Technological possibilities for adding value on defective skins: Practical valorization of leather products with chromium and dyeing spots

Abstract

In this study, decolorization via a bleaching operation was carried out on chrome and dye spotted skins with several bleaches to valorize the defective leather materials. Sodium perborate, hydrogen peroxide, sodium thiosulfate, sodium percarbonate, and oxalic acid were applied as bleaching agents in production. Sodium perborate by itself was applied and combined with other bleaching agents. Combined processes were also supported with an emulsifier agent for uniform distribution and deeper penetration of bleaching agents into the fibers. Color changes and differences on leather products were determined by Konica Minolta CM-3600d test apparatus spectrophotometrically before and after the applications. Bleached leathers were processed again, re-dyed, and finished after the bleaching operations. Then, bleaching performances of the single and combination applications were also evaluated for the leather strength indicators by analyzing tensile and tear strength of the final products as well as chromium (VI) oxidation of basic chromium (III) salts used in tanning. Antibacterial properties of the final bleached products were also investigated. As a result, sodium perborate was found to provide the most effective and safe bleaching without any physical and chemical deformation by creating antibacterial activity on leather.

Keywords

Leather defects bleaching oxidation valorization strength antibacterial

Chromium tannage is the most widely used technique by leather industry worldwide. 90% of leather products are tanned by basic chromium sulphates (BCSs) globally because of its advantages provided.^1–3 On the other hand, some wrong applications, inattention to dosages, improper recipes of the operators and defects in chromium tanning procedure cause the inhomogeneity appearance in dyeing process of leathers. These leathers are defined as poorer for the aesthetic properties and ended up with very low prices in sales for leather factories. For this reason, skin defects on surface should be eliminated using practical and effective ways.⁴

Chromium surface spots/stains typically appear as large and blue/green stains/spots often in liquor mark patterns. These stains occur when the chromium salts used in the tanning process are not evenly distributed or properly fixed in the leather. Some common reasons for this kind of chromium spots/stains are uneven applications of chromium solutions, patches of uneven pH, and heat/temperature damage effects. Degraded skin collagen takes up more chromium as the breaking of the collagen bonds creates more binding sites for chromium fixation. Such defects can occur due to lack of process control and improper/wrong applications of the operators.¹ Temperature/heat damage caused by undiluted process acids can also induce darker marks to show up after tanning application. The most significant problem is defined as the pooling of chromium solution. If chromium solutions are allowed to stand on the skin after the unloading from the vessel, the chromium tannage will go on to fix. Washing the leathers properly at the end of the tanning process and preferentially horsing/stacking to drain is needed. Careless washing leaves tanning liquors on the skins and this fixes the chromium spots on the surface during drying in batches. Chromium staining appears as very small spots. These spots can be also caused by application of strong alkaline powders in the tanning process leading to the rapid/sudden chromium fixation in contaminated surface regions because of the incompatible pH ambient for the tanning agent. On occasion, these types of intensive spots/stains can also allow the dyestuff precipitations on. Chromium soaps occurring in the tanning bath may cause pinky spots on the wet blue surfaces that can then go on to cause trouble in the dyeing processes. Chromium soap is caused by decomposition of the natural fats to the free fatty-acids which react with chromium to provide pinkish colors.⁵

The stained appearance in the dyeing process caused by fast fixation, inaccurate temperature, and insoluble dyestuffs in the dyeing floats and improper pH adjustment during the neutralization process can lead to undesirable impurities on the surface of the leather. Thus, homogeneity in the surface colors is destroyed. Because of these reasons, leathers having such colored impurities lose their appeal, are sold with very low prices, and bring trouble for the companies. That is why a bleaching operation is needed for the fiber-based products preparation to eliminate the colored impurities, develop aesthetic appeal, maintain dyeing properties, and enhance product quality.^6,7

Bleaching of fibers on the industrial scale is by far the most significant application of the various chemicals. Nearly all oxidizing and reducing compounds could be theoretically used in bleaching applications: however, just a handful of compounds are applied in technical practice.⁸ Sodium perborate (SP) and sodium percarbonate, as oxidative bleaching agents, have been widely applied as fabric bleach compositions.⁹ SP and hydrogen peroxide (HP) bleaches can release the oxygen, and their combined processes are more effective and synergistic as reported by some authors.¹⁰ Creating an antimicrobial effect has also been reported to be possible for bleaching procedures.^11–13 This has been another target of defective leathers for adding value via bleaching operations.

The main objective of this study is to investigate the effectiveness of various bleaching agents and their combinations in decolorizing and valorizing defective leather materials, specifically focusing on leather with chromium and dyeing spots. This includes an in-depth analysis of SP, HP, sodium thiosulfate (ST), sodium percarbonate (SPC), and oxalic acid (OA) in individual and combined applications. We aim to assess these agents based on their decolorization efficacy, impact on the physical integrity of the leather, and any additional properties imparted, such as antibacterial effects. Our approach involves systematic experimentation with these bleaching agents under varied conditions, followed by quantitative and qualitative assessments using spectrophotometry, tensile and tear strength tests, and chromium (VI) oxidation analysis. The objective extends to understanding how these bleaching processes can be optimized for practical application in the leather industry, potentially offering a solution to enhance the value of defective leather products.

Mainly, this research is related to the use of different and combined bleaches for safely eliminating of the contaminants on dyed and/or wet blue skin areas without any physical decomposition and to gain economical value for the defective products again with an antibacterial surface.

Experimental

Material

Chrome-tanned domestic sheep skins were used as material. The skins had chrome and dye spots on their surface. SP, HP, OA, SPC, and ST were purchased from Sigma-Aldrich Co., Ltd and used as bleaches.

Bleaching process of the defective skin surfaces

Water was used on the wet-blue skin weight as 300% ratio (liquor ratio: 1:3 on the skin weight) at 38°C for 60 min. SP and SPC with 1%, 3%, and 5% of solutions were applied alone to start, then combined with the other bleaches. A 1% emulsifier was also used in the combined bleaching processes for uniform distribution and deeper penetration of bleaching agents into the fibers. Moreover, it was also used for aiding in the removal of dirt, stains, and other impurities from the fiber surface, which not only improved the bleaching effect but also prepared the fibers for subsequent processes like dyeing. Each experiment was carried out in triplicate.

Colorimetric analyses

A Konica Minolta brand spectrophotometer was used to measure the colors of the skin samples which were applied with bleaches, and the changes of the surface colors was checked in relation to the original leather samples. Color differences in the leathers processed by bleaches and the original sample were measured with the formula of CIE L* a* b* 76:¹⁴

Δ E = \sqrt{{(Δ L)}^{2} + {(Δ a)}^{2} + {(Δ b)}^{2}}

(1)

According to CIE L* a* b* colorimetric system (Figure 1), L values which are increasing and decreasing indicate increasing or decreasing whiteness/lightness levels of color. The increases or decreases in +a, −a, +b, and −b directions define the increases or decreases in red, green, yellow, and blue color indicators, respectively. The measurements were carried out three times in different places on the leathers.

Figure 1.

Commission Internationale De L’Eclairage (CIE) L* a* b* color system.¹⁴

The percentage reflectance values (at λ_max = 400 nm) were recorded and color strength values (K/S value) were calculated according to the Kubelka-Munk formula (equation (2)).

K / S = {(1 - R)}^{2} / 2 R

(2)

where K indicates the scattering coefficient, S is the absorption coefficient, R is the reflectance (R is the decimal fraction of the reflectance of dyed leather, R = 1.0 at 100% reflectance).

Re-processing applications of the bleached skins

The recipe for the wet blue (chromium tanned skins) leathers after the bleaching operation is provided in Table 1. Dye spotted skins were only re-dyed and washed after the bleaching. The recipe is part of standard and conventional leather processing. Neutralization is used to increase the pH level for anionic chemicals to be used in dyeing, fat liquoring, and retanning processes. Dyeing is applied for colorization of the leathers. The fat liquoring process is for softening of the collagen fibers of the skin. Then, some fillers are used during the retanning process to get the homogenous touch/feeling on the leather surface via binding with skin fibers. The last washing was applied before piling/stacking of the leathers. Three skin samples were produced without any bleaching as the control group for the comparison.

Table 1.

Recipe for process after the bleaching application

Processes	%	Substance	Temperature (^oC)	Duration	Explanations
Wash	100	Water	30
	0.5	HCOOH		45 min	Drain the drum
Neutralization	100	Water	35
	1	HCOONa		10 min
	1	NaHCO₃		3 × 15 min + 45 min	pH: 5.0–5.2 and drain
Wash	300	Water	35	10 min	Drain the drum
Dyeing, fat liquoring and retanning	100	Water	40
	5	Dyeing agent		60 min
	8	Natural and synthetic fat liquoring agent		60 min
	2	Phenolic syntan		20 min
	2	Synthetic tannin		20 min
	1	HCOOH		3 × 15 min + 45 min	Drain the drum
Wash	300	Water	20	3 × 15 min + 45 min	Drain the drum

Leather strength analyses

Before the analyses, each finished leather was conditioned to the EN ISO 2419¹⁵ standard and sampling was carried out to the EN ISO 2418¹⁶ standard (Figure 2).

Figure 2.

Sampling from HGJK square for physical tests.

Each product was put to the strength tests with triple repetition (three vertical, three horizontal samples). The Shimadzu AG-IS brand apparatus was employed for each analysis. Thickness measurements of the leathers was carried out according to EN ISO 2589,¹⁷ tensile and tear strength to EN ISO 3376¹⁸ and EN ISO 3377-2¹⁹ standards, respectively.

Chromium(VI) oxidation analyses

Chromium(VI) oxidation analyses in leathers were applied three times to IUC 18 standard (EN ISO 17075 2007)²⁰ before and after the bleaching application. This applied method is dependent on the chromium reaction with the used diphenyl carbazide analysis chemical.

Antibacterial activity tests

Analyses and procedures on antibacterial properties of the final bleached leathers were carried out at M. Kh. Dulaty Taraz Regional University Laboratories. The assessment of antibacterial properties was performed on bleached leather in accordance with ASTM Standard E 2149-01, targeting E. coli ATCC 25922 and S. aureus ATCC 25923. The initial bacterial suspension was diluted to a turbidity of 0.5 McFarland standard (equivalent to a concentration of 1.5–3.0 ×10⁸ cfu/ml) using sterilized Ringer solution. The concentration of bacterial dilution was verified using spectrophotometry, measuring absorbance at 625 nm. This solution was utilized to create the operational bacterial dilution for the experiments by appropriately diluting it in a sterile buffer (0.3 mM KH₂PO₄, pH = 7.2 ± 0.1), achieving a final concentration of 1.5–3.0 × 10⁵ cfu/ml. To assess antibacterial activity, sterile 250 ml flasks were employed, each containing 50 ml of the operational bacterial dilution, into which 2 cm × 2 cm bleached leather samples were introduced. Subsequently, these flasks were positioned in a temperature-controlled bath at 37°C, with orbital stirring.

Following 1 h of continuous stirring, 1 ml of the solution was aseptically retrieved to ascertain the bacterial concentration utilizing standard plate count techniques on nutrient agar. The colony counting results were transformed into colony forming units per milliliter (cfu/ml) and utilized to compute the percentage reduction of bacteria.

The reduction in colony count percentage was calculated using equation (2).

Reduction (%, \frac{cfu}{m l}) = \frac{B - A}{B} \times 100

(3)

where B–A = death rate constant, A = cfu/ml for the flask containing the treated substrate after the specified contact time, and B = “0” contact time cfu/ml for the flask used to determine “A” before the addition of the treated substrate.

Statistical analysis evaluations

The analysis data were checked by using ST, SPC, and OA methods statistically, descriptive statistical, and Duncan tests in the statistical software package. Each data value was represented as mean for three independent measures. Comparisons of means were analyzed when p < 0.05.

Results and discussion

The bleaching processes of the defective skins were carried out with 1%, 3%, and 5% SP and SPC agents, firstly. Six experiments were applied on the dyed skin samples to optimize the chemical dosage proportions for beginning operations. Kasiri et al. have discussed how factors like ozone dosage, dye concentration, solution pH, and temperature can affect the efficiency of decolorization in leather dyeing and finishing processes.²¹ The time duration for leather bleaching is selected to ensure thorough reaction without overexposure, which could damage the leather. Mistik and Yükseloglu explored varying time durations (20, 30, and 60 min) at different temperatures to determine the optimal conditions (60 min) for HP bleaching of cotton fabrics.²² The temperature for leather bleaching processes is often optimized based on the reaction kinetics of the bleaching agents and the thermal stability of the leather. When we evaluate materials, they are chromium tanned and can resist a higher temperature. After pre-trial studies and literature survey, the process parameters were determined as 300% water, 38°C, and 60 min duration for each combination. Table 2 provides the effectiveness of SP and SPC bleaches with three different percentages.

Table 2.

Sodium perborate (SP) and sodium percarbonate (SPC) effects on the dyed skin surfaces

Experiments	Process parameters	Bleaches	ΔE values
E1	300% water38°C60 min.	1% SP	2.39 ± 0.41
E2		3% SP	5.44 ± 0.52
E3		5% SP	5.49 ± 0.46
E4		1% SPC	1.52 ± 0.33
E5		3% SPC	4.00 ± 0.48
E6		5% SPC	4.11 ± 0.57

Bleaching effectiveness was improved via the increased bleach proportions of both SP and SPC. When proportions were increased from 1% to 5%, the bleaching effect was also increased with the ΔE indicators for evidence. The best efficient and optimal bleaching was performed with 3% SP with E2 as seen in Table 2 and it was not necessary for greater dosage with 5% of E3 experiment. The difference in color of the skin surfaces in the bleached and control samples was 5.44 ± 0.52. Because of the increase from 5.44 to 5.49, there was no sense in increasing the using ratio from 3% to 5%. the lowest bleach effect was also provided by 1% SPC with 1.52 ΔE value.

After demonstrating SP’s greater effect compared to SPC, combined bleaching applications were experimented via SP agent. In a study by Onem et al., optimal parameters for combined bleaching processes included specific time durations for ozonation and a certain water pick-up value for maximum decolorization effect.²³ This indicates that combined processes are often formulated based on achieving the best possible outcomes in terms of effectiveness and efficiency. The combined bleaching processes are also formulated by considering the environmental impact and the material quality consistency. For instance, combining processing technologies and waste management techniques can enhance environmental sustainability, as highlighted by Dixit et al.²⁴ Table 3 indicates 3% SP combined bleaching. Combinations were with HP, OA, and ST agents. An emulsifier (E) was also used for each combination application.

Table 3.

Combined processes via sodium perborate (SP) bleaching agent

Experiment	Bleaches	ΔE values
E7	3% SP	1.90 ± 0.37
	1% E
	0.5% HP
E8	3% SP	2.15 ± 0.40
	1% E
	0.5% OA
E9	3% SP	1.93 ± 0.36
	1% E
	0.5% ST

E: emulsifier; HP: hydrogen peroxide; OA: oxalic acid; ST: sodium thiosulfate.

The OA bleaching combination gave a better effect than HP and ST agents with E8. The HP combination effect provided 1.90 ΔE value. The OA combination showed 2.15 color difference. The color difference was 1.93 for the ST combination in terms of the ΔE value. Meanwhile, SP itself provided superior effectiveness for the skin surfaces according to each combination. E2 was selected as the optimal process with 5.44 ΔE value for the following operations to be applied on the chrome defective skins of wet-blues. The 3% SP bleach and its combinations were carried out for the chrome tanned skins as well, and Table 4 gives each bleach application on both dyed and wet blue leathers.

Table 4.

Colorimetric indicators on wet-blue and dyed skin surfaces

Exp.	Wet-blue skins			ΔE values
Exp.	Bleaches			ΔE values
E10	3% SP	–	–	18.61 ± 1.97
E11	3% SP	1% E	0.5% HP	10.74 ± 1.02
E12	3% SP	1% E	0.5% OA	11.02 ± 1.11
E13	3% SP	1% E	0.5% ST	10.91 ± 1.07
	Dyed skins
Exp.	Bleaches			ΔE values
E2	3% SP	–	–	5.44 ± 0.52
E7	3% SP	1% E	0.5% HP	1.90 ± 0.37
E8	3% SP	1% E	0.5% OA	2.15 ± 0.40
E9	3% SP	1% E	0.5% ST	1.93 ± 0.36

E: emulsifier; HP: hydrogen peroxide; OA: oxalic acid; SP: sodium perborate; ST: sodium thiosulfate.

Table 4 and Figure 3 provides the relative evaluation of the bleach application for the wet blue and also dyed skins. The best effect of the process was provided on chrome tanned skins via the only SP treatment. Colorimetric differences between the control and bleached surfaces was 18.61 ± 1.97 for 3% SP. The OA combined process was superior according to the alternative combination applications on chrome tanned and dyed skin surfaces with 11.02 ΔE and 2.15 ΔE, relatively. The only SP process was also the more advantageous for economical costs compared to the combined applications.

Figure 3.

Comparative evaluation of bleaching on wet-blue and dyed skin surfaces. E: emulsifier; HP: hydrogen peroxide; OA: oxalic acid; SP: sodium perborate; ST: sodium thiosulfate.

In the study, after comparing the bleaching effect between SPC and SP, it was determined that SP was a more effective bleaching agent. After that two reductive bleaches (OA and ST) and two oxidative bleaches (SP and HP) were studied in different combinations for efficient color elimination in dyed and chromium tanned leather.

The reaction mechanisms of bleaches when applied to fibers are fundamental to enable understanding of the colorimetric differences observed between control (untreated) and bleached surfaces. The color elimination mechanism of bleaches involves a series of combination of chemical and physical reactions that break down or alter the structure of colored compounds (chromophores) present in the material.²⁵ When we consider the leather material, our discussion focuses on the chemical change causing the decomposition of colored compounds in dyed and chromium tanned leather by various mechanisms.

ST, often used as a reductive bleaching agent, works quite differently from oxidative bleaches like HP or SP. The primary mechanism of reductive bleaches involves the chemical reduction of colored compounds by adding electrons to the chromophores. This reduction process alters the chemical structure of these molecules, typically breaking down the conjugated double-bond systems that are responsible for their color.²⁶ OA is an organic compound that is used in various applications, including as a bleaching agent. Unlike typical bleaching agents, OA works primarily through chelation and reduction. OA can act as a chelating agent. It can bind with metal ions present in stains or on the surface of materials, forming a soluble complex that can be washed away. Moreover, it can act also a reductive bleach by adding electrons into the colored compounds that eliminate the color effect on the material.²⁷

HP and SP are both oxidative bleaching agents, but they have some distinct differences in their chemical composition, mechanism of action. HP is a simple peroxide, a compound with an oxygen-oxygen single bond (H-O-O-H). It acts directly through oxidation.²⁶ When applied to a substrate (like fabric or leather), it breaks down, releasing oxygen radicals species such as hydroperoxyl/superoxide radicals, amino radicals, and hydroxyl radicals.²⁸ These radicals break the chemical bonds of the chromophores, effectively decolorizing the stain or material. However, SP is a compound containing a perborate anion. It is a source of ‘reactive oxygen' in many laundry detergents, releasing HP upon dissolution in water. Compared to HP, SP appears to be a less damaging bleaching agent, suggesting a milder chemical interaction with the substrate.²⁹ When dissolved in water, it breaks down into HP and borate. The HP then acts as the active bleaching agent. Similarly, the perborate ion in SP decomposes into oxygen radicals and borate ions in the presence of water, enabling it to bleach fibers through oxidation. The reactive oxygen molecules released by bleaches break up the chemical bonds of chromophores of the colored compounds. This is represented by two mechanisms for the oxidative cleavage reaction of azo dyes, involving either -C-N- or -N=N- cleavage. Decoloration can occur by breaking up the chromophore groups, generally by destruction of one/more of double bonds within the conjugated systems. The modified chromophore groups do not reflect color or reflect a color outside the visible light region.²³ When we evaluated the color differences between the bleaches, SP stands out with its different mechanisms of action explained above. In the study, the most effective bleaching after SP was obtained in the group with SP and OA addition. This is thought to be due to the chelating property of OA. These results was also evaluated with color strength values.

Figure 4 demonstrates the color strength values (K/S) of bleached leathers of dyed and wet-blue skins. A lower K/S value defines more whiteness properties of leathers. That means better bleaching process by oxidation. Color differences between products and K/S values after bleaching applications support each other. Higher K/S values define color strength. Measurement of K/S values was for better proof and supportive explanations of the bleaching results. Even though it gives different data, it serves the same purpose working independently.

Figure 4.

Color strength (K/S) values of bleached wet-blue and dyed leather samples. E: emulsifier; HP: hydrogen peroxide; OA: oxalic acid; SP: sodium perborate; ST: sodium thiosulfate.

On the wet-blue skins and dyed samples, only SP applications showed the closest whiteness value in Figure 4. The color strength of the sample was 9.86 with K/S indicator on the wet-blue skins. The highest K/S and color strength was 13.25 with the SP + E + HP combination. This whiteness value also proved the best bleaching operation with a lower K/S value than other combination applications. The K/S value was 22.43 on the dyed samples with only SP application, 24.75 with SP + E + OA, 26.54 with SP + E + ST and 27.92 with SP + E + HP combinations. Higher K/S values demonstrate the color strength and lower bleaching effect. A better effect was obtained with only SP application compared to the combinations, but there was a lower bleaching effect of SP than the operations on wet-blue leathers. The OA combination process was superior than other combination applications on the skin surfaces for the K/S indicators as well for both dyed and wet-blue samples. The K/S results showed that the effects of the combinations were lower than the single use of SP bleach and SP bleach is the most suitable bleach in the study for decolorizing stains on leather.

Some authors have reported that it is probable for bleach operations to lead to some physical deformation of the treated fibers.^30,31 Therefore, leather’s mechanical durability after the bleaching applications was also examined in our study. The most effective bleach was provided on chromium defective skins, hence wet blue bleached skins, and then finished leathers were exposed to tear and tensile strength analyses. Table 5 shows the analysis data for the finished/final leathers that were bleached and processed again.

Table 5.

Strength analysis results of the bleached and re-processed skins

Processes	Tensile strength values (N/mm²)	Tear strength values (N)
No bleach application	10.24 ± 2.16^a	36.02 ± 4.25^a
SP bleach	9.87 ± 2.24^a	34.98 ± 3.96^a
Combined bleaching 1	9.13 ± 0.98^b	28.54 ± 4.01^b
Combined bleaching 2	9.81 ± 1.99^a	31.15 ± 2.97^b
Combined bleaching 3	9.21 ± 2.02^b	28.65 ± 2.84^b

SP: sodium perborate.

a,b

Significantly different values (p < 0.05).

The tensile test results provide the significant quality indicator of tested products and play an important role in reporting of performance characteristics of materials.^32–34 Tear strength of leather products is also of considerable importance in producing the desired leather good materials. Knowledge about the utility of the wearing products can be provided via the determination of their tear and tensile properties.³⁵

Bleach can oxidize protein in leather, leading to potential damage of fibers,³⁶ but the SP bleaching application did not affect both analyzed strength values of leather products shown in Table 5. Tensile strength value decreased from 10.24 N/mm² to 9.87 N/mm² with the SP bleach application compared to the control sample (without any bleach application), but this decrease was not significant, as indicated in Table 5. The OA combined process also did not reduce the tensile strength as statistical evaluation from 10.24 ± 2.16 to 9.81 ± 1.99. The HP and ST combination operations decreased the tensile test values significantly in range according to p < 0.05, shown in Table 5. Each combined application decreased the tear load values of bleach applied samples with considerable differences in values as small amounts. The SP bleach application decreased the tear load from 36.02 N to 34.98 N. This decline could be ignored when the positive approach of bleaching was considered. The top reductions for strength test values were noted for HP bleach application. As a consequence, the best effective bleach was provided by the SP agent with no physical deformation of the leather products.³⁷

The SP agent has been reported as a gentler bleach, not causing damage to fabric samples and dyes. Thus, it has been applied as the major fabric bleach in the European region and is commonly an important component. The SP agent provided the best bleaching effect via eradicating the surface defects of the skin samples and did not decrease the tear and/or tensile strength as significant factors for the final product properties.³⁷ Studies have also shown that SP is biocompatible, meaning it does not cause significant morphological or functional alterations in biological systems. This property is important in leather treatment to ensure that the bleaching process does not adversely affect the leather's integrity or pose environmental hazards.³⁸

Tanning of around 85–90% produced leather products are by BSCs worldwide. Because oxidative bleaches could potentially convert chromium (III) salts in leather to chromium (VI) by oxidation reaction, analysis of this oxidation possibility was needed. The formation of such matter in leather could lead to the leather products as garments, upholstery, and shoe linings being directly in contact with the human body and seriously impact human health. Therefore, eco-benign and safe decolorizing applications for leather materials should also be studied for bleaching. Chromium (VI) oxidation formation in leather products was evaluated after bleaching operations. The obtained results indicate that chromium (VI) of the samples was under 3 ppm for each bleach application. Leather products having that limited value are regarded as safe since the international regulations state chromium (VI) had to be under 3 ppm level to abstain from the ecological risks for the produced leather materials.^39–41 This result also proves the safety of the bleaching process and the product as the environmentally friendly decolorization method.

Table 6 provides the effect of different bleaching methods on antibacterial activity for leather.

Table 6.

Antibacterial effects of bleaching methods for leather

Processes	E. coli reduction rate (%)	S. aureus reduction rate (%)
No bleach application	–	–
SP bleach	76.4	76.7
Combined bleaching 1	85.6	86.4
Combined bleaching 2	76.9	77.3
Combined bleaching 3	76.2	76.1

SP: sodium perborate.

The outcomes presented in Table 6 revealed that leather samples subjected to various bleaching methods exhibited notable antimicrobial efficacy against two bacteria. In contrast, the leather samples from the control group did not exhibit any antibacterial activity. Combined bleaching 1 application showed the best antibacterial effect on the final leather products. When the analysis results were evaluated, it was determined that the source of the antibacterial effect on the bleached leathers was SP.

SP, a chemical compound commonly used as a bleaching agent and disinfectant, does possess antibacterial properties.⁴² It releases HP when in contact with water, which is known for its antimicrobial properties.⁴³ HP exhibits antibacterial properties through a variety of mechanisms, primarily due to its strong oxidizing nature. It generates oxidative stress within bacterial cells by releasing reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and superoxide radicals (O₂·^-). These radicals can damage cellular components like DNA, proteins, and lipids, disrupting essential cellular processes and structures. This can lead to mutations, cell cycle arrest, and ultimately bacterial cell death.⁴⁴ Upon comparing the analysis outcomes, increasing the amount of HP in the bleaching application causes an increase in the antibacterial effect on the leather. OA in combined bleaching 2 application and ST in the combined bleaching 3 application show a similar bacterial reduction on E. coli and S. aureus compared with SP bleaching.

Bleaching agents demonstrate a commendable ability to hinder bacterial growth and deliver effective antibacterial properties to leather without relying on harmful synthetic biocides. It was observed that as the HP content increases, the populations of both gram-positive and gram-negative bacterial cells on the leather decreases. These obtained results will be helpful for valorization of leather products and of interest to the consumers.

Conclusions

Impurities on the leather surfaces cause some problems in the wet and coating processes to create a homogeneous appearance. These leathers are defined as poor for their aesthetic properties and result in much lower prices in the trade of leather factory and companies. For this reason, such surface defective leathers must be prevented before the finishing process via efficient methods.

SP, HP, ST, SPC, and OA were applied as bleaching agents for this research study. SP by itself was applied and combined together with other bleaching agents. Combined processes were also supported with an E agent. Color changes and differences on leather products were determined spectrophotometrically before and after the applications.

According to the data obtained, SP provided the most effective and safe bleaching for the elimination of the impurities/spots without any physical and chemical deformation by creating antibacterial activity on leather. Chrome and dye stained/spotted skins were effectively disposed of with no loss of strength or in the class/quality indicative of skin/leather properties and without any oxidation problem (Cr⁺³ to Cr⁺⁶) as a safety indicator of international regulations. The SP bleaching application can gain economical value for good quality leather materials produced by maintaining the chemical and physical composition, and as the safer and gentler bleach agent. The antibacterial property on the leather surfaces is another advantage of this application method for valorization of final products.

This research was conducted in collaboration with a leading leather manufacturing company, addressing a critical industrial challenge. The findings have not only provided a practical solution for effectively eliminating the prevalent issues related to leather impurities but have also significantly contributed to adding value to defective leather products. Numerous companies facing similar challenges have since adopted the methodologies developed in this study, reporting substantial improvements in product quality and satisfaction with the outcomes. Our work not only addresses a specific industrial challenge but also contributes to the broader discourse on sustainable and efficient leather processing.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by Scientific Research Projects Coordination Unit of Ege University. The authors would like to thank to Ege University for the provided financial support (Project No: FHD-2021-23207). The authors would also like to thank Ata Dilek Leather and Automotive Company for the technical assistance and financial support they provided.

ORCID iD

Ali Yorgancioglu

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