and only 4% and 3.5% dyestuffs gave higher levels of color intensity compared to the conventional system, while 3% and 2.5% dyestuffs gave leathers with lower color intensity. Accordingly, it was determined that the use of 3.5% dyestuff is the optimum parameter for vacuum productions. Thus, it turns out that the use of dyestuff can be reduced by 30% in vacuum ambient compared to the conventional system. Finally, optimization of the dyeing/process time was carried out. The results obtained after the productions and analyses are given in Table 7.
Leather and float analyses after dyeing processes carried out in a vacuum drum in dependence on the process time
| 45 mins | 94.05 ± 2.87 | 15.18 ± 1.52 |
| 30 mins | 92.02 ± 1.96 | 12.38 ± 2.39 |
| 15 mins | 80.11 ± 3.69 | 10.77 ± 1.10 |
As shown in Table 7, the float exhaustion at the end of the 45-minute process was determined as 94.05%, at the end of the 30-minute process as 92.02%, and after the 15-minute process as 80.11%. Considering that the float exhaustion rate in the conventional system was 71.18% and the
The uptake of the dyestuff by collagen matrix is demonstrated with Figure 3 as computer simulation.
Opened collagen matrix by vacuum and full dyeing of leather
Figure 4 shows the residual floats after conventional dyeing and after production in vacuum ambient with optimized parameters.
Residual floats of conventional dyeing (left) and vacuum dyeing (right)
The color difference of the floats in Figure 5 shows the differences in exhaustions. After the dyeing studies with acid dyestuff, it was aimed to investigate the behavior of metal complex dyestuff under both conventional and optimized vacuum process conditions, for the purpose of which the same dyeing recipe was tested by changing only the type of dyestuff. Moreover, cotton dyeing was also applied under a vacuum with half water according to the conventional system. Table 8 presents the results of leather analyses with metal complex dyestuff and cotton dyeing results obtained using the conventional process and vacuum ambient.
Results of leather dyeing with metal complex dyestuff and cotton dyeing carried out in a conventional and vacuum cabinet
| Conventional system (metal complex dye, leather) | 74.05 ± 4.47 | 14.81 ± 3.35 |
| Vacuum system (metal complex dye, leather) | 96.79 ± 2.24 | 15.23 ± 0.74 |
| Conventional system (hypericum dye, cotton) | 92.21 ± 1.93 | 6.91 ± 1.23 |
| Vacuum system (hypericum dye, cotton) | 98.76 ± 2.01 | 7.06 ± 1.19 |
Conventional dyeing with acid dyestuff has a float exhaustion of 71.18% and a
In the last stage, the rubbing fastness properties of leather and cotton products dyed with the optimized vacuum cabinet parameters and, leather and cotton products dyed with the conventional system were compared. It was examined whether dyestuff was sufficiently bound to the surface of the material in vacuum ambient. Analyses were performed on acid dyed leather, metal complex dyestuff treated leather and hypericum treated cotton. Table 9 shows the results of rubbing fastness analyses of leathers and cottons dyed in a vacuum and by the conventional method.
Rubbing fastness values of dyed samples
| Conventional system (acid dye, leather) | 4/5 |
| Conventional system (metal complex dye, leather) | 4/5 |
| Conventional system (hypericum dye, cotton) | 4 |
| Vacuum system (acid dye) | 4/5 |
| Vacuum system (metal complex dye) | 4/5 |
| Vacuum system (hypericum dye, cotton) | 4 |
When Table 9 is examined, it is seen that the rubbing fastness values of the leathers dyed with both acid dye and metal complex dye in vacuum ambient are equivalent to the values of the leathers produced with the conventional system (4/5). For cotton dyeing, the fastness to rubbing values of the products dyed with the conventional and vacuum systems were also found to be same (4). Therefore, as a result of the dyeing processes carried out in the vacuum system, it was concluded that there was no problem in terms of the binding of dyestuff to fibre type materials in the ambient where less water was used.
The success criteria for vacuum dyeing processes focused on achieving high float exhaustion, reducing dyestuff usage, and shortening dyeing times. These objectives were successfully realized. The primary goal was to enhance color intensity within the matrix by increasing the float uptake as compared to conventional methods. While similar or slightly improved color intensities were achieved, this was accomplished with reduced water consumption, shorter processing times, and lower dye usage. Despite higher float uptake, the reduction in dye concentration resulted in color intensities comparable to those of traditional methods.
Throughout the experiments, maintaining color intensity values equivalent to or better than those obtained with conventional systems remained a key objective, with no compromise on quality. The principle of meeting or exceeding the standards of classical methods was upheld in all vacuum system productions. By comparing vacuum system outcomes with those of conventional methods, the success criteria and optimal vacuum production conditions were effectively identified. Achieving all target values within a vacuum atmosphere marked the study’s success.
The findings demonstrate that dyeing processes with higher exhaustion, significant time savings, and reduced water and dyestuff consumption are achievable under vacuum conditions. Vacuum dyeing proved successful and advantageous for both leather and cotton products, with no issues observed in the binding of dyestuffs to fiber materials. As sustainable production systems gain prominence, these promising results suggest substantial potential for industrial adoption. Various initiatives are being planned to capitalize on these findings and advance sustainable dyeing practices.