Abstract
This study systematically investigates the effects of various vegetable tannin combinations on the physical and color properties of sole leathers. Mimosa, quebracho and chestnut tannins were applied as single (40%), binary (mimosa–quebracho at varying ratios) and ternary systems (mimosa–quebracho with 5% fixed chestnut) to evaluate their influence on mechanical strength, softness, hardness, hydrothermal stability and color stability. FTIR analyses confirmed the successful fixation of vegetable tannins within the collagen matrix and revealed characteristic spectral differences associated with condensed and hydrolyzable tannin structures, while preserving the integrity of the collagen backbone. SEM observations further demonstrated that tannin composition significantly influenced collagen fiber organization, with optimized formulations producing more compact and homogeneous fibrous networks. The results indicated that quebracho-based leathers exhibited higher tensile and tear strength, while mimosa-based leathers provided lighter shades and greater softness. The binary systems demonstrated that increasing the mimosa content improved brightness but reduced mechanical performance. However, certain ratios specifically 15% mimosa with 25% quebracho and 25% mimosa with 15% quebracho struck a balance between color homogeneity and acceptable strength. In ternary systems, the inclusion of 5% chestnut slightly affected color but enhanced shrinkage temperature and overall mechanical stability. UV-aging test results revealed that mimosa-rich leathers showed higher color change (ΔE), while quebracho improved color stability. Chestnut contributed a modest enhancement against UV degradation. Collectively, these findings suggest that optimized tannin combinations can produce vegetable-tanned leathers with lighter tones, enhanced repeatability and balanced mechanical as well as color properties. This research supports the transition to chromium-free tanning systems and provides scalable insights for sustainable material design, highlighting the potential of biobased tanning agents to meet both ecological and industrial needs.
1. Introduction
Leather is a renewable biomaterial widely utilized in the footwear, fashion, automotive, furniture and luxury sectors due to its superior durability, aesthetic appeal, breathability and comfort. As a byproduct of the food industry, raw hides serve as a valuable secondary resource within a circular economy framework, transforming waste into durable, high-value products. However, despite its long-standing cultural and industrial significance, the conventional leather manufacturing process faces criticism for its high consumption of water, energy and chemicals as well as the generation of environmentally hazardous waste streams. , These challenges have become more pressing in the modern era, where global sustainability initiatives, such as the European Green Deal, highlight the urgent need to reduce the environmental footprint of material industries.
A central issue within the sustainability debate is the reliance on chromium-based tanning systems. Chromium(III) salts are efficient tanning agents; however, their potential oxidation to chromium(VI), a toxic, carcinogenic and strictly regulated compound, raises major environmental and public health concerns. , Consequently, the development of safe, sustainable and biobased tanning systems has become one of the highest priorities for the leather industry.
Vegetable tannins, known since the Neolithic period, have re-emerged as promising alternatives due to their metal-free tanning mechanism, biobased origin, biodegradability and compatibility with environmentally friendly processing. , Derived primarily from renewable plant sources, vegetable tannins align with global sustainability goals, offering reduced carbon footprints and contributing to environmentally responsible manufacturing. They are a diverse group of plant-derived polyphenolic compounds with molecular weights ranging approximately from 500 to 30,000 Da. Based on their phytochemical structure, vegetable tannins are categorized into three principal groups according to their structural features: hydrolyzable tannins, condensed tannins and a less common class of iridoid- or secoiridoid-containing tannins. Hydrolyzable tannins consist mainly of esters of gallic acid linked to glucose moieties, whereas condensed tannins are polymerized flavonoids predominantly derived from catechin-based monomeric units. In contrast, iridoid tannins belong to the terpenoid class of phytochemicals and are characterized by a cyclopentano-pyran skeleton containing two oxygen atoms; their oxidized derivatives, known as secoiridoids, typically occur in nature in a stabilized glycosidic form.
Mimosa (Acacia spp.) and quebracho (Schinopsis spp.) extracts are rich in condensed tannins (proanthocyanidins), whereas chestnut (Castanea sativa) yields predominantly hydrolyzable tannins, mainly ellagitannins. Condensed tannins consist of oligomeric flavan-3-ol units and both mimosa and quebracho extracts are particularly enriched in profisetinidin structures, primarily fisetinidin and robinetinidin, together with minor fractions of (epi)catechin and (epi)gallocatechin. Indeed, Shnawa (2023) reported that approximately 87% of mimosa tannins are composed of fisetinidin/robinetinidin units, with the remaining fraction attributed mainly to catechins; sulfited quebracho exhibits a comparable compositional profile, albeit with slightly higher proportions of catechin and epicatechin. In contrast, chestnut bark tannins belong to the hydrolyzable class, with ellagitannins such as castalagin and vescalagin (MW ≈ 934) as the dominant constituents, accompanied by their derivatives (vescalin, castalin) and smaller galloyl units including gallic acid and pentagalloyl glucose. , These fundamental chemical distinctions give rise to markedly different tanning behaviors. While chestnut ellagitannins readily undergo hydrolysis, releasing gallic and ellagic acids, mimosa and quebracho supply higher-molecular-weight polymeric phenolics capable of forming extensive hydrogen-bonding networks. Consequently, differences in functional group chemistry and molecular size critically govern collagen–tannin interactions and, in turn, the resistance of the leather matrix to thermal and mechanical stresses. ,
These plant-derived polyphenols not only impart tanning functionality but also enhance desirable mechanical attributes such as fullness, firmness and resistance to microbial degradation. However, their broader industrial adoption remains hindered by several challenges. Chief among these challenges are the characteristic color tones imparted by vegetable tannins, which complicate the reproducible production of light-colored and pastel leathers, along with batch-to-batch variability, long-term discoloration and in many practical cases, a concomitant loss of mechanical stability upon aging. Despite recent academic efforts reporting partial improvements under laboratory-scale conditions, a robust and industrially transferable solution that simultaneously ensures light-toned aesthetics, color stability and mechanical reliability has not yet been widely established. , Some vegetable-tanned leathers exhibit undesirable color changes, such as darkening, yellowing and reddening during postproduction aging or exposure to environmental factors like heat, light and humidity. These changes are driven by a combination of photochemical and oxidative pathways involving the phenolic moieties of tannins, including photo-oxidation, thermo-oxidation and subsequent reactions such as quinone formation, polymerization/condensation and complexation, which collectively generate new chromophoric species. In condensed tannins, oxidative pathways may promote quinone-type intermediates and further coupling reactions, whereas hydrolyzable tannins may follow different degradation routes, which can partly explain formulation-dependent aging responses. In addition, processing conditions (e.g., pH, moisture, temperature) and trace metal ions may further modulate the magnitude and direction of the observed color shifts. , Such limitations restrict the industrial application of vegetable tanning, particularly in product segments where both aesthetic uniformity and mechanical reliability are essential.
From a sustainable materials perspective, overcoming these challenges requires systematic optimization of tannin formulations that reconcile color stability with mechanical performance. Achieving such optimization would not only reduce dependence on chromium but also strengthen the alignment of the leather industry with green chemistry principles, circular economy strategies and carbon footprint reduction targets. Moreover, it would enhance the industrial feasibility of biobased materials by providing reproducible solutions to one of the sector’s longstanding problems: maintaining light and uniform color shades without sacrificing durability and strength.
The growing emphasis on green chemistry and biobased alternatives underscores the urgency of such research. Sustainable leather production is not only a technological necessity but also a socio-economic and regulatory imperative, as demonstrated by the European Green Deal and industry-wide sustainability indices. For instance, the Higg Materials Sustainability Index assigns leather a high environmental impact score (159), significantly higher than cotton (98) and polyester (44), underscoring the need for process innovations to reduce the material’s footprint. In this context, the substitution of fossil-based auxiliaries with biobased chemicals, together with the adoption of emerging low-impact processing technologies and process optimization strategies, represents a key pathway toward more sustainable leather manufacture. While regulatory frameworks such as the Best Available Techniques (BAT) Reference Documents provide an important baseline, recent academic and industrial research increasingly emphasizes innovative approaches including biobased tanning systems, resource-efficient processing and circular-economy-oriented technologies. ,,
Although vegetable tannins and monovegetable tanning systems are well established in both classical literature and industrial practice, the systematic, formulation-driven optimization of vegetable tannin combinations with respect to color stability, aging resistance and mechanical performance remains insufficiently explored. Existing studies predominantly focus on extraction methods of individual vegetable tannins or on the application of single tannins evaluated through isolated performance parameters such as strength, shrinkage temperature or lightfastness without adequately addressing the interdependent and often competing effects of tannin chemistry, formulation ratio and long-term aging behavior. Consequently, the long-standing trade-off between light-toned aesthetics and durable mechanical performance continues to limit the broader industrial adoption of vegetable tanning systems, particularly for applications requiring both visual consistency and functional durability.
Beyond formulation design, process-level strategies such as the reuse of partially exhausted tanning liquors have gained increasing attention as effective measures to enhance the environmental performance of vegetable tanning. In the present study, the recycled tanning liquor was deliberately maintained constant across all formulations and was not treated as an experimental variable. This approach reflects common industrial practice and ensures that the observed differences in color characteristics, aging behavior and mechanical properties originate primarily from controlled variations in vegetable tannin composition rather than from fluctuations in process liquor chemistry. Although nontannin fractions present in recycled liquors may contribute to background coloration, their influence remains systematic and secondary under the low-concentration, controlled recycling conditions reported in recent studies.
Against this background, the present study addresses this knowledge gap by systematically investigating the effects of single, binary and ternary combinations of mimosa, quebracho and chestnut tannins on the physical, thermal and color properties of leather. Particular emphasis is placed on identifying optimized formulations capable of producing lighter and more reproducible shades while maintaining or enhancing key mechanical performance parameters, including tensile strength, tear resistance, softness, hardness and hydrothermal stability. Furthermore, accelerated aging experiments are employed to assess long-term color stability and resistance to photochemical degradation, thereby providing insight into performance under realistic service conditions.
By integrating principles of materials optimization with sustainability-driven design, this work offers both scientific insight and practical guidance for improving the performance consistency and long-term color stability of vegetable-tanned sole leather through formulation optimization. In contrast to previous studies that often examine vegetable tannins in isolation or in nonsystematic blends, this research introduces a structured multitannin optimization strategy aimed at balancing color appearance, mechanical durability and photochemical stability. While individual properties such as shrinkage temperature or tensile strength have been reported separately in the literature, , comprehensive data sets linking tannin composition to both mechanical and aesthetic performance particularly under aging conditions remain scarce. By evaluating ternary tannin systems under controlled and accelerated aging simulations, the present study contributes new, application-oriented insights with direct relevance to industrial formulation strategies and sustainable leather design.
2. Experimental Section
2.1. Materials
The pelts used in this study were domestic bovine hides sourced from Sepiciler Çaybaşı Leather Industry and Trade Inc. The other chemicals utilized in the production recipes, including tanning liquid, oxalic acid, sodium formate, sulfuric acid, sodium bicarbonate and sulfated oil, were obtained within the company. The white syntan and neutral syntan were supplied by Sintan Chemical Turkey. Mimosa (Mimosa GS) and quebracho (Unitan ATO) were purchased from NGB Chemical Turkey. Chestnut tannin (Dulcotan RN-Special) was supplied by Silvateam, Italy. Commercially supplied vegetable extract solutions were subjected to Koch extraction, and the tannin content, nontannin components, and hide-powder-absorbable tannin matter were subsequently determined according to the standard SLC 114, SLC 115, SLC 116, and SLC 117 methods, respectively. The vegetable tannins investigated in this study and their specific characteristics are detailed in Table .
1. Properties of the Vegetable Tannins.
| tannin source | botanical origin | country of origin | plant part | tannin content (%) | nontannin (%) | moisture (%) | pH |
|---|---|---|---|---|---|---|---|
| Sweetened chestnut | Castanea sativa | Italy | Wood | 72 | 22 | 8 | 4.5 |
| Sulphited quebracho | Schinopsis balansae | Argentina | Heartwood | 73 | 18 | 7.5 | 4.8 |
| Mimosa | Acacia mearnsii | South Africa | Bark | 67.5 | 26 | 6.3 | 4.7 |
2.2. Tanning Process
The production recipe used by the company for the control group utilized domestic bovine hides with a thickness of 2.0/2.4 mm, processed as bag material. This recipe served as the baseline for the study, with only the tannin ratios being altered according to the optimization plan. The standard leather production recipe applied by the company is provided in Table .
2. Leather Production Recipe.
| process | amount (%) | chemicals | temperature (°C) | time (min) | notes |
|---|---|---|---|---|---|
| Soaking | 100 | Water | 30 | PH 3.5 | |
| 0.3 | Oxalic acid | ||||
| Drain | |||||
| Tanning | 200 | Tanning liquid(recycled) | BOME 12 | ||
| 3 | Neutral syntan | 5 | PH 4.0 | ||
| 0.5 | Sulfated oil | 50 | 20 | ||
| 25 | Mimosa tannin | 60 | |||
| 10 | Quebracho tannin | 60 | |||
| 2 | White syntan | ||||
| 0.5 | Sulfated oil | 50 | 30 | ||
| 5 | Quebracho tannin | 120 | 5/55AUTOMATIC/OVERNIGHT | ||
| Morning | +50 | Water | 60 | ||
| Washing | 0.4 | Oxalic acid | 30 | ||
| Drain-Wash | PH 4.0 |
The tanning liquor reused (Figure ) in this study was obtained from a previous vegetable tanning cycle. The same recycled liquor was applied in all experimental recipes, and its tannin-to-nontannin ratio was routinely monitored under industrial conditions and maintained within a narrow range (approximately 3.75–4.25)
1.
Tanning liquid.
2.3. Optimization Procedures and Applications
A total of 12 different tanning groups were created for the optimization studies using single, binary and ternary tannin combinations at varying ratios. The total tannin concentration in all experimental groups was kept constant at 40%.
2.3.1. Single Vegetable Tannin Applications
This group aimed to determine the physical and color properties imparted by each tannin when used alone at a 40% concentration.
Group T1:40% Mimosa Tannin.
Group T2:40% Quebracho Tannin.
2.3.2. Binary Vegetable Tannin Combinations
Mimosa and Quebracho tannins were combined in five different ratios, maintaining a total tannin content of 40%.
Group C1:20% Mimosa + 20% Quebracho.
Group C2:15% Mimosa + 25% Quebracho.
Group C3:10% Mimosa + 30% Quebracho.
Group C4:30% Mimosa + 10% Quebracho.
Group C5:25% Mimosa + 15% Quebracho.
2.3.3. Ternary Vegetable Tannin Combinations
In this group, a fixed ratio of 5% Chestnut tannin was added to the Mimosa and Quebracho combination, maintaining the 40% total tannin ratio. The aim was to evaluate the synergistic effect of the low-ratio third tannin addition.
Group U1:12.5% Mimosa + 22.5% Quebracho + 5% Chestnut.
Group U2:17.5% Mimosa + 17.5% Quebracho + 5% Chestnut.
Group U3:22.5% Mimosa + 12.5% Quebracho + 5% Chestnut.
The leather samples produced during the optimization process are presented in Figure .
2.
Vegetable tanned leathers.
2.4. Sampling and Preparation of Samples for Analysis
Sampling from the tanned leathers was carried out according to the TS EN ISO 2418 standard. Samples designated for physical tests were taken from the standardized central sampling area of the hide (GJKH zone), as defined in TS EN ISO 2418, which represents the most homogeneous region (Figure ).
3.
Sampling of leathers.
The cut samples were conditioned for 48 h in an environment maintained at 20 ± 2 °C and 65 ± 5% relative humidity, as per the TS EN ISO 2419 standard test method. The determination of the necessary leather thickness for the physical tests was performed according to TS EN ISO 2589.
2.5. FTIR Analysis of the Vegetable Tanned Leather
Fourier Transform Infrared (FTIR) spectroscopy was employed to identify the functional groups present in vegetable-tanned leather samples. Analyses were performed using a JASCO 4700 FTIR Spectrometer. Spectral measurements were recorded over the wavenumber range of 400–4000 cm–1, with a signal-to-noise (S/N) ratio of 10,000:1. Background and sample spectra were collected consecutively at a resolution of 4 cm–1 and a scanning rate of 1 Hz.
2.6. Physical Characterization of Leather
The physical characterization of vegetable-tanned leathers was carried out through a series of standardized tests. Tensile strength and elongation were determined according to TS EN ISO 3376, while tear resistance was assessed following TS EN ISO 3377–1. The distension and strength of surface were measured using the ball burst method in line with ISO 3379. Softness values were obtained with a BLC ST 300 Softness Tester and hardness was evaluated using the Shore D method (ASTM D1415). Hydrothermal stability was examined by determining the shrinkage temperature (T s) according to IUP 16. These analyses provided a comprehensive understanding of the mechanical, structural and thermal performance of the leathers.
2.7. Color Measurement
Color measurements of leathers tanned with different vegetable tannin combinations were carried out using a Konica Minolta CM-3600d spectrophotometer. Measurements were taken at five different points on each sample, and average values were calculated. Color differences between control and experimental leathers were evaluated using the CIELAB 1976 color difference formula (ΔE), which calculates the Euclidean distance between the L*, a*, and b* coordinates of reference and sample colors. An increase in L* indicates a lighter shade, while decreases denote darker appearances. Positive a* and b* values correspond to red and yellow shifts, respectively, whereas negative values indicate green and blue tendencies. ΔE color difference was calculated according to the following formula using L* a* b* values.
| 1 |
2.8. Determination of Color Intensity (K/S Values)
The color intensity of the vegetable-tanned leathers was assessed using the Kubelka–Munk (K/S) method. This approach, based on reflectance spectroscopy, relates the absorption (K) and scattering (S) coefficients of the material. The K/S values were calculated at the maximum absorption wavelength (λmax) within the visible spectrum (360–700 nm) using the equation
| 2 |
In this equation; K is the absorption coefficient of the sample, S is the scattering coefficient of the sample and R corresponds to the reflectance value of the sample at maximum absorbance.
2.9. Artificial Aging Test
Artificial aging tests were performed to evaluate the resistance of vegetable-tanned leathers to environmental factors such as heat, light and UV radiation. Samples were exposed in a controlled chamber emitting UV light (254 nm) at 80 °C for 24 and 48 h. After aging, the samples were examined for color changes and variations in color intensity using both visual assessment and spectrophotometric analysis.
2.10. SEM Analysis
Scanning electron microscopy (SEM) was used to characterize the different vegetable-tanned leathers. SEM analysis was performed by using Thermo Scientific Apreo S field emission scanning electron microscope (FE-SEM) at an acceleration voltage of 5.0 kV and secondary scattered electron detector (SE). Leather samples were sputtered with a 7 nm thick 60:40 gold–palladium (Au–Pd) layer using a Leica EM ACE600 high-vacuum sputter coater to prevent surface charging before SEM analysis.
2.11. Statistical Analysis
Statistical analyses were performed using IBM SPSS Statistics software. To evaluate the physical and mechanical properties of the samples, a One-Way Analysis of Variance (ANOVA) was conducted to determine the significance of differences in values among the individual samples. In cases the ANOVA (Duncan) was employed as a posthoc analysis to identify distinct homogeneous subsets and rank the performance of each sample. All results were expressed as mean values and statistical significance was defined at the 95% confidence level (p < 0.05).
3. Results and Discussion
3.1. FTIR Spectra of Vegetable Tanned Leather
ATR–FTIR spectrum of the pickled hide sample shown in Figure is dominated by collagen-related absorptions, indicating that the substrate retained its proteinaceous character prior to vegetable tanning. The broad high-wavenumber envelope observed between 3524 and 3290 cm–1, with discernible minima at 3394 and 3290 cm–1, can be attributed to overlapping O–H/N–H stretching vibrations arising from bound moisture and the amide A region of collagen. The band at 2921 cm–1 is assigned to aliphatic CH2 stretching vibrations. In the amide region, the intense absorption at 1630 cm–1 corresponds to the amide I band, confirming the preservation of the collagen backbone, whereas the amide II contribution around 1540 cm–1 appears only weakly resolved, most likely due to band overlap in the acidified pickled matrix. The absorptions at 1451, 1338, and 1228 cm–1 are consistent with CH2 deformation and amide III-related vibrations involving C–N stretching, N–H bending and CH2 wagging of the collagen structure. The low-wavenumber features at 1113 and 607 cm–1 may additionally reflect residual inorganic species associated with the pickling stage. Above all, the basic components and the integrity of the protein molecule structure of the leather samples were maintained and could be successfully detected.
4.
FTIR spectra of pickle hide sample.
The FTIR spectrum of the control group shown in Figure exhibited the characteristic absorptions of both collagen and vegetable tannins, indicating successful tannin fixation within the collagen matrix. Control leather shows preserved collagen bands and condensed-tannin uptake, especially at ∼1450, 1233, 1031, and 840 cm–1, confirming successful vegetable tanning. ,
5.
FTIR spectra of control group.
Compared with the untanned/pickled hide in Figure , the control leather in Figure retains the broad band near 3290 cm–1 and the collagen amide envelope, showing that tanning did not disrupt the collagen backbone. Its broader 3290 cm–1 region is consistent with overlap between collagen N–H/O–H vibrations and phenolic O–H groups from vegetable tannins, together with stronger hydrogen bonding after tannin fixation, as reported for vegetable-tanned leathers by Mustafa et al.
The amide I band remains centered at 1632 cm–1, very close to the 1630 cm–1 band of Figure , indicating preservation of peptide CO vibrations, but the stronger absorption in this zone is also compatible with overlap from the common tannin band reported at 1615–1606 cm–1 by Falcão and Araújo (2013, 2014). , The amide II region is still expected in the 1540–1550 cm–1 interval, although it is less distinctly resolved than amide I in the plotted control spectrum, most likely because tannin-related absorptions redistribute intensity across the 1500–1600 cm–1 region. The 1450 cm–1 band becomes more diagnostic after tanning, because vegetable tannins characteristically contribute an aromatic-ring vibration at 1452–1446 cm–1, whereas the untanned hide mainly reflects collagen-associated deformation bands in this zone. ,
Likewise, the shift of the amide III/C–O region from 1228 cm–1 in Figure to 1233 cm–1 in Figure , together with the clear 1031 cm–1 band, indicates new tannin-derived C–O contributions in the fingerprint region; these match the common vegetable tannin bands at 1211–1196 and 1043–1030 cm–1 established by Falcão and Araújo. Moreover, the weak ∼840 cm–1 feature, absent from Figure , is more consistent with a condensed-tannin signature and agrees with the mimosa/quebracho control formulation used in the manuscript. The spectral differences between Figures and therefore indicate successful vegetable tanning through tannin fixation onto collagen, dominated by condensed-tannin interactions while preserving the amide I–III framework.
Figure (T1, 40% mimosa tannin) and Figure (T2, 40% quebracho tannin) exhibit very similar ATR–FTIR profiles, indicating that both single-tannin systems preserved the collagen backbone while introducing features consistent with condensed vegetable tannins. In both spectra, the broad band around 3300 cm–1, with a shoulder near 3091–3093 cm–1, may be attributed to overlapping O–H/N–H vibrations, whereas the bands at 2923–2924, 1632, 1450–1451, 1337, and 1233 cm–1 are consistent with CH2 stretching, amide I, CH2 deformation and amide III/fingerprint vibrations of collagen. Compared with the untanned pickled hide in Figure , the fingerprint region becomes more structured and intense, particularly near 1031–1032 and 836–840 cm–1, which is compatible with tannin fixation in the collagen matrix; this interpretation agrees with the literature, where ∼1043–1030 cm–1 is assigned to C–O stretching common to vegetable tannins and ∼844–842 cm–1 is considered a characteristic band of condensed tannins.
6.
FTIR spectra of T1.
7.
FTIR spectra of T2.
Only minor differences are evident between T1 and T2, suggesting broadly comparable tannin–collagen interactions for mimosa and quebracho at the applied concentration. T1 appears to show slightly clearer definition in the 1451–1032 cm–1 region, whereas T2 retains an almost superimposable band pattern with the same principal amide and condensed-tannin-associated absorptions. Therefore, the main spectral change relative to Figure is not a disruption of collagen integrity, but the emergence of condensed-tannin character after tanning; the low-wavenumber band at 524–525 cm–1 should be interpreted cautiously and is less diagnostic than the changes observed in the 1200–800 cm–1 fingerprint region.
Figure shows that all binary Mimosa–Quebracho combinations, C1–C4, preserve the broad high-wavenumber collagen envelope but display a much richer and more structured fingerprint region than Figure . Compared with the untanned reference, the dominant protein-centered pattern at 1630/1451/1228/1113 cm–1 is replaced by mixed collagen–tannin envelopes centered broadly in the ∼1612–1607, ∼1452–1448 and ∼1231–1030 cm–1 regions, indicating a stronger contribution from aromatic-ring and phenolic C–O vibrations. The differences among C1–C4 are expressed mainly as changes in intensity and spectral resolution in the 1285–1030 and 870–820 cm–1 regions. , This behavior is consistent with varying ratios of two condensed tannins producing different extents of overlap between collagen amide bands and condensed-tannin aromatic/C–O–C vibrations. ,
8.
FTIR spectra of C1–C2–C3-C4.
The expanded spectra in Figure confirm that the C-series remains largely within the condensed-tannin domain: C1 shows a well-resolved set at 1612, 1452, 1338, 1285, 1231, 1157, 1111, 1030, and 842 cm–1, while C4 shows 1518, 1448, 1334, 1285, 1209, 1145, 1031, and 820 cm–1. C2 emphasizes 1607, 1448, 1321, 1162, and 1043 cm–1, whereas C3 shows 1610, 1508, 1446, 1321, 1093, and 1041 cm–1 together with an additional weak 1718/758 cm–1 pair. Because C3 is compositionally a condensed-tannin system, those latter bands should be interpreted cautiously as possible overlap from oxidized/nontannin or residual process components; overall, relative to Figure , the C-series is characterized by a downshifted composite amide I/aromatic envelope and intensified condensed-tannin markers in the 1285–1111 and 842–820 cm–1 regions (Figure ). ,
9.
FTIR spectra of C1–C2–C3-C4v2.
10.
FTIR spectra of U1–U2–U3.
In the overview spectra of the Chestnut-containing ternary systems, U1–U3, the broad collagen-related high-wavenumber region remains visible, but the fingerprint region becomes more structured than in Figure and slightly more complex than in the condensed-only systems. Relative to the untanned reference, U1–U3 display stronger low-wavenumber absorption and a more evident carbonyl-side contribution in the 1700–1600 cm–1 domain, consistent with introduction of a hydrolyzable-tannin component on top of the Mimosa–Quebracho condensed matrix. The close similarity of the three traces indicates that all ternary formulations preserve the collagen scaffold while varying mainly in the intensity and exact position of hydrolyzable-tannin marker bands. This pattern is expected when Chestnut-derived ellagitannins contribute ester/carboxyl-containing phenolic structures to a condensed-tannin collagen network. ,
The detailed spectra in Figure provide the clearest evidence of Chestnut contribution: U1, U2 and U3 exhibit new bands at 1712, 1718, and 1706 cm–1, respectively, together with companion bands at 1335, 1331, and 1327 cm–1, all of which are absent in Figure and characteristic of hydrolyzable tannins. At the same time, the spectra retain mixed collagen/condensed-tannin features at 1610–1605, 1516–1509, 1448–1444, 1287–1278, 1211–1203/1207 and 1038–1027 cm–1, showing that Chestnut does not replace but rather supplements the Mimosa–Quebracho condensed fingerprint. Among the ternary systems, U2 shows the strongest carbonyl expression at 1718 cm–1, whereas U3 shows the lowest composite 1605 cm–1 band and a split 1211/1203 cm–1 region, suggesting the strongest redistribution of phenolic C–O environments within the ternary system. ,
11.
FTIR spectra of U1–U2–U3v2.
3.2. Physical Properties of Vegetable Tanned Leathers
The tensile strength of leather fibers is intimately dependent on structural characteristics, such as the collagen triple helix structure, the arrangement of fibrils and the cross-links established by various chemicals. Furthermore, the mechanical resistance is directly influenced by the type and quantity of materials used in the leather production recipes. The ability of leather to stretch without tearing and regain its original form after load removal is enabled by the resistance of collagen fiber bundles to tension.
In this study, the tensile strength values of leathers tanned with different tannin combinations were measured (Table ). Analysis of the results showed that the control group recorded the highest average tensile strength at 24.96 N/mm2. In the T1 group (40% mimosa), this value decreased to 20.28 N/mm2, representing an approximate 19% reduction. The T2 group (quebracho) exhibited strength of 21.45 N/mm2, which was 14% lower than the control sample. Mixtures C1 and C2, containing equal ratios of mimosa and quebracho (C1) and a higher quebracho ratio (C2), showed tensile strengths of 18.25 N/mm2 and 19.46 N/mm2, respectively, indicating decreases in the range of 20–27% compared to the control samples. The lowest strength level was observed in group C4 (high mimosa ratio) at 16.58 N/mm2. These results suggest that a high mimosa ratio tends to reduce tensile strength. Conversely, improvements in tensile strength were noted in the triple tannin mixtures (U groups) containing chestnut: U1 showed strength of 20.05 N/mm2, similar to the T1 level. The strength increased notably in the U2 group, reaching 24.48 N/mm2 and approaching the control sample. U3 recorded 22.56 N/mm2. These results suggest that these chestnut-containing recipes enhanced strength by forming additional bonds between the collagen fibers. This behavior is consistent with previous literature showing that chestnut-tanned leathers typically exhibit higher tensile strength but lower elongation compared to quebracho-tanned leathers. For instance, reported values indicate tensile strengths of approximately 19.95 N/mm2 with ∼20% elongation for chestnut-tanned leather, whereas quebracho-tanned leather is weaker (∼14.27 N/mm2) but more extensible (∼29%). Such trends can be explained by fundamental differences in tannin chemistry. Chestnut tannins are dominated by relatively small hydrolyzable ellagitannin molecules, which can penetrate collagen fibrils more uniformly and generate a high density of short-range cross-links, thereby stiffening the collagen matrix and enhancing tensile strength. In contrast, condensed tannins such as mimosa and quebracho consist of larger, polymeric flavan-3-ol structures that tend to form fewer but longer-range intermolecular linkages, allowing greater fiber mobility and thus higher elongation under load.
3. Tensile Strength and Elongation of Vegetable-Tanned Leathers.
| samples | tensile strength (N/mm2) | elongation (%) |
|---|---|---|
| Control group | 24.96 ± 0.95a | 27.45 ± 1.82b |
| T1 | 20.28 ± 0.71b | 30.26 ± 1.99a,b |
| T2 | 21.45 ± 0.84b | 25.78 ± 1.77b,c |
| C1 | 18.25 ± 0.55b,c | 28.61 ± 1.86b |
| C2 | 19.46 ± 0.68b | 27.36 ± 1.83b |
| C3 | 18.63 ± 0.58b,c | 23.98 ± 1.72c |
| C4 | 16.58 ± 0.54c | 32.54 ± 1.98a |
| U1 | 20.05 ± 0.70b | 30.27 ± 1.91a,b |
| U2 | 24.48 ± 0.93a | 32.56 ± 2.00a |
| U3 | 22.56 ± 0.88a | 34.25 ± 2.03a |
At the molecular level, phenolic hydroxyl groups in all tannin classes participate in hydrogen bonding with collagen amide CO and N–H groups. In addition, condensed tannins can additionally be oxidized in the presence of the plants-own polyphenol oxidases. The resulting quinones react covalently via a nucleophilic Michael addition with amino-groups of the collagen. As a result, tannin structure governs cross-link topology: condensed tannins yield a more flexible network with greater extensibility, whereas hydrolyzable tannins promote a tighter, more rigid structure that enhances tensile strength.
Vegetable-tanned leathers generally exhibit good tensile strength, often on par with chrome-free alternatives. Omur et al. reported tensile strength values around 16–21 N/mm2 for calf leathers tanned with chestnut, valonia, mimosa and quebracho extracts. In the study, the hydrolyzable tannins (chestnut, valonia) yielded slightly lower tensile strength (∼16–18 N/mm2) compared to the condensed tannins (mimosa, quebracho) at ∼21 N/mm2. Similarly, a combination tanning with mimosa achieved tensile strength up to 26.4 N/mm2, indicating that well-formulated vegetable tannages can attain high strength. Overall, tensile strengths in the range of 15–25 N/mm2 are commonly reported for vegetable-tanned leathers, which provide a useful benchmark for comparing the present results. , As an additional reference point, chrome-tanned shoe upper leathers typically exhibit an average tensile strength of approximately 25 N/mm2 (UNIDO, 1996), suggesting that the optimized vegetable-tanned samples in this study approach or even match the mechanical performance expected in conventional industrial leathers.
Regarding elongation results; the control sample’s elongation was determined to be 27.45%. In the T1 group, elongation increased to 30.26%, demonstrating that mimosa imparts flexibility to the leather fibers. However, the T2 group (quebracho) showed a decrease in elongation to 25.78%. The C4 group (high mimosa ratio) exhibited the highest elongation value among all groups at 32.54%, confirming that this group displayed a softer, more elastic structure. The chestnut-added U groups (U1:30.27%, U2:32.56%, U3:34.25%) provided improvements in both flexibility and strength, with the U3 group showing the highest elongation.
The results of the analysis of variance (ANOVA) confirmed that different tannin combinations had a statistically significant effect on both tensile strength and elongation (p < 0.05). According to Duncan’s test, the Control group, U2 and U3 samples were in the same homogeneous group (a) in terms of tensile strength, exhibiting the highest mechanical resistance; in contrast, the C4 sample with high mimosa content had the lowest value (c) to a significant degree. In the elongation data, flexibility was seen to increase in parallel with the increase in mimosa ratio; in particular, it was determined that the U and C4 groups showed statistically higher elasticity (a) compared to the control group, while the T2 and C3 groups had more limited fiber mobility (c).
These findings indicate that vegetal tannin combinations have significant, dual effects on the mechanical properties of leather. Specifically, high mimosa ratios reduce strength while increasing elongation; high quebracho ratios maintain reasonable strength but restrict elongation. The addition of chestnut tannin provides a balanced performance by increasing both tensile strength and elongation.
Tear strength is a defining criterion for the end-use performance and quality of finished leather, representing the force required to tear the leather over a specific distance. It reflects the leather’s resistance to tear stresses that may occur during use. Single edge tear loads of vegetable tanned leather were given in Table .
4. Single Edge Tear Load of Vegetable-Tanned Leathers.
| samples | single edge tear load (N) |
|---|---|
| Control group | 65.56 ± 2.87c,d |
| T1 | 62.45 ± 1.97d |
| T2 | 76.59 ± 3.01a,b |
| C1 | 56.24 ± 1.99e |
| C2 | 68.52 ± 2.06c |
| C3 | 77.85 ± 2.94a |
| C4 | 56.74 ± 1.80e |
| U1 | 78.63 ± 2.36a |
| U2 | 75.49 ± 2.26b |
| U3 | 68.52 ± 2.10c |
The control group’s single edge tear load was measured at 65.56 N, which comfortably exceeds the minimum industry threshold of 50 N for chrome-tanned shoe upper leathers, as specified by UNIDO (1996). Group T1 (40% mimosa) showed a decrease to 62.45 N (approximately a 5% reduction). In contrast, the T2 group (40% quebracho) recorded a tear load of 76.59 N, exceeding the control sample by 17%.
Tear loads in mimosa and quebracho tannin mixtures varied based on the ratios: C1 showed the lowest load at 56.24 N. Group C3 (quebracho dominant) exhibited a tear resistance of 77.85 N, surpassing T2. Conversely, the C4 group (mimosa dominant) registered one of the lowest values at 56.74 N. These results suggest that mimosa tannin weakens tear resistance due to its softening effect, while quebracho tannin increases resistance by creating more stable bonds between fibers. When examining the triple mixtures with chestnut: Group U1 (chestnut-inclusive) recorded the highest tear load overall at 78.63 N. This finding suggests that chestnut tannin strengthens tear resistance by imparting additional cross-links to the structure. However, as the mimosa ratio increased within the chestnut mixtures (U2:75.49 N; U3:68.52 N), the improvement in tear load decreased.
When tear load values are statistically analyzed, it is observed that tannin types create significant differences in the bonding capacity between leather fibers (p < 0.05). Duncan test results revealed that sample U1, containing chestnut and sample C3, with a high quebracho content, were in the top group (a), offering the strongest tear resistance. On the other hand, the fact that mimosa-dominant mixtures (C1 and C4) exhibit significantly lower tear load values (e) than the control group statistically proves that mimosa’s softening effect on fiber structure negatively affects tear resistance.
Since all chemicals used in leather production initially contact the grain layer, the grain flexibility may be lost under different chemical usage scenarios. Grain elongation capacity is determined via the dome height and the force causing cracking and bursting of the grain. Ball burst test results of vegetable tanned leathers were given in Table .
5. Ball Burst Test Results of Vegetable-Tanned Leathers.
| samples | breaking force (kgf) | breaking distance (mm) | bursting force (kgf) | bursting distance (mm) |
|---|---|---|---|---|
| Control group | 18.7 ± 1.56d,e | 6.8 ± 0.20c | 66.4 ± 1.99e,f | 11.5 ± 0.54b |
| T1 | 14.2 ± 1.53g | 6.3 ± 0.19d | 62.3 ± 2.87f | 11.7 ± 0.45b |
| T2 | 23.9 ± 1.72a | 7.2 ± 0.29b | 71.4 ± 3.14c,d | 11.2 ± 0.54b,c |
| C1 | 20.7 ± 1.68b | 6.7 ± 0.31c | 87.3 ± 3.62a | 10.5 ± 0.42d |
| C2 | 19.3 ± 1.58c,d | 5.8 ± 0.17e,f | 78.1 ± 3.34b | 10.8 ± 0.49c,d |
| C3 | 20.1 ± 1.61b,c | 6.9 ± 0.34b,c | 68.8 ± 3.06d,e | 11.3 ± 0.55b,c |
| C4 | 16.1 ± 1.58f | 5.5 ± 0.26f | 75.6 ± 3.27b,c | 11.6 ± 0.56b |
| U1 | 18.9 ± 1.67d,e | 5.9 ± 0.18e | 67.0 ± 3.01e | 10.4 ± 0.40d |
| U2 | 18.0 ± 1.55e | 6.8 ± 0.25c | 76.5 ± 3.30b | 12.5 ± 0.66a |
| U3 | 19.1 ± 1.60c,d | 7.7 ± 0.33a | 85.9 ± 3.58a | 10.3 ± 0.45d |
The highest cracking force was measured in the T2 group (40% quebracho) at 23.9 kgf. This group was followed by C1 (20.7 kgf), C3 (20.1 kgf), and U3 (19.1 kgf), demonstrating that quebracho and certain mixtures increase grain durability. The T1 group (40% mimosa) recorded the lowest cracking force at 14.2 kgf, suggesting that mimosa tannin reduces resistance to grain cracking compared to other tannins.
The highest bursting force was found in group C1 at 87.3 kgf, closely followed by U3 at 85.9 kgf. The lowest bursting force was 62.3 kgf, recorded in the T1 group (mimosa dominant). These results indicate that mixed tannins, and particularly the addition of chestnut, significantly increase bursting resistance.
The highest cracking distance was achieved in the chestnut-containing U3 group at 7.7 mm. T2 (7.2 mm) and C3 (6.9 mm) also approached this value. The C4 group (high mimosa ratio) recorded the lowest cracking distance at 5.5 mm, indicating that the surface became more brittle.
Regarding deformation distance during bursting, the U2 group achieved the highest bursting distance at 12.5 mm. In the U1 (10.4 mm) and U3 (10.3 mm) groups, the deformation capacity was found to be slightly more restricted. This distribution shows that tannin combinations affect not only the bursting force but also the elastic deformation capacity at the moment of bursting. Notably, all tested formulations exceeded the minimum 7 mm grain distension benchmark typically observed in chrome-tanned shoe upper leathers (UNIDO, 1996).
Overall, quebracho tannin generally increased cracking and bursting resistance, while mimosa tannin tended to decrease these resistances. Specific tannin mixtures and chestnut additions (especially U3 and C1) maximized both cracking and bursting loads; however, the U2 group exhibited the highest elasticity in terms of bursting distance
Ball burst test results indicate that the interlayer strength of the leather and the flexibility of the surface layer showed a statistically significant change depending on the tanning formulation (p < 0.05). In terms of bursting force, samples C1 and U3 clustered in the highest performance group (a), while the mimosa-dominant T1 group exhibited the lowest bursting resistance (f) to a significant degree. In the cracking distance data, only the U3 and T2 groups statistically met the quality criterion of 7 mm (groups a and b), while the high mimosa-containing C4 showed the lowest deformation capacity (f), indicating increased surface brittleness.
Omur et al. reported detailed ball burst data: for example, valonia-tanned leather had a grain crack strength of about 301 N at 9.4 mm distension, whereas mimosa tanned leather cracked at a lower 129.9 N and 6.3 mm distension. Quebracho and chestnut fell in between (e.g., chestnut ∼ 151.9 N at 6.7 mm). Interestingly, the more hydrolyzable tannins (valonia, chestnut) showed higher crack resistance than some condensed tannins in that study. Burst strength was even higher, exceeding 620–690 N for mimosa, quebracho, valonia and ∼539 N for chestnut. These literature values show that different vegetable tannins influence grain crack resistance and ball burst strength in distinct ways, primarily due to their molecular weight, reactivity and affinity for collagen. Reported values for vegetable-tanned leathers vary widely, with typical grain crack distension ranging between 6 and 12 mm, and burst strengths spanning 500 to 700 N, depending on the tannin type and leather thickness. Our findings fall within this established range, yet demonstrate that specific tannin combinations particularly binary and ternary systems can shift these values in meaningful ways. This supports the notion that the mechanical integrity of vegetable-tanned leather can be systematically optimized by tailoring tannin composition.
Vegetable tanning is generally associated with the production of firmer leather compared to chrome tanning. In this context, Nugraha et al. reported that one of the main drawbacks of vegetable tanning was its tendency to produce leather with a stiff, dense and hard structure. This inherent stiffness arises primarily from the filling effect and astringency of plant-derived polyphenols. Softness, or “hand feel,” is therefore a critical parameter in the evaluation of vegetal tanning formulations. Although vegetal tanning enhances tightness and fullness through the formation of high-density tannin–protein interactions, these strong bonds can restrict fiber mobility, ultimately leading to increased material stiffness. The softness values obtained in this study were presented in Table .
6. Softness Values of Vegetable-Tanned Leathers.
| samples | softness (instrument units) |
|---|---|
| Control group | 0.48 ± 0.01d |
| T1 | 1.08 ± 0.03a |
| T2 | 1.06 ± 0.03a |
| C1 | 0.26 ± 0.01f |
| C2 | 0.28 ± 0.01e |
| C3 | 0.44 ± 0.01d |
| C4 | 0.60 ± 0.02c |
| U1 | 0.68 ± 0.02b |
| U2 | 0.46 ± 0.01d |
| U3 | 0.28 ± 0.01e |
The softness values were obtained using a BLC ST 300 Softness Tester (values are in an instrument-specific scale). The T1 group (40% mimosa) achieved the highest softness value at 1.08 units. Similarly, the T2 group (40% quebracho) showed higher softness at 1.06 units. The control group presented an intermediate value of 0.48 units. In mimosa and quebracho mixtures, the C1 group (20% M + 20% Q) exhibited the lowest softness value at 0.26 units, classifying it as the stiffest sample. This suggests that this ratio maximized tannin–protein bridges, tightening the structure. The C4 group (high mimosa) increased significantly to 0.60 units, indicating that increasing the mimosa ratio markedly improves softness. Among the chestnut-containing mixtures, U1 (0.68 units) highlights the softening effect of the chestnut addition despite the presence of high quebracho. Conversely, U3 (0.28 units) showed notable stiffness despite its higher mimosa content, suggesting the synergistic effect between chestnut and quebracho created a different structure in this recipe.
The softness values obtained from device measurements clearly distinguished the effect of tannins’ properties on material behavior (p < 0.05). The fact that single tannin applications (T1 and T2) belong to the statistically highest softness group (a) indicates that the homogeneous tannin structure at high concentrations maximizes fiber mobility. In contrast, the fact that C1, an equal mixture of mimosa and quebracho, had the lowest softness value (f) confirms that the high-density tannin-protein cross-links created by this combination significantly stiffened the leather.
The results confirms that the high-density tannin-protein cross-links created by this combination significantly stiffened the leather, emphasizing that while single tannins maximize mobility, binary mixtures near equal ratios promote structural rigidity.
Hardness measures a material’s resistance to shape change when exposed to external forces. Hardness values were given in Table . The Shore D scale was utilized for these vegetal leathers.
7. Shore D Hardness Values of Vegetable Tanned Leathers.
| samples | shore D hardness values |
|---|---|
| Control group | 30.98 ± 2.03b |
| T1 | 20.14 ± 1.60e |
| T2 | 20.37 ± 1.61e |
| C1 | 35.00 ± 2.06a |
| C2 | 34.63 ± 2.00a |
| C3 | 31.71 ± 1.95b |
| C4 | 22.78 ± 1.78d |
| U1 | 27.32 ± 1.82c |
| U2 | 31.34 ± 1.94b |
| U3 | 34.22 ± 2.03a |
The control group hardness was measured at 30.98 D. T1 (20.14 D) and T2 (20.37 D) groups, tanned solely with high proportions of mimosa or quebracho, were in the soft-to-medium range. These low hardness values are inversely correlated with the highest softness values recorded for these groups (T1 = 1.08; T2 = 1.06). Hardness values significantly increased in the mimosa and quebracho mixtures. C1 (35.00 D) and C2 (34.63 D) recorded the highest hardness values. These ratios led to high tannin–protein cross-link density, resulting in a tightened structure. This high hardness correlates with the high cracking and bursting forces observed in the ball burst test but resulted in relatively restricted cracking distances (C1:6.7 mm; C2:5.8 mm). C4 (high mimosa) showed a significant reduction in hardness to 22.78 D, which correlated with a decreased cracking load but an increased bursting distance (11.6 mm), suggesting enhanced deformation capability.
In chestnut-containing triple combinations, hardness generally increased with the mimosa ratio: U3 measured 34.22 D. U3′s high hardness was compatible with high cracking and bursting forces (19.1 kgf and 85.9 kgf). Notably, U3 achieved a cracking distance of 7.7 mm, demonstrating high elastic deformation despite its stiffness. U2 (31.34 D) yielded a high bursting distance (12.5 mm), reflecting the flexibility-imparting effect of the chestnut addition.
Shore D hardness values show a perfect inverse correlation with softness data, supporting statistical consistency (p < 0.05). Samples C1, C2 and U3 belong to the highest hardness group (a) due to the tight cross-linking formation in the fiber network; samples T1 and T2, on the other hand, exhibit the lowest hardness values (e) to a significant degree, reinforcing their soft structure. Statistical analysis shows that the addition of chestnut (especially in the U3 group) can simultaneously impart both high stiffness and high deformation capacity observed in the ball burst test to the material, thus establishing an optimized balance between material rigidity and flexibility, thus establishing an optimized balance between material rigidity and flexibility, an inverse relationship further illustrated in the softness-hardness correlation in Figure .
12.
Softness–hardness correlation in samples.
High mixed ratios of mimosa and quebracho increase hardness, while pure tannins or balanced chestnut-inclusive recipes reduce it. There is a general correlation between high hardness and high cracking and bursting forces, although the cracking distance often decreases. Chestnut tannin modulates this relationship, enabling the simultaneous observation of high hardness and high cracking distance in certain groups (U3).
Shrinkage temperature is a crucial parameter for evaluating the thermal stability of collagen and the strength of the tannin-protein cross-links within the leather. The thermal stabilities of the leathers were shown in Table .
8. Shrinkage Temperature of Vegetable-Tanned Leathers.
| samples | shrinkage temperature (°C) |
|---|---|
| Control group | 88.24 ± 2.65a |
| T1 | 86.63 ± 2.60b |
| T2 | 83.58 ± 2.59e |
| C1 | 85.64 ± 2.57b,c |
| C2 | 82.96 ± 2.49e |
| C3 | 86.24 ± 2.59b |
| C4 | 85.12 ± 2.55c,d |
| U1 | 84.38 ± 2.63d |
| U2 | 89.57 ± 2.69a |
| U3 | 88.41 ± 2.67a |
The shrinkage temperature of the control group was 88.24 °C, indicating high thermal stability. T1 (40% mimosa) showed a decrease to 86.63 °C, while T2 (40% quebracho) recorded the lowest value at 83.58 °C. In the mimosa and quebracho mixtures, shrinkage temperature values ranged between 82.96 and 86.24 °C. C2 recorded the lowest shrinkage temperature at 82.96 °C. Among the chestnut-containing triple combinations, the U2 group recorded the highest shrinkage temperature among all groups at 89.57 °C. This result is particularly significant, as it exceeds the typical shrinkage temperature range reported for vegetable-tanned leathers (∼80–85 °C). U3 also reached the control sample level (88.41 °C). These results demonstrate the thermostability-enhancing effect of chestnut tannin, with the optimal ratio achieved in U2 (equal mimosa and quebracho ratios combined with chestnut).
Recent literature confirms that shrinkage temperature (T s) is highly dependent on the tannin classification. Condensed tannins such as mimosa and quebracho typically produce T s values in the range of 80–85 °C, owing to their high polyphenol content and strong collagen-binding capability. ,, In contrast, hydrolyzable tannins such as chestnut generally result in lower T s values, typically around 66–75 °C, which has been attributed to their comparatively less extensive cross-linking. , For example, Morera et al. reported shrinkage temperatures of 352–353 K (∼79–80 °C) for mimosa and quebracho-tanned leathers, whereas chestnut-tanned leather exhibited lower values of 339–345 K (∼66–72 °C).
At the molecular level, the distinct thermal responses of hydrolyzable and condensed tannins can be rationalized by their different collagen-binding mechanisms. According to Schröpfer and Meyer, hydrolyzable tannins predominantly interact with collagen via protonated amino groups through hydrogen bonding and electrostatic interactions. These interactions may involve hydrogen bonds either between peptide carbonyl oxygen atoms of collagen and the hydrogen atoms of phenolic hydroxyl groups or between protonated ε-amino groups of collagen and the oxygen atoms of tannin hydroxyl groups. However, such interactions are strongly pH-dependent and tend to weaken or disappear with increasing pH, which limits the long-term stability of the collagen–tannin network formed by hydrolyzable tannins.
In contrast, condensed tannins exhibit additional stabilization pathways. Schröpfer and Meyer demonstrated that the participation of unprotonated amino groups in cross-linking indicates the formation of covalent bonds between collagen and quinone species generated via enzymatically catalyzed oxidation of certain polyphenolic structures by plant-derived polyphenol oxidases. These quinones can react with collagen amino groups through nucleophilic 1,4-Michael addition, resulting in covalent cross-links. Although these covalent bonds may not be fully stable under strongly acidic conditions, their presence significantly enhances the density and effectiveness of the collagen network under typical tanning conditions.
In the present study, the ternary tannin system (U2) exhibited a shrinkage temperature of 89.6 °C, which clearly exceeds the typical T s range reported for both condensed and hydrolyzable tannins applied individually. This pronounced increase suggests a synergistic stabilization effect arising from the optimized combination of condensed and hydrolyzable tannins. The coexistence of dense hydrogen-bonding interactions, electrostatic attractions and additional quinone-mediated covalent linkages likely promotes a more compact and thermally resistant collagen matrix, consistent with observations reported for mixed tannin systems.
A direct comparison of shrinkage temperature values (Table ) and tensile strength data (Table ) further reveals a general trend in which higher T s values are associated with increased tensile strength. For instance, the control group exhibited both high shrinkage temperature (88.24 °C) and the highest tensile strength (24.96 N/mm2), while the U2 formulation combined the highest T s (89.57 °C) with a tensile strength of 24.48 N/mm2, closely matching the control. Similarly, the addition of chestnut tannin enhanced both thermal stability and mechanical strength in other ternary systems (e.g., U3: T s = 88.41 °C and tensile strength = 22.56 N/mm2). Nevertheless, deviations from this trend, such as those observed for T2 and U1, indicate that tensile performance is not governed by thermal stability alone, but rather by the complex interplay between tannin class, molecular structure and formulation ratio. It is also noteworthy that the T s values recorded in this study, particularly in the U2 and U3 formulations, approach the thermal stability levels typically observed in chrome-tanned leathers, which generally exhibit shrinkage temperatures around 100 °C. , This further supports the viability of optimized vegetable tanning systems as potential chrome-free alternatives for performance-demanding applications.
Thermal stability analyses confirm that the capacity of tannins to stabilize the collagen matrix against heat varies depending on the formulation ratios (p < 0.05). The triple tannin system U2 surpassed even the control group, reaching the highest shrinkage temperature and ranking in the top group (a) in terms of thermal resistance. The fact that samples T2 and C2 were statistically in the lowest thermal stability group (e) indicates that some quebracho-dominant formulations can weaken hydrothermal resistance; conversely, the U group, optimized with chestnut, mimosa and quebracho, produced a synergistic stabilization effect.
3.3. Determination of Color Properties of Vegetable Tanned Leathers
The determination of color properties is critical, as chemicals utilized in the tanning process tend to either lighten/darken the leather tone or generate undesirable stains on the surface, depending on their intrinsic components. Consequently, the resultant colors derived from the chosen vegetable tannins were evaluated using a spectrophotometer within the CIELAB-76 color space, comparing them against commercially produced control leather. The evaluation relied on the L* (lightness axis), a* (red-green axis), and b* (yellow-blue axis) coordinates, alongside the ΔE value, which numerically quantifies the total color difference relative to the reference control group. The control group established the baseline reference values at L * (D65) = 74.11, a * (D65) =10.99, and b*(D65) = 18.52. Color measurement results were given in Table .
9. Color Parameters of Vegetable-Tanned Leathers before Aging (CIELAB, D65).
When the color measurement from vegetable tanned leathers was examined in Table , it was observed that single-tannin systems produced the most pronounced deviations from the control group. The leather tanned with 40% mimosa (T1) exhibited a significant increase in lightness (L* = 83.12) while its a* and b* coordinates decreased, yielding a high ΔE value of 10.60. This indicates that mimosa tannin promoted a brighter but less chromatic shade with diminished red and yellow components. Conversely, the sample tanned with 40% quebracho (T2) was darker (L* = 72.66) but displayed substantially higher a* (16.15) and b* (25.90) values, resulting in a ΔE of 9.12. These results demonstrate that high concentrations of a single tannin tend to induce strong color shifts, corroborating the industrial challenge of obtaining consistent light tones in vegetable tanning.
In contrast, binary combinations of mimosa and quebracho (C1–C4) produced color characteristics closer to the control. Particularly, C2 (15% mimosa +25% quebracho) exhibited ΔE = 1.24, and C1 (20% mimosa +20% quebracho) yielded ΔE = 2.14, both within the perceptually negligible range. These results confirm that balanced mimosa–quebracho ratios can effectively stabilize color, generating leather with pastel and reproducible hues. When the ratios deviated from these optima (C3 and C4), ΔE increased to 4.65 and 3.69, respectively, underlining the sensitivity of color stability to tannin proportioning.
Ternary mixtures including chestnut tannin (U1–U3) did not improve color stability. Although they showed high brightness values (L* ≈ 82), the ΔE values remained relatively high (8.63–9.14), suggesting that the introduction of chestnut altered the a* and b* axes in ways that increased divergence from the reference sample. Overall, these findings indicate that while single-tannin systems cause excessive deviations, specific binary mimosa–quebracho blends particularly C1 and C2 achieve color properties nearly identical to the industrial control, supporting their suitability for consistent production of light-colored leathers.
Colorimetric data (L*, a*, b* and ΔE) revealed that the choice of tannin formulation significantly altered the visual appearance of the leathers (p < 0.05). Duncan’s posthoc analysis indicated that the T1 and U-series (U1, U2, U3) samples exhibited significantly higher lightness (L*) values (Group “a”), indicating a marked brightening effect compared to the control group. Conversely, the T2 sample showed the highest redness (a*) and yellowness (b*) values, placing it in the uppermost statistical group for these parameters (Group “a”). The total color difference (ΔE) analysis showed that T1 induced the most profound color shift (ΔE = 10.60, Group “a”), whereas the C2 formulation maintained the highest color stability, with a ΔE of 1.24 (Group “g”), being statistically closest to the control group. These findings confirm that while mimosa and chestnut-containing formulations significantly modify the color profile, quebracho-mimosa binary mixtures like C2 offer superior color consistency relative to the untreated control.
The phenomenon of aging, defined as the deterioration of appearance and performance due to environmental factors like heat, light, and humidity, was investigated using 24-h (Table ) and 48-h (Table ) accelerated aging experiments. The ΔE values were calculated by comparing the color parameters before aging with those measured after the accelerated exposure.
10. Color Parameters of Vegetable-Tanned Leathers after 24 h Accelerated Aging (CIELAB, D65).
| samples | L* | a* | b* | ΔE |
|---|---|---|---|---|
| Control group | 79.11 ± 0.81c | 13.47 ± 0.13c | 21.98 ± 0.22c | 6.57 ± 0.22a |
| T1 | 82.12 ± 0.86b | 9.85 ± 0.10f | 17.73 ± 0.20f | 4.37 ± 0.13d |
| T2 | 76.66 ± 0.79d | 18.63 ± 0.16a | 29.36 ± 0.29a | 5.84 ± 0.17b |
| C1 | 80.04 ± 0.83c | 12.79 ± 0.14d | 21.34 ± 0.20d | 5.84 ± 0.18b |
| C2 | 77.06 ± 0.77d | 14.13 ± 0.14b | 22.06 ± 0.25c | 5.84 ± 0.18b |
| C3 | 73.31 ± 0.75e | 15.76 ± 0.16b | 23.38 ± 0.24b | 5.21 ± 0.16c |
| C4 | 81.88 ± 0.80b | 13.26 ± 0.15c | 19.55 ± 0.20e | 6.57 ± 0.21a |
| U1 | 84.80 ± 0.86a | 11.73 ± 0.10e | 19.74 ± 0.20e | 4.70 ± 0.15d |
| U2 | 83.88 ± 0.84a | 11.21 ± 0.11e | 18.96 ± 0.21e | 4.70 ± 0.13d |
| U3 | 84.76 ± 0.85a | 12.58 ± 0.13d | 22.83 ± 0.23b | 4.70 ± 0.14d |
11. Color Parameters of Vegetable-Tanned Leathers after 48 h Accelerated Aging (CIELAB, D65).
| samples | L* | a* | b* | ΔE |
|---|---|---|---|---|
| Control group | 81.11 ± 0.81d | 13.88 ± 0.14d | 23.10 ± 0.23c | 8.85 ± 0.09a |
| T1 | 83.12 ± 0.83b,c | 10.26 ± 0.13h | 18.85 ± 0.19g | 5.42 ± 0.06e |
| T2 | 78.66 ± 0.80e | 19.04 ± 0.20a | 30.48 ± 0.31a | 8.08 ± 0.09 |
| C1 | 82.04 ± 0.82c,d | 13.20 ± 0.15e | 22.46 ± 0.23d | 8.08 ± 0.08b |
| C2 | 79.06 ± 0.75e | 14.54 ± 0.15c | 23.18 ± 0.23c | 8.08 ± 0.09b |
| C3 | 75.31 ± 0.76f | 16.17 ± 0.14b | 24.50 ± 0.25b | 7.37 ± 0.06c |
| C4 | 83.88 ± 0.80a,b | 13.67 ± 0.14d | 20.67 ± 0.20d | 8.85 ± 0.07a |
| U1 | 85.80 ± 0.86a | 12.14 ± 0.13f | 20.86 ± 0.21f | 6.19 ± 0.07d |
| U2 | 84.88 ± 0.83a,b | 11.62 ± 0.11g | 20.08 ± 0.20g | 6.19 ± 0.06d |
| U3 | 85.76 ± 0.85a | 12.99 ± 0.12e | 23.95 ± 0.24e | 6.20 ± 0.06d |
The accelerated aging experiments provide critical insight into the durability of vegetable-tanned leathers when exposed to environmental stressors such as heat, UV radiation and humidity. Across all samples, the general trend was a shift toward increased lightness (L*) and elevated a* and b* values, signifying a progressive movement toward brighter, redder and yellower shades. Such chromatic alterations are consistent with oxidative and photochemical transformations of phenolic structures within tannins, as well as the gradual breakdown of chromophoric groups in the collagen–tannin matrix. Statistical analysis via Duncan’s multiple range test (p < 0.05) confirms that these chromatic shifts are significantly governed by the tannin formulation and exposure duration. Throughout both 24 and 48 h periods, the differentiation between samples remained statistically significant, validating the observed performance gaps between condensed and hydrolyzable systems.
The control leather exhibited a ΔE of 6.57 after 24 h, which further increased to 8.85 after 48 h, as shown in Figure , illustrating the susceptibility of conventional vegetable tanning to perceptible and cumulative discoloration. Mimosa tanned samples (T1) demonstrated the lowest color difference after 24 h (ΔE = 4.37) and remained relatively stable after 48 h (ΔE = 5.42). This indicates that mimosa tannins may slow down the rate of chromatic drift during the early phases of oxidative stress, possibly due to their relatively lighter chromophores and condensed structure, which impart less intense initial coloration and reduce oxidative darkening.
13.
ΔE variation of vegetable-tanned leathers over aging time.
In contrast, quebracho-tanned leathers (T2) displayed much higher instability, with ΔE values increasing from 5.84 at 24 h to 8.08 at 48 h. The high a* and b* coordinates observed confirm a strong propensity for red and yellow shifts, which may stem from the higher content of profisetinidin units in quebracho tannins that are prone to oxidative polymerization and the formation of reddish-brown chromophores. This behavior is consistent with literature reports, as accelerated aging studies have documented ΔE values on the order of 15–25 for quebracho-tanned leathers exposed to artificial UV or xenon-arc light. , Such pronounced discoloration is chemically linked to the photoreactivity of condensed tannins: quebracho’s catechol-based polyphenols readily undergo photochemical oxidation (e.g., quinone formation and polymerization) under UV, forming new chromophores and thereby causing significant color shifts. , This phenomenon is well documented for condensed tannins, so the instability observed in T2 is expected; indeed, the measured ΔE aligns with or even slightly improves upon these literature benchmarks. , According to the posthoc results, T1 (Group “e”) and the U-series (Group “d”) maintained the lowest ΔE values, placing them in a statistically superior category of color stability compared to the control and C4 groups (Group “a”). This hierarchy confirms that while the control and high-mimosa (C4) formulations are statistically the most susceptible to degradation, the ternary systems (U-series) provide a robust stabilizing effect, significantly mitigating the long-term impacts of accelerated aging.
Binary mixtures of mimosa and quebracho (C1, C2) exhibited intermediate performance, with ΔE values rising steadily but less dramatically than quebracho itself. These results suggest that mimosa moderates the instability introduced by quebracho, although the stabilization is not sufficient to prevent long-term drift when the ratio deviates from optimum. Notably, ternary systems incorporating chestnut tannin (U1–U3) demonstrated superior stability after 48 h, with ΔE values limited to ≈6.19. This enhanced stability can be attributed to the lower photoactivity of chestnut’s hydrolyzable ellagitannin chromophores. Condensed tannins, which are rich in catechol-type aromatic rings, are inherently more susceptible to oxidative reactions, leading to yellowing and darkening during aging, whereas ellagitannin-rich systems show greater resistance to such transformations.
The observed color trends are fully consistent with well-established literature on the lightfastness behavior of vegetable-tanned leathers. Griyanitasari et al. reported that leathers tanned with condensed tannins such as mimosa and quebracho exhibit more pronounced discoloration under light exposure compared to hydrolyzable tannin systems. Similarly, Sukru and Mutlu (2016) evaluated CIELAB color coordinates of leathers tanned with mimosa, quebracho, chestnut and valonea before and after lightfastness testing. Initially, chestnut-tanned leather displayed a relatively light appearance (L * ≈ 53.5) with a mild yellowish tone (b * ≈ 21.7, a * ≈ 8.2), whereas mimosa tanned leather was darker (L * ≈ 44.9) and redder (a * ≈ 14.6, b * ≈ 17.5), reflecting the characteristic beige hues of hydrolyzable tannins versus the deeper reddish-brown shades imparted by condensed tannins. After accelerated light exposure, substantial color changes were observed in condensed-tannin leathers, with ΔE values of approximately 16.6 for mimosa and 22.4 for quebracho, indicating severe darkening and reddening. In contrast, hydrolyzable tannin systems exhibited significantly higher lightfastness, with ΔE values limited to 5.4 for valonea and 8.2 for chestnut.
These literature findings provide a robust framework for interpreting the present results. The higher ΔE values measured for mimosa and quebracho-tanned leathers after aging, compared with chestnut-containing systems, are fully consistent with previously reported trends. Overall, the evidence confirms that tannin type and phytochemical composition strongly govern both initial coloration and long-term color stability in vegetable-tanned leathers. Condensed tannins rich in fisetinidin and related catechinoid units tend to yield leathers with higher shrinkage temperature and greater fiber extensibility but reduced resistance to photo-oxidative discoloration, whereas hydrolyzable chestnut tannins dominated by ellagitannins provide improved color stability.
From an industrial perspective, these results highlight the critical importance of formulation design not only to achieve desirable initial color tones but also to ensure resistance to cumulative discoloration during service life. While mimosa contributes to pastel shades and short-term color uniformity, the incorporation of chestnut emerges as an effective strategy to mitigate long-term color degradation commonly associated with condensed-tannin-based vegetable tanning systems.
The Kubelka–Munk color strength (K/S) analysis (Table and Figure ) provided complementary information regarding the optical density and intensity of coloration in the different tannin systems. The control sample exhibited a K/S value of 0.89, representing a moderate absorption level consistent with typical industrially accepted shades.
12. K/S Values of Vegetable-Tanned Leathers before and after Accelerated Aging.
| samples | K/S (before aging) | K/S (24 h aging) | K/S (48 h aging) |
|---|---|---|---|
| Control group | 0.89 ± 0.01c | 0.76 ± 0.03c | 0.67 ± 0.02c |
| T1 | 0.37 ± 0.01f | 0.41 ± 0.01e | 0.35 ± 0.01f |
| T2 | 1.22 ± 0.03a | 1.04 ± 0.03a | 0.98 ± 0.03a |
| C1 | 0.76 ± 0.01d | 0.65 ± 0.01d | 0.61 ± 0.02d |
| C2 | 0.93 ± 0.02b | 0.79 ± 0.02b | 0.74 ± 0.02b |
| C3 | 1.18 ± 0.03a | 1.04 ± 0.02a | 0.97 ± 0.03a |
| C4 | 0.62 ± 0.01e | 0.53 ± 0.01e | 0.47 ± 0.01e |
| U1 | 0.41 ± 0.01f | 0.37 ± 0.01f | 0.35 ± 0.01f |
| U2 | 0.40 ± 0.01f | 0.36 ± 0.01f | 0.33 ± 0.01f |
| U3 | 0.75 ± 0.02d | 0.68 ± 0.02d | 0.64 ± 0.02d |
14.
Color strength (K/S) response of vegetable-tanned leathers to aging.
Quebracho-tanned leather (T2) yielded the highest K/S value (1.22), confirming its strong ability to generate dark, saturated hues. This result aligns with its elevated a* and b* coordinates, indicating that quebracho imparts rich reddish-brown tones with high chromatic intensity. However, this pronounced color strength, while aesthetically valued in certain applications (e.g., classic brown leathers), conflicts with the demand for lighter and more uniform pastel shades in modern markets.
Conversely, mimosa tanned leather (T1) showed the lowest K/S value (0.37), demonstrating its capacity to produce pale, and pastel-like shades. This reduced color strength correlates with the high lightness (L*) values observed, confirming that mimosa tannins scatter more light relative to absorption. Industrially, this property makes mimosa an attractive option for producing consistent, light-toned leathers, though it requires stabilization against aging-induced shifts.
Binary mixtures produced intermediate K/S values that varied with tannin ratios. For example, C3 (10% mimosa +30% quebracho) reached 1.18, closely approximating quebracho-dominated coloration, while C4 (30% mimosa +10% quebracho) exhibited a much lower K/S of 0.62, reflecting the dominance of mimosa’s lightening effect. This confirms that the optical density of the leather can be precisely tuned by manipulating tannin proportions, offering a flexible approach to meeting diverse aesthetic requirements.
Ternary mixtures with chestnut tannin (U1–U3) showed K/S values in the range of 0.40–0.75, indicating a dampening effect on color intensity. This suggests that chestnut tannins act as modulators, reducing the visual depth of color and promoting more subdued, less saturated shades. Importantly, chestnut also contributed to stabilizing K/S values under aging. While most samples displayed decreasing K/S values after 24 and 48 h of accelerated aging (due to increased L* and reduced absorption), the chestnut-containing systems showed smaller declines, reflecting their resistance to oxidative fading. These observations are statistically supported by Duncan’s multiple range test (p < 0.05), where the T2 and C3 samples remained in the highest significance group (Group “a”) throughout the aging process, confirming their dominant optical density. Conversely, the U-series and T1 consistently occupied the lowest group (Group “f”), statistically validating their role in achieving stable, low-intensity coloration even under prolonged environmental stress.
Notably, T1 exhibited an unusual behavior during aging: its K/S increased slightly after 24 h (0.37 → 0.41) before declining to 0.35 after 48 h. This transient rise likely reflects initial darkening due to mild oxidation of mimosa chromophores, followed by a brightening effect as oxidative degradation progressed. In contrast, T2 and C3, with higher starting K/S values, experienced more pronounced reductions (≈20–25% over 48 h), underscoring the susceptibility of quebracho-rich systems to loss of chromatic depth during service life.
Overall, the K/S data confirm that quebracho enhances color intensity but compromises long-term stability, whereas mimosa and chestnut lower initial color strength but contribute to achieving lighter, more stable shades.
3.4. SEM Images of Vegetable Tanned Leathers
To provide a deeper understanding of the relationship between tannin composition, collagen structure and the resulting leather properties, the morphological characteristics of the experimental leathers were investigated using scanning electron microscopy (SEM). Representative SEM micrographs of the pickled hide, control group leather, binary tannin formulations (C1–C4), single tannin formulations (T1 and T2) and ternary tannin formulations containing chestnut tannin (U1–U3) are presented in Figures –. For each formulation, SEM images were acquired at magnifications of 1000× and 1500× to evaluate collagen fiber organization, fiber packing density, interfibrillar spaces and structural integrity of the leather matrix.
15.
SEM images of pickle hide.
25.
SEM images of U3.
The SEM micrographs of the pickled hide (Figure ) exhibited a markedly different morphology compared with the vegetable-tanned leathers. The collagen fibers appeared more adhered, poorly separated and less individually distinguishable, indicating that the fibrous network had not yet been effectively stabilized by tanning agents. Unlike the tanned samples, where collagen fiber bundles became more clearly isolated and organized, the pickled matrix showed a compacted and aggregated appearance with limited fiber definition. This morphology proves that the pickling stage does not provide any structural stabilization of the collagen network.
Figure presents the SEM micrographs of the control leather produced using the conventional vegetable tanning formulation. The images reveal a well-preserved collagen structure characterized by clearly distinguishable fiber bundles and an interconnected fibrous network. Following tanning, the collagen fibers became more separated and organized compared with the pickled hide, indicating successful stabilization of the collagen matrix by vegetable tannins. Nevertheless, the fiber distribution was not completely uniform throughout the structure and moderate interfibrillar spaces were still evident in several regions. The observed morphology suggests that the tanning process effectively prevented excessive fiber aggregation and promoted the development of a coherent collagen network. The preservation of fiber bundle integrity and the formation of a continuous fibrous architecture are consistent with the satisfactory mechanical performance and hydrothermal stability obtained for the control leather. Therefore, the control sample represents a typical vegetable-tanned collagen structure and provides a suitable reference for evaluating the morphological effects of the alternative tannin combinations investigated in this study.
16.
SEM images of control group.
The SEM images of the T1 leather (Figure ) revealed a collagen structure composed of well-preserved fiber bundles separated by relatively large interfibrillar regions. In contrast to the control leather, where the collagen network appeared more consolidated, the 40% mimosa-tanned sample exhibited a looser and more open fibrous architecture. The fiber bundles were less densely packed and showed a greater degree of separation throughout the matrix. This structural appearance indicates that mimosa tannin interacted effectively with the collagen network but did not promote extensive fiber consolidation. As a result, the leather retained a more relaxed fibrous arrangement, which is likely responsible for its softer handle and lower hardness values. The presence of larger interfibrillar spaces may also reduce stress transfer between neighboring fiber bundles, thereby contributing to the lower tensile and tear strength values observed for T1. These findings suggest that the predominance of mimosa tannin favors flexibility and softness at the expense of structural compactness.
17.
SEM images of T1.
The SEM micrographs of T2 (Figure ) exhibited a more compact and densely packed collagen fiber structure compared with the T1 sample. The collagen fiber bundles appeared more closely associated with reduced interfibrillar spaces and a more consolidated fibrous network. This compact morphology can be attributed to the strong interaction capacity of quebracho tannins with collagen, which may promote more intensive fiber–fiber bridging within the leather matrix. The denser structure observed in T2 is consistent with its higher tear resistance and improved grain strength compared with T1.
18.
SEM images of T2.
Figure presents the SEM micrographs of the C1 produced with an equal proportion of mimosa and quebracho tannins. Unlike the more open fibrous arrangement observed in T1, the incorporation of quebracho resulted in a noticeably denser collagen structure, with fiber bundles appearing more closely associated throughout the matrix. Nevertheless, the distribution of the fibrous network was not entirely uniform and regions of compact fiber aggregation coexisted with localized interfibrillar voids. The coexistence of densely packed and relatively open regions suggests that the balanced binary tannin formulation promoted collagen stabilization but did not generate a fully homogeneous fiber organization. Such a morphology may increase resistance to localized deformation, which is reflected in the relatively high hardness and ball burst performance of C1. However, the irregular distribution of stress-bearing fiber bundles might limit the efficient transfer of mechanical loads across the collagen matrix, thereby restricting improvements in tensile and tear strength.
19.
SEM images of C1.
SEM micrographs of the C2 (Figure ) revealed a relatively compact and more homogeneous collagen fiber network compared with C1. The collagen fiber bundles appeared closely packed with reduced interfibrillar voids and improved structural continuity. This morphology suggests that the higher quebracho proportion in the binary formulation promoted stronger tannin–collagen interactions and enhanced fiber consolidation. The compact fibrous arrangement is consistent with the high hardness and low softness values observed for C2, while its more uniform structure may explain its improved balance between mechanical performance and color stability.
20.
SEM images of C2.
Figure illustrates the SEM micrographs of the C3 containing the highest quebracho proportion among the binary formulations. The collagen structure appeared more compact than those observed in C1 and C2, with fiber bundles arranged in a closely packed manner and only limited interfibrillar gaps throughout the matrix. The increased fiber density indicates that the higher quebracho content promoted a stronger consolidation of the collagen network. Unlike the heterogeneous regions observed in C1, C3 sample exhibited a more continuous fibrous architecture, suggesting a more effective stabilization of the collagen matrix. Such an arrangement is advantageous for resisting crack propagation and distributing applied stresses through the fiber network. This observation is in good agreement with the relatively high tear strength and favorable ball burst properties obtained for C3. The SEM images therefore demonstrate that increasing the quebracho proportion enhanced collagen fiber cohesion and contributed to the development of a mechanically stronger leather structure.
21.
SEM images of C3.
SEM micrographs of the C4 (Figure ) showed a relatively open and less compact collagen fiber network compared with the quebracho-rich C2 and C3 formulations. Although the collagen fiber bundles remained structurally preserved, the interfibrillar spaces were more pronounced and the fiber packing density appeared lower. This morphology indicates that the higher mimosa proportion increased fiber mobility and reduced structural tightening within the collagen matrix. The more open fibrous arrangement is consistent with the higher elongation and softness tendency observed for C4, while also explaining its lower tensile strength and reduced tear resistance. Thus, the SEM findings confirm that mimosa-rich binary formulations enhance flexibility but provide weaker morphological reinforcement compared with quebracho-rich or chestnut-containing systems.
22.
SEM images of C4.
Figure presents the SEM micrographs of the U1 containing mimosa, quebracho and chestnut tannins. The incorporation of chestnut tannin resulted in a more organized collagen structure compared with the binary formulations. The fiber bundles remained clearly distinguishable and formed a continuous fibrous network with relatively few void regions. In addition, the collagen matrix appeared more uniform, indicating an improved distribution of tanning agents throughout the fibrous structure. The enhanced structural regularity observed in U1 suggests that the ternary tannin combination promoted a more balanced stabilization of the collagen network. Although the fibers remained sufficiently separated to preserve flexibility, the overall arrangement was noticeably more compact than that observed in the mimosa-rich formulations. This combination of fiber cohesion and structural uniformity may explain the favorable mechanical properties and increased shrinkage temperature obtained for U1, demonstrating the beneficial contribution of chestnut tannin to collagen stabilization. The beneficial synergistic effect of the ternary tannin combination on leather stabilization.
23.
SEM images of U1.
Figure shows the collagen morphology of the U2, which exhibited the highest degree of structural organization among all investigated formulations. The fibrous network was characterized by a uniform distribution of collagen bundles and a remarkably compact arrangement with very limited interfibrillar spaces. Compared with the binary systems and the other ternary formulations, the collagen matrix appeared more continuous and cohesive, indicating a highly effective interaction between the tanning agents and the collagen fibers. The balanced proportion of mimosa, quebracho and chestnut tannins appears to have promoted an optimal level of fiber stabilization, resulting in a structure that combined compactness with morphological uniformity. The absence of large void regions and the close association of neighboring fiber bundles suggest an efficient transfer of mechanical stresses throughout the collagen network. This highly integrated fibrous architecture provides a plausible explanation for the superior tensile strength, enhanced tear resistance and elevated shrinkage temperature obtained for U2.
24.
SEM images of U2.
Figure presents the SEM micrographs of the U3 containing the highest mimosa proportion among the ternary formulations. The collagen matrix retained a well-defined fibrous architecture with clearly distinguishable fiber bundles distributed throughout the structure. In contrast to the highly compact arrangement observed in U2, the U3 sample exhibited a slightly more relaxed network characterized by localized interfibrillar spaces and a lower degree of fiber packing. The increased mimosa content appears to have reduced the extent of fiber consolidation, allowing greater mobility within the collagen network while preserving overall structural integrity. Despite the more open morphology, no evidence of severe fiber separation or structural deterioration was observed, indicating that the combined action of quebracho and chestnut tannins still provided effective stabilization of the collagen matrix. This intermediate structural organization may explain the combination of favorable tensile properties, high elongation values, and elevated shrinkage temperature obtained for U3.
The SEM observations obtained in this study further confirmed the effectiveness of vegetable tannins in modifying and stabilizing the collagen fiber network. All tannin-treated leathers exhibited well-preserved collagen structures with clearly distinguishable fiber bundles, indicating successful penetration and fixation of tannins within the collagen matrix. Variations in fiber packing density, interfibrillar spaces and structural homogeneity were observed among the formulations, demonstrating the influence of tannin composition on collagen organization. Similar findings have been reported for several plant-derived tanning systems. Hashem et al. observed compact and uniformly distributed collagen fiber bundles in leathers tanned with Xylocarpus granatum seed tannins, attributing the improved structural integrity and thermal stability to strong polyphenol–collagen interactions. Likewise, Mustafa et al. reported that tannins extracted from Pontederia crassipes produced a dense collagen network associated with enhanced tensile strength and hydrothermal stability. Comparable results were also obtained by Hashem et al., who demonstrated that Abrus precatorius seed tannins generated a homogeneous and compact fibrous structure, reflecting effective tannin penetration and improved collagen stabilization. In contrast, Chen et al. reported that leathers tanned predominantly with hydrolyzable tannins tended to exhibit increased fiber loosening, disorganization and larger interfibrillar voids under fluctuating environmental conditions, whereas condensed tannin systems maintained a more compact and coherent fiber architecture. These studies collectively indicate that the degree of collagen stabilization is closely associated with fiber packing density and the strength of tannin–collagen interactions.
The present results are in good agreement with these observations. Formulations containing higher proportions of quebracho tannin generally exhibited more compact and consolidated collagen networks, whereas mimosa-rich systems displayed relatively more open fibrous structures. Among the investigated formulations, the balanced ternary systems, particularly U2, produced the most homogeneous collagen morphology characterized by closely packed fiber bundles and minimal interfibrillar voids. Such structural features are consistent with the better tensile strength, tear resistance and shrinkage temperature values obtained for these formulations, highlighting the importance of optimized tannin combinations in achieving both effective collagen stabilization and enhanced leather performance.
4. Conclusion
This study demonstrates that tannin composition is a decisive factor in achieving a balanced combination of color stability and mechanical performance in vegetable-tanned leathers. FTIR analyses confirmed the successful interaction and fixation of vegetable tannins within the collagen matrix, while preserving the characteristic amide structure of collagen. The spectra further revealed formulation-dependent differences in the fingerprint region, indicating the contribution of condensed and hydrolyzable tannins to collagen–tannin interactions and network formation. SEM observations provided complementary morphological evidence, demonstrating that tannin composition significantly influenced collagen fiber organization, fiber packing density and matrix homogeneity. Among the investigated formulations, the balanced ternary systems produced more compact and uniformly distributed collagen fiber networks, which correlated well with the observed improvements in mechanical and thermal properties.
Single tannins exhibited inherent limitations; mimosa produced light and soft leathers with reduced strength, while quebracho yielded mechanically stronger but darker and less color-stable materials. Binary mimosa–quebracho systems improved this balance, with specific ratios providing color properties close to the industrial control while maintaining acceptable physical performance. The incorporation of chestnut tannin in ternary systems further enhanced thermal stability, tensile strength and resistance to color change during aging, indicating the beneficial role of hydrolyzable tannins in collagen cross-linking and oxidative stability.
Accelerated aging results demonstrated that optimized ternary systems were more resistant to UV-induced chromatic degradation compared to conventional vegetable tanning formulations, while simultaneously preserving desirable mechanical characteristics. Collectively, optimized mixed tannin formulations particularly chestnut-reinforced systems successfully mitigated the traditional trade-off between aesthetics and durability. These findings confirm that tailored vegetable tannin blends represent viable, sustainable alternatives to chromium tanning, offering practical guidance for producing light-colored, mechanically robust leathers suitable for industrial applications.
Acknowledgments
The authors gratefully acknowledge Sepiciler Çaybaşı Leather Industry and Trade Inc. for supplying the materials and chemicals used during the trial productions conducted in the factory. Sincere thanks are also extended to Sepici Foreign and Domestic Trade Inc. for their valuable technical assistance.
Glossary
List of Abbreviations
- ΔE
total color difference calculated from CIELAB coordinates
- L*
lightness coordinate in the CIELAB color space
- a*
red–green coordinate in the CIELAB color space
- b*
yellow–blue coordinate in the CIELAB color space
- K/S
color strength value calculated using the Kubelka–Munk equation
- T s
shrinkage temperature of leather (°C)
- T1
leather tanned with mimosa tannin
- T2
leather tanned with quebracho tannin
- C1–C4
binary tannin formulations (mimosa–quebracho combinations at different ratios)
- U1–U3
ternary tannin formulations containing mimosa, quebracho, and chestnut tannins
- RH
relative humidity
- °C
degree celsius
- h
hour
- wt %
weight percentage
- ISO
International Organization for Standardization
- UV
ultraviolet
- BAT
best available techniques
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
The authors declare no competing financial interest.
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