The Use of Collagen Hydrolysate from Chromium Waste in the Optimization of Leather Retanning

Abstract

Leather tanning generates substantial amounts of solid waste and effluents, posing significant environmental challenges due to the presence of hazardous chromium compounds. The aim of this study was to develop and optimize a method for recycling chromium-tanned leather waste by utilizing it as a raw material in the retanning process. Collagen hydrolysate was extracted from chrome-tanned leather shavings through acid hydrolysis and subsequently incorporated, together with melamine, into novel retanning compositions. The experimental design, based on the Kleeman method, involved varying the hydrolysate content (25%, 30%, 35%) and melamine concentration (2.5%, 3.0%, 3.5%, 4.0%) to assess their impact on the physicochemical properties of retanned wet-blue leathers. An innovative aspect of the study was the integration of the Kateskór computer program, employing the Kleeman experimental planning method, to optimize the formulation of retanning compositions. This computational approach enabled the precise determination of hydrolysate and melamine quantities required to achieve leather properties that meet both producer and consumer expectations. The optimized formulation identified the hydrolysate content in the range of 28.78–29.63% and melamine in the range of 3.61–3.68% as optimal for obtaining leathers with the desired mechanical strength, shrinkage temperature, and water vapor permeability. The study presents a practical model of a circular economy within the leather industry, aligning with the European Green Deal Strategy by promoting resource efficiency and minimizing hazardous waste. The proposed methodology provides a viable pathway for sustainable leather production through waste valorization and process optimization.

1. Introduction

Leather tanning involves converting raw hides, which are by-products of the meat industry, into finished leathers that can be used for the production of footwear, clothing, leather goods, etc. Tanned leathers are soft, flexible, durable, resistant to decomposition by proteolytic bacteria, and exhibit high hydrothermal resistance [1]. These properties are mainly attributed to chrome tanning, which has become the most common tanning method since the 1930s, accounting for 80% to 90% of global production [2,3]. However, the tanning industry continues to raise environmental concerns due to the large volumes of wastewater and tannery waste it generates [4]. Statistics show that one ton of raw hides produces about 650 kg of solid waste, approximately 25% of which contains chromium salts [5]. In the case of tanned leather waste, conventional disposal methods such as incineration are not practical, as they release nitrogen oxides, hydrogen cyanide, and ammonia and may lead to the oxidation of Cr³⁺ to the hazardous Cr⁶⁺ form [6]. This high waste output hampers the sustainable development of the leather industry and negatively impacts the environment. One way to mitigate environmental harm is to implement a circular economy model in leather tanning [7].

The reuse of waste products has been recognized as a promising cleaner technology and has attracted interest from both academia and industry over the past decade [8]. In the tanning industry, such wastes include scraps of raw hides, after liming and tanning, and shavings produced during mechanical processing of tanned leathers. Of particular importance is the recovery of valuable protein—collagen—from solid tannery waste in the form of hydrolysate or gelatin. These biopolymers are increasingly used by researchers in leather tanning, especially in the retanning process as fillers [9,10,11,12,13]. The aforementioned researchers used polyphenols and vegetable tannins (Quebracho and Tara) to modify gelatin combined with whey protein concentrate and gelatin with chitosan [9,10]. According to these authors, the application of modified biopolymers as filling agents yields leathers of satisfactory quality. Gelatin was also enzymatically modified with casein and sodium caseinate [11,12], as well as whey protein [13]. The resulting products were used for filling and surface finishing of leathers. The conducted studies showed that enzymatic modification positively affects collagen cross-linking and the properties of the tanned leather.

In a study [14], a tanning agent was developed based on gelatin obtained from chrome shavings via alkaline hydrolysis, coordinated with zirconium and aluminum. The same authors found that the product used for retanning wet-blue and wet-white leathers increased the exhaustion of dye and fat from the bath. Like gelatin, collagen hydrolysate from tanned leather waste is also widely used by scientists in leather processing [3,15,16,17,18,19,20,21,22,23]. The authors of [15] developed a multifunctional composition based on collagen hydrolysate and black liquor from the pulp industry for tanning. According to them, the resulting leathers were soft, filled in loose areas, and exhibited high strength and uniform dyeing.

Other researchers [16] used a retanning composition based on collagen hydrolysate and oxazolidine. The hydrolysate was obtained from chrome shavings via enzymatic hydrolysis in an alkaline medium. Optimization of the composition revealed that samples retanned with 15% hydrolysate and 6% oxazolidine exhibited the highest strength and fullness. According to scientists [17,18], collagen hydrolysate obtained through alkaline-enzymatic hydrolysis of chrome shavings used for retanning helps mask leather defects and improves grain tightness. Other authors used enzymatic hydrolysis to obtain collagen hydrolysate from tanned leather waste [19]. The extracted hydrolysate was subjected to a cross-linking reaction with casein, and the resulting product was introduced into the leather. The quality of the leather was assessed by the authors as satisfactory. In a study [3], collagen hydrolysate was obtained by in situ dechroming using oxalic acid, followed by trypsin treatment. The hydrolysate was then modified with ethylene glycol diglycidyl ether (EGDE). The resulting chrome-free tanning agent based on biomass was used to produce a chrome-free wet-white semi-finished product.

Similar research based on collagen obtained via in situ hydrolysis of chrome-containing leather waste was conducted by Liu et al. [20]. The obtained hydrolysate was used to modify an oxidized extract from valonia algae, and the resulting product was applied to retanning wet-white leather. In [21], chrome shavings were subjected to alkaline hydrolysis, and the resulting hydrolysate was modified with acrylic acid and acrylamide. The biopolymer used in retanning improved the mechanical properties of the leathers. Other researchers [22] also used collagen hydrolysate in leather tanning processes. They applied acid hydrolysis to chrome-tanned leather shavings to obtain hydrolysate, which was used in a composition with acrylic resins for surface finishing of leathers.

Crudu M. et al. [23] developed a method for obtaining collagen hydrolysate from wet-white leather waste using acid hydrolysis. The resulting product, used to retanning wet-white semi-finished leather, produced crust leathers with properties comparable to those tanned with commercial chrome tanning agents. The circular economy model in the leather industry also includes the management of untanned leather waste. In [5], an attempt was made to obtain collagen hydrolysate from raw goat hide trimmings. The hide scraps underwent initial alkaline hydrolysis in the presence of hydrogen peroxide, followed by thermal hydrolysis at 100 °C. According to the authors, the resulting tanning agent produced leathers with good mechanical properties, full grain, and fine texture.

There was also an attempt to recycle waste from the fleshing of raw hides, which was treated with proteolytic enzymes [24]. The resulting hydrolysate was polymerized with methacrylic acid, and the product was used for leather retanning. Other scientists, in turn, subjected waste from the fleshing of raw hides to hydrolysis using alkaline protease [25]. The resulting hydrolysate was introduced into the hide, which was then subjected to chrome tanning. According to the authors, the amount of chrome bound to the hide increased by at least 10%. A crucial stage in all experimental research is the appropriate experimental design. Choosing the proper optimization method suitable for the research problem leads to significant reductions in time and resource investment. One such experimental technique is the simplex algorithm, also known as the simplex method [26]. It is a method for solving linear programming tasks through successive improvements (optimization). Another method, response surface methodology (RSM), enables optimization by determining process parameters under which product quality is deemed satisfactory.

Modern researchers have access to various analytical software tools to support their efforts. An example is the Design of Experiments module in the advanced data analysis software STATISTICA. There are also graphical planning methods and computer programs based on them. Another approach involves conducting experiments in quantities derived from the number of selected parameters and their possible combinations [16].

The interest of researchers in utilizing tannery waste through the extraction of collagen hydrolysate and its reuse in leather tanning inspired this study. Therefore, in line with the circular economy model, the aim of the study was to obtain collagen hydrolysate from chrome-tanned leather shavings and use it in combination with melamine for retanning wet-blue leathers. The tool used to determine optimal amounts of hydrolysate and melamine to achieve leather of appropriate quality was the Kateskór (v. 1.0) computer program, developed based on the Kleeman experimental optimization method [27].

2. Materials and Methods

In this study, collagen hydrolysate was obtained from chrome-tanned leather waste (chrome shavings), which was then used in a melamine composition to retan the wet-blue intermediate. The retanned leathers were fatliquored with the commercial agent Roksol STE. As a result of these tanning processes, a crust intermediate product (crust leathers) was obtained.

2.1. Materials

The following materials and chemicals were used for the research:

• Roksol STE (producer: PCC Exol SA, Brzeg Dolny, Poland)—a fatliquoring agent composed of a mixture of anionic emulsifiers and aliphatic compounds containing carboxyl groups. The product is characterized by a pH of 5.5–7.4 in a 10% aqueous emulsion and an active substance content ranging from 45% to 60%. When applied to leather, it functions as a lubricant, facilitating the relative movement of collagen fibers, which imparts the desired functional properties to the leather, such as softness and elasticity.
• Melamine (producer: Grupa Azoty Zakłady Azotowe, Puławy, Poland)—an aromatic compound from the amine group, classified as a triazine derivative and a cyanamide trimer. When introduced into the leather structure, it primarily functions as a filler in areas with a looser fibrous arrangement, such as the flanks and groin regions. Melamine exhibits the ability to form weak non-covalent bonds with functional groups of collagen as well as with other reagents used in the chemical processing of leather.
• Chrome shavings (producer: tannery in Radom, Poland)—solid leather waste generated during the shaving of chrome-tanned leather, characterized by a moisture content of 43% and a chromium oxide (Cr₂O₃) content of 3.7% relative to dry mass. In this study, chrome shavings were utilized as a raw material for the production of collagen hydrolysate.
• Semi-finished wet-blue leather (producer: tannery in Radom, Poland)—chrome-tanned leather after the shaving process, with a thickness of 1.4–1.6 mm, moisture content of 47%, and chromium oxide (Cr₂O₃) content of 3.8% relative to dry mass.
• Collagen hydrolysate—a product obtained through the acid hydrolysis of chrome-tanned leather shavings (the detailed procedure for collagen hydrolysate preparation is provided below). When applied to the leather structure, it exhibits a retanning effect by binding to functional groups within the collagen matrix, as well as a filling effect by occupying voids in areas with a looser fibrous structure.
• Semi-finished crust leather—dry leather obtained after the retanning and fatliquoring of the wet-blue semi-finished product.

2.1.1. Obtaining Collagen Hydrolysate

Collagen hydrolysate for the testing was produced by means of acid hydrolysis of chrome-tanned leather shavings. The steps for obtaining the hydrolysate are outlined below:

• first, 100 g of chrome-tanned leather shavings and 250 mL of water were added to a 1000 mL three-necked flask,
• a thermometer, mechanical stirrer, and reflux condenser were placed in the necks of the flask,
• the stirrer and cold-water flow through the reflux condenser were turned on, and the mixture in the flask was heated to 100 °C using a heating mantle,
• after reaching the desired temperature, 10 mL of concentrated H₂SO₄ was slowly added to the reaction mixture along the walls of the flask in 5 portions every 5 min and stirring and heating were continued for another 40 min (a total of 60 min from the addition of the first portion of sulfuric acid),
• then, after lowering the temperature of the reaction mixture to 80 °C in the same reactor, precipitation of Cr(OH)₃ was started by adding 50 mL of 10% Ca(OH)₂ suspension for about 30 min,
• after precipitation, the resulting mixture was left for 24 h to allow the chromium hydroxide precipitate to settle,
• the mixture was then filtered using a vacuum filtration system with a Büchner funnel.

2.1.2. Retanning of the Wet-Blue Semi-Finished Product

Following the principle of experimental planning according to Kleeman, six variants of wet-blue hide tanning (

Table 1. Amounts of collagen hydrolysate and melamine (input parameters) in individual retanning variants.

Retanning Variant	Amount of Collagen Hydrolysate (X) [g]	Amount of Melamine (Y) [g]
1	25	2.5
2	35	2.5
3	30	3.0
4	30	3.5
5	35	4.0
6	25	4.0

) with varying amounts of collagen hydrolysate and melamine (input parameters) were carried out according to the scheme shown in Figure 1.

Wet-blue leathers were used for the tests. Leather samples were prepared for six retanning variants, three samples for each variant from different topographic locations of the leather half. The samples were selected using the asymmetric method, according to the scheme presented in Figure 2.

The process of retanning the wet-blue intermediate product and the post-retanning fatliquoring of the leathers was carried out according to the methodology shown in

Table 2. Methodology of retanning the wet-blue semi-finished product.

Operation	Type of Agent	Amount [%]	Temp [°C]	Times [min]	pH/Comments
Washing I	Water	200	30	10	Pour out the bath
Washing II	Water	200	35	10	Pour out the bath
Neutralization	Water Sodium bicarbonate	150 1, 3	35	40	Bromocresol green pH = 5.0–5.2
Washing	Water	200	35	10	Pour out the bath
Rettaning	Water Melamine Collagen hydrolysate	150 X: 2.5/3.0/3.5/4.0 Y: 25/30/35	35	90
Fatliquoring	Roksol STE Formic acid	10 1		60 2 × 10 +20	Add emulsifier with hot water Pour out the bath
Washing	Water	200	20	10	Pour out the bath

. In accordance with the procedure shown in Table 2, the wet-blue hides not subjected to the tanning process were also fatliquored. The determined parameters of the fatliquored leathers made it possible to determine how retanning (according to the planned variants) affects selected properties of the leathers. The tanning and fatliquoring processes were carried out in Wacker-type laboratory glass drums.

After the tanning and fatliquoring process, the crust leathers were dried in a free state and mechanically softened.

2.2. Methods

In this study, the physicochemical properties of collagen hydrolysate obtained from chromium shavings and the leathers retanned with the obtained hydrolysate and melamine were evaluated. Methods used for the evaluation and characterization of the resulting collagen hydrolysate solution are as follows.

2.2.1. An Organoleptic Evaluation

An organoleptic evaluation of the obtained hydrolysate solution was made using the senses of sight and smell. The evaluation was carried out by specialists and practitioners in the field of tanning.

2.2.2. Measurement of pH

Measurement of pH values of the analyzed collagen hydrolysate solution was conducted with the use of a CP-551 stationary pH meter from Elmetron (Zabrze, Poland) with a universal electrode.

2.2.3. Determination of Chromium Content

Cr content in collagen hydrolysate was determined using a Perkin Elmer (Waltham, MA, USA) AAS-3100 flame atomization atomic absorption spectrometer (F-AAS). An air–acetylene flame was used for the tests. Acetylene was the reducing agent, and air was the oxidizing gas. Metal standard solutions prepared from a 1000 µg/mL stock solution, BDH Spectrosol (Leics, UK), were used for calibration. The instrument’s operating parameters are given in

Table 3. Atomic absorption spectrometer parameters during Cr determination.

Determined Metal	Cr
Wavelength [nm]	357.9
Slit width [nm]	0.2

.

2.2.4. Determination of Dry Mass

Determination of the dry mass of the collagen hydrolysate solution was conducted in a WPS 50 SX laboratory moisture analyzer from RADWAG (Radom, Poland). Before employing the aforementioned device for dry matter determination, a method validation was conducted by comparing the results obtained using the standardized reference method with those obtained using the applied method. A series of measurements was performed, which yielded consistent and repeatable results, thereby allowing the use of the moisture analyzer as a reliable instrument for accurate and precise determination of dry matter content in the hydrolysate. The adopted method also significantly reduces the analysis time, which is justified and advantageous from the perspective of subsequent process steps [28]. In the conducted analyses, the following parameters were used for dry matter determination of the hydrolysate: sample weight 10 g, drying temperature 103 °C. The drying process was carried out until stable results were achieved, with stability defined as three consecutive measurements showing no variation. The measurement interval was set at 30 s. The dry mass of the collagen hydrolysate solution was expressed as a percentage.

2.2.5. Determination of Density

Density of the collagen hydrolysate solution was determined by means of a Gay-Lussac pycnometer at 20 °C.

The density of collagen hydrolysate was calculated from the formula:

$$d_c={\displaystyle \frac{m_2-m_1}{m_3-m_1}}\cdot 0.9982 [\mathrm{g}/\mathrm{c}{\mathrm{m}}^3]$$

(1)

where:

• d_c—density of the tested liquid,
• m₁—mass of an empty pycnometer,
• m₂—mass of the pycnometer with the tested liquid,
• m₃—mass of the pycnometer with distilled water,
• 0.9982—water density (g/cm³).
• A detailed description of the procedure is included in the PN-EN ISO 2811-1:2016-04 standard.

2.2.6. Determination of Dynamic Viscosity

Dynamic viscosity of the collagen hydrolysate solution was determined using a Höppler viscometer type KF 40 from RheoTec Messtechnik GmbH (Ottendorf-Okrilla, Germany).

The dynamic viscosity is calculated by:

η = t·(dk − dc)·Ks [Pa·s]

(2)

where:

• η—dynamic viscosity,
• t—time of a ball falling (s),
• d_k—ball density (kg/m³),
• d_c—density of test fluid (kg/m³),
• K_S—constant of the ball used (9.03 × 10⁻⁹ Pa·m³/kg).

The following methods were used for the evaluation of crust leathers and fatliquored wet-blue leathers. Crust leathers prepared for testing were conditioned in accordance with the EN ISO 2419-2005 standard. The leathers were subjected to the following investigation:

2.2.7. Softness

The determination of leather softness was carried out according to the PN-EN 17235 standard using the ST 300/IVP/36 TEST INSTRUMENT (Ho Chi Minh, Vietnam) device. A ring with a diameter of 25 mm was used for the tests. The measure of leather softness is the depth of its deflection on a scale from 1 to 10 mm.

2.2.8. Shrinkage Temperature

The measurement of the leather shrinkage temperature was performed according to PN EN ISO 3380:2005 using a custom-built device (Casimir Pulaski Radom University, Radom, Poland). The test consists of determining the temperature at which shrinkage of the leather sample begins in slowly heated glycerin.

2.2.9. Water Vapor Permeability

Water vapor permeability of the leathers was determined according to a patented, authorial method (PATENT No. 210759 for the invention) using a special vessel (PROTECTION LAW No. 64479) and moisture analyzer by RADWAG (Radom, Poland). The test was carried out at 40 °C for 1 h. Details of water vapor permeability assays according to this method are published in ref. [29].

2.2.10. Water Absorption

The determination of water absorption was conducted under static conditions using a Kubelka apparatus. A detailed description of the procedure is provided in the PN-EN ISO 2417 standard.

2.2.11. Mechanical Properties

Tensile strength and maximum elongation of the leather were determined according to PN-EN ISO 3376 standard. The measurement was carried out using a ZwickRoell (Ulm, Germany) tensile testing machine.

2.2.12. Statistical Analysis

Results of crust leather properties and parameters of collagen hydrolysate are arithmetic means of at least three determinations. Statistical analysis was performed using Student’s t-test. Statistical differences between the samples were estimated at the significance level of α = 0.05 (5%).

The optimization of the retanning process was carried out using the Kateskór (v. 1.0) software based on the Kleeman method, which reduces the number of experimental variants to a maximum of six [30]. This method involves selecting a research area in the form of a rectangle that contains points where the mathematical relationship between properties and variable factors is expressed as the function Z = f (x, y). These points lie on a spherical surface in three-dimensional space. The curves that bound the sides of the rectangle, resulting from the intersection of the spherical surface with horizontal planes corresponding to equal values of the studied properties, are projected onto a horizontal coordinate plane. Figure 3 illustrates the relationships between the properties and variable parameters in three-dimensional space.

The Kleeman method, utilized for optimization, is a classical six-point experimental design and optimization procedure. It was originally developed in the 1960s at the Tanning Institute in Freiburg to rapidly fine-tune leather tanning processes. Currently, it is widely applied in food chemistry, cosmetology, materials engineering, and in modeling colloidal systems. It serves as a tool for simultaneously optimizing two process variables (factor A, factor B) at three levels (−1; 0; +1) and for estimating factor interactions and their quadratic effects with a minimal number of trials (N = 6).

The method assumes that the objective function $Y=f(A,B)$ is quadratic. With two variables, five coefficients of the equation can be fully identified with just six measurements:

$$Y={\beta }_0+{\beta }_{1}A+{\beta }_{2}B+{\beta }_{11}{A}^2+{\beta }_{22}{B}^2+{\beta }_{12}AB$$

(3)

The coordinate system forms a three-dimensional figure—a rotational five-vertex “Kleeman’s wheel” (a tetrahedron with a central point). Factor levels are coded to meet orthogonality requirements, which simplifies the calculation of coefficients using the least squares method.

The ${\beta }_i$ coefficients are determined using the Vandermonde matrix, and then the optimum is localized by simultaneously setting the derivatives to zero:

$${\displaystyle \frac{\mathsf{\partial }Y}{\mathsf{\partial }A}}=0, {\displaystyle \frac{\mathsf{\partial }Y}{\mathsf{\partial }B}}=0$$

The Kleeman method is an effective engineering tool for rapid optimization of processes with two variables. By reducing the number of necessary trials to 6, it minimizes experimental costs while providing a reliable prediction of the optimum without the need for extensive statistical analysis.

The Kleeman method is very helpful in determining the parameters of the finished product. It makes it possible, using a minimum number of experiments, to obtain a graphical compromise optimum of the parameters that maximally corresponds to the task at hand (Figure 4).

3. Results and Discussion

The tanning industry generates significant amounts of hazardous waste. Improper management of this waste causes serious environmental problems and ultimately has a negative impact on human health [31]. Hence the need for solutions that will allow the leather industry to reduce its negative impact on the environment while meeting consumer expectations [32]. One such solution is the circular economy, which is more economical than the traditional economic model (lower consumption of energy and non-renewable resources) and more beneficial to the environment (reuse of waste, lower emissions) [33]. The application of a circular economy is particularly important in the case of leather waste tanned with chromium compounds. The use of collagen hydrolysate from leather waste reduces the consumption of commercial tanning agents. Furthermore, chromium hydroxide precipitated during the hydrolysis of waste, after appropriate treatment with sulfuric acid, can be reused as an additional component of commercial tanning agents in chrome tanning processes. As a result of anaerobic fermentation of both chrome-tanned and untanned leather waste, biogas can also be obtained [34,35].

The collagen hydrolysate (used as retanning agent) obtained according to the method described in Section 2.2.1 of this paper was tested. The test results have been presented in

Table 4. Characteristics of collagen hydrolysate.

Property	Value of Parameter
Color of solution	Yellow
Odor	No odor
Density [g/cm3]	1.04 ± 0.08
Dynamic viscosity [mPa·s]	1.98 ± 0.15
Dry mass [%]	12.9 ± 0.5
pH	7.5 ± 0.2
Content of chromium [mg Cr/L]	3.05 ± 0.25

.

Based on the obtained results, it was found that the collagen hydrolysate obtained had organoleptically typical features of protein hydrolysates (Table 4) [36]. The low dry mass content in the collagen hydrolysate produced indicates that the solution is substantially diluted. This is caused during the progress of the hydrolysis, in which a lot of water is added to the reaction flask. In turn, the low collagen hydrolysate concentration in the solution directly contributes to its low density and dynamic viscosity. Our testing implies a chromium content in the resultant collagen hydrolysate of 3.05 mg/L, which is equivalent to 23.6 μg Cr/g protein. This result is several times greater than the one obtained by Shanthi et al. [37]. The chromium content is not of key significance, however, if the hydrolysate is used for retanning chrome leather.

Table 5. Results of crust leather properties (output parameters) used to optimize the retanning process of wet-blue leathers according to the Kleeman method.

Retanning Variant	Properties of Crust Leather (Output Parameters)
	Softness [1–10 mm]	Shrinkage Temp. [°C]	Water Vapor Permeability [%]	Water Absorption [%]	Tensile Strength [MPa]	Maximum Elongation [%]
1	4.0 ± 0.2	108 ± 2	63.8 ± 3.6	139 ± 12	13.8 ± 3.8	74.9 ± 8.9
2	4.0 ± 0.3	107 ± 1	67.0 ± 4.1	151 ± 18	13.6 ± 2.5	74.4 ± 7.4
3	4.1 ± 0.1	108 ± 2	65.6 ± 4.3	154 ± 15	15.5 ± 3.3	68.8 ± 7.8
4	4.3 ± 0.2	107 ± 2	65.6 ± 3.4	152 ± 16	18.7 ± 2.7	75.9 ± 6.4
5	4.6 ± 0.2	111 ± 1	68.3 ± 2.8	151 ± 10	19.5 ± 4.0	82.1 ± 9.0
6	4.7 ± 0.3	110 ± 1	71.0 ± 3.8	173 ± 21	17.1 ± 1.9	82.7 ± 8.1
Wet-blue	5.3 ± 0.2	105 ± 3	67.9 ± 4.0	142 ± 13	13.3 ± 2.2	91.2 ± 7.9

presents the properties of leather (output parameters) retanned according to the variants designed using the Kleeman method. This table also includes the properties of fatliquored wet-blue leather that was not subjected to the retanning process.

An analysis of the data compiled in Table 5 indicates that the softness of crust leather (wet-blue after retanning and fatliquoring) is lower than that of wet-blue leather subjected only to fatliquoring. The lowest softness was observed in crust leather retanned according to variants 1 and 2, in which the amount of melamine in the retanning composition was the smallest.

On the other hand, analysis of the shrinkage temperature results showed that leather retanned with compositions containing various amounts of collagen hydrolysate and melamine had higher hydrothermal resistance than wet-blue leather. The greatest increase in hydrothermal resistance was recorded for leathers retanned using variants 5 and 6, by 6 °C and 5 °C, respectively. Other researchers [20] also observed an increase in the shrinkage temperature of wet-white leather retanned with a biopolymer based on collagen hydrolysate.

The results of water vapor permeability presented in Table 5 show that the tested leathers have similar values for this parameter. Only in the case of retanning with the variant 6 composition (25% collagen hydrolysate, 4% melamine) was an increase of 3.1% in water vapor permeability observed compared to wet-blue leather.

Analysis of water absorption test results showed that retanning with compositions containing various amounts of collagen hydrolysate and melamine, except for variant 1, increased the amount of absorbed water compared to wet-blue leather. For variant 6, a 31% increase in water absorption was observed.

The data in Table 5 also show that the tensile strength of crust leather depends on the composition used for retanning the wet-blue leather. For variants 1 and 2, the effect was minor, while variant 5 showed an increase of up to 46% compared to wet-blue. Similarly, other researchers [21] found that retanning with a collagen-hydrolysate-based product increased leather tensile strength by 23%. A positive effect of collagen-hydrolysate-based retanning compositions on the mechanical properties of leather was also reported by Afsar A. et al. [16].

Analysis of the data presented in Table 5 indicates that the maximum elongation of leathers after retanning is lower than that of wet-blue leather. The greatest decrease in elongation for crust leather compared to wet-blue was observed in variants 1 and 2, by 16.3% and 16.8%, respectively.

The amounts of collagen hydrolysate and melamine (input parameters) listed in Table 1, as well as the properties of the crust leather (output parameters) summarized in Table 5, were used to select the optimal retanning parameters for wet-blue leather. After entering these parameters into the Kateskór computer program, graphs were generated showing contour lines corresponding to equal values of the output parameters (Figure 5).

The optimization method used in this study allows for selecting limiting values of the measured crust leather parameters, which were also entered into the software. In the presented study, it was assumed that the retanned and fatliquored leather (crust) should meet the following property requirements (output parameters): softness—at least 4.2; shrinkage temperature—at least 107 °C; water vapor permeability—at least 66%; water absorption—below 160%; tensile strength—at least 18 MPa; maximum elongation—below 80%.

Figure 5 presents the influence of the amount of collagen hydrolysate and melamine (input parameters) used in retanning wet-blue leather on the resulting properties of the crust leather (output parameters). The curves corresponding to the selected limit values of the labeled output parameters are marked with a blue color. The curves in Figure 5a show that the softness of the leathers depends mainly on the amount of melamine introduced into the leather. With an increase in the amount used for tanning, the softness of the leathers increases, while the effect of the amount of collagen hydrolysate on this parameter is observed to be negligible. Analyzing the curves shown in Figure 5b, the effect of both the amount of collagen hydrolysate and melamine used in the tanning process on the shrinkage temperature can be seen. The distribution of the curves shows that the highest shrinkage temperature is reached by retanned leather according to variant 5 and the lowest by variant 4. From Figure 5c, it can be seen that increasing the amount of melamine slightly increases the water vapor permeability of the leather. When the amount of melamine is at its highest (4%), water vapor permeability reaches 71%. According to the author [38], the use of a collagen hydrolysate and plant tannin (Mimosa) composition for retanning wet-white leather yields similar permeability values. Figure 5d shows that the highest water absorption was observed for leather obtained using variant 6 (173%). The amount of melamine introduced into the leather significantly influences this parameter. The curves in Figure 5e indicate that, in this experiment, both the variable amounts of melamine and collagen hydrolysate used in retanning were key parameters determining the tensile strength of the leather. It was observed that, as the amounts of these agents increased, the tensile strength of the leather also increased. Similar relationships were obtained by Afsar, A. et al. [16]. According to them, all collagen hydrolysate and oxazolidine compositions used for retanning wet-blue semi-finished leather increased its tensile strength. The results in Figure 5f show that the key parameter affecting the maximum elongation of the leather was the variable amount of melamine introduced during retanning. Collagen hydrolysate did not significantly influence this parameter. In summary, the relationships presented in Figure 5 indicate that, as the amount of melamine used in retanning increases, so do the softness, tensile strength, and maximum elongation of the leather. The influence of the collagen hydrolysate amount on these parameters is minor. In contrast, at lower levels of collagen hydrolysate, increasing the amount of melamine results in higher water absorption and water vapor permeability. The influence of these parameters on the shrinkage temperature of the leather is observed to be negligible.

Figure 6 shows a set of lines corresponding to the selected limiting values of the output parameters. Obtained using a computer program based on the Kleeman method, the graph shows that the amounts corresponding to the compromise optimum of the parameters for the wet-blue leather retanning process with collagen hydrolysate and melamine are as follows: for collagen hydrolysate content, 28.78–29.63 g; for melamine content, 3.61–3.68 g (yellow field in Figure 6). The selection of optimal parameters for the process of retanning leathers with collagen hydrolysate using other methods was also carried out by Castell J. et al. [26] and Afsar A. et al. [16]. However, the methods they used required more trials. The method used in the present study limits the number of trials to only six, which significantly shortens the duration of the experiment while allowing accurate results to be obtained.

4. Conclusions

The conducted study demonstrated the feasibility of reusing chrome tanned leather waste (wet-blue) in the leather retanning process, contributing to the effective implementation of a closed-loop economy model within the leather industry. This approach aligns with the principles of sustainable development by reducing waste generation and promoting resource efficiency.

The application of process optimization techniques allowed for the selection of the most advantageous retanning variant in terms of achieving the desired leather properties while simultaneously reducing process duration and material consumption. Specifically, it was found that leathers retanned according to variant 5 (35% collagen hydrolysate, 4% melamine) exhibited the highest shrinkage temperature and tensile strength, making this composition suitable for applications requiring enhanced mechanical durability. In contrast, variant 6 (25% collagen hydrolysate, 4% melamine) resulted in the highest water vapor permeability and water absorption, thus representing the optimal choice for leathers where breathability and moisture management are critical.

Quantitative optimization of the wet-blue retanning process revealed that, in order to achieve the predefined target leather characteristics, the amounts of collagen hydrolysate and melamine should be precisely adjusted to the ranges of 28.78–29.63 g and 3.61–3.68 g, respectively. The specified ranges provide a reliable guideline for technologists to conduct the retanning process efficiently while ensuring the production of leather with the planned performance attributes.

The findings underscore the potential for significant material savings and process efficiency improvements through precise formulation, offering a practical methodology for the sustainable advancement of leather manufacturing practices.

5. Summary

The proposed method facilitates the advancement of sustainable leather production through the recycling of leather waste and the optimization of processing operations. In the long term, this approach is expected to mitigate the environmental impact associated with hazardous tanning waste containing chromium compounds. Moreover, the utilization of collagen hydrolysate derived from leather waste generated during finishing processes enables a reduction in the use of commercial tanning agents. Additionally, chromium hydroxide precipitated during waste hydrolysis, following appropriate treatment with sulfuric acid, can be reutilized as a supplementary component of commercial tanning agents in chromium tanning processes. Consequently, this strategy not only offers economic advantages but also contributes to the reduction of mineral resource consumption, including chromite ores and petroleum, used in the manufacture of tanning agents.