A novel approach for preparing aldehyde-free melamine resin and investigation of its retanning performance

Melamine resin (MR), traditionally synthesized using melamine and formaldehyde, is widely used in the leather industry. However, the emission of free formaldehyde poses a significant challenge for conventional MR. To address the issues of aldehyde in MR, extensive research has been conducted. This paper introduces a novel aldehyde‑ free MR (LTSL) retanning agent synthesized using cyanuric chloride, l‑lysine, and sodium sulfanilate. The chemical structure of LTSL was analyzed via Fourier transform infrared spectroscopy, nuclear magnetic resonance, and X‑ray photoelectron spectroscopy. The presence of amino, carboxyl, and sulfonic acid groups in LTSL enhanced its stor‑ ability and imparted LTSL with an amphoteric character. The isoelectric point of LTSL was optimized to reach 4.37, and LTSL exhibited an appropriate size distribution with an average particle size of 254.17 nm and achieved high absorption rates of 87.77% and 95.84% for retanning and fatliquoring agents, respectively. Consequently, the thick‑ ness rate of LTSL reached up to 37%, with no detectable formaldehyde. Notably, LTSL also demonstrated excel‑ lent physical and mechanical properties, primarily attributed to the coordination and electrostatic interactions between the chrome‑tanned collagen fiber and amino/carboxyl groups in LTSL. This research presents an innovative approach for developing an aldehyde‑free MR retanning agent, significantly contributing to the sustainable develop‑ ment of leather manufacturing.


Introduction
Melamine resin (MR) has long been a crucial retanning agent in leather manufacturing, providing a desirable filling effect and fullness feeling [1].However, traditional synthesis of MR involves the condensation of melamine and formaldehyde-a process that leads to continuous release of free formaldehyde.This problem stems from two primary factors.First, the quantity of formaldehyde used in MR synthesis is substantially higher than required, resulting in residual free formaldehyde.Second, the hydroxymethyl amino compounds generated from the reaction between formaldehyde and melamine are unstable and prone to decompose, leading to the sustained release of formaldehyde [2,3].Given that formaldehyde is recognized as an environmental contaminant and a human toxicant [4,5], it is imperative to either prevent or minimize the presence of formaldehyde or other harmful aldehydes in MR.
Significant efforts to reduce free formaldehyde or other free aldehydes in MR and leather have been made.One method involves adjusting the formaldehyde-melamine ratio, which-while being cost effective-tends to reduce MR synthesis efficiency [6].Further, formaldehyde scavengers have been employed to decrease formaldehyde content in MR and leather; additives containing hydroxyl and/or amino groups have been proven effective in this regard [7].Alternative dialdehydes, such as glyoxal, glutaraldehyde, and furfural, have also been explored as formaldehyde substitutes.For instance, Ashraf et al. synthesized formaldehydefree MR using melamine, glyoxal, and metanilic acid [8].Similarly, crop-oriented saccharide-derived aldehydes from sucrose were used to prepare more environmentally friendly MR [9].However, these methods have not fundamentally resolved the issue of formaldehyde usage.
On the other hand, the development of the leather industry has driven a growing demand for waterborne MR.Yet, the highly reactive hydroxymethyl groups within MR tends to condensate during storage, thereby resulting in turbidity, represents a significant impediment to the development of waterborne MR.In a recent endeavor, Ning et al. engineered a novel waterborne MR by integrating glutaraldehyde, melamine, and collagen hydrolysates, demonstrating superior storability and retanning efficacy [10].Similarly, Sun et al. enhanced MR's hydrophilicity and mitigated the hydrolysis of methylene ether bonds by incorporating epichlorohydrin and lysine via a Mannich-type reaction [11].These investigations underscore the potential of augmenting MR hydrophilicity to improve its storability.Nonetheless, the burgeoning leather industry's requirements extend beyond mere hydrophilicity, aspiring to endow MR with additional functionalities, such as amphoteric properties, to elevate its performance.
Cyanuric chloride (2,4,6-trichloro-[1,3,5]-triazine, TCT), known for its relative low toxicity, high reaction, cost-effectiveness, and wide application in leather industry [12,13], presents a potential alternative.The structure of TCT, featuring a triazine ring and three reactive chlorine groups, allows for selective reactions with various nucleophiles such as amine and hydroxyl groups at specific temperatures [14,15].This unique property renders TCT an excellent candidate to replace formaldehyde in MR preparation.
Besides, other utilization of biomass also brought inspiration.Ilaria et al. synthesis a side-chain poly [2]pseudorotaxane by threading β-CD (cyclic oligosaccharides composed of seven α-1,4-linked glucose units) macrocycles onto the sulfonated 4,4'-dihydroxydiphenyl sulphone pendants attached on the main chain of MIDA DD (one of the most widely used tanning polymer, which may release bisphenol S in leathermaking), which could substantially reducing the residual bisphenol S in leather [16].In addition, Yu et al. utilized oxidized starch to design a type of environment-friendly polymer ligand tanning agent, which remarkably upgraded the tanning performance [17].These reports revealed the significant role of biomass in cleaner leather manufacture.
L-lysine (Lys), which is readily derived from fermented feedstock [18], is a proteinogenic amino acid [19] with significant structural, catalytic, and functional importance and wide application in chemical synthesis [20,21].With its two active amino groups and a carboxyl group, Lys readily reacts and polycondenses with TCT.It is expected to confer amphoteric and hydrophilic properties to MR products.Being a natural product, Lys is generally regarded as environmentally friendly and biocompatible material [22].
In this research, TCT was first reacted with sodium sulfanilate (SE) to enhance its hydrophilicity, followed by condensation with Lys to synthesize a new type of MR (LTSL).The chemical structure of LTSL was characterized via Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS).In addition, the Zeta potential, average particle size, and storability of LTSL were examined.Chrome-tanned leather was then employed to assess the retanning performance of LTSL.For comparison, the retanning performances of a formaldehyde-containing commercial MR (FMR) and formaldehyde-free commercial MR (CMR) were also evaluated.

Materials
Sodium sulfanilate (SE) was sourced from Guangdong Weng Jiang Chemical Reagent Co., Ltd.TCT was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., and L-lysine (S configuration, Lys) from Shandong Usolf Chemical Technology Co., Ltd.Wet-blue leather was prepared in our laboratory using a standard leather-making process.The commercial products, FMR (containing formaldehyde) and CMR (formaldehyde free), were also used in this study.

Synthesis of LTSL
SE (19.5 g, 0.1 mol), TCT (18.44 g, 0.1 mol), NaOH (4 g, 0.1 mol) and pure water (80 mL) were added to a threenecked round-bottom flask equipped with a stirrer.This mixture was stirred at 5 °C for 12 h.After the first step reaction, Lys (29.2 g, 0.2 mol) and pure water (70 mL) was added to the flask.This solution was stirred at 30 °C for 1 h.After the second step reaction, the temperature was raised to 60 °C for 2 h.During the whole reaction, the generated HCl dissolved in water and was neutralized by of NaOH.The product of the three-step reaction was named as TS, TSL and LTSL, respectively.Figure 1 illustrates the synthesis process of LTSL.

Fourier transform infrared (FTIR) spectroscopy analysis
The analysis involved preparing a tablet by combining freeze-dried samples of TCT, TS, TSL, and LTSL with potassium bromide in specific ratios.A Fourier transform infrared spectrometer (Nicolet IS10, Thermo Scientific, USA) was used to analyze the molecular structures of these samples, scanning across a range of 4000-400 cm −1 .

NMR spectroscopy analysis
For NMR analysis, freeze-dried TCT samples were dissolved in deuterated acetone and the freeze-dried TS, TSL, and LTSL samples were dissolved in D 2 O.The 13 C NMR spectra of these solutions were obtained using a Bruker Advance II 400 MHz NMR spectrometer (Bruker, Swiss) at a temperature of 25 °C.

XPS spectroscopy analysis and optical rotation
The structural characterization of TS, TSL, and LTSL was performed via XPS.A monochromator Al Kα X-ray source (1486.6 eV) with an X-ray photoelectron spectrometer (Thermo Fisher Nexsa, USA) was utilized for this purpose.

Optical rotation
The optical rotation of LTSL was measured by polarimeter (A.Krüss P8000-T, Germany).

Elemental analysis (EA)
The elemental analysis of C, H, O, N, S in Lys and LTSL was determined by elemental analyzer (elementar vario EL, Germany).

Storability test
To assess storability, LTSL was subjected to pH adjustments of 5, 6, and 7, and then stored at 80 °C to simulate extended storage conditions.The samples were observed daily to evaluate their stability during this period.

Particle size and isoelectric point (pI) determination
The average particle size of 5 g/L solutions of LTSL, CMR, and FMR along with LTSL in practical concentration, LTSL coexisting with NaCl, FMR in practical concentration, FMR coexisting with NaCl was measured using a NanoBrook Omni instrument (Brookhaven, USA) [23].The pI of these resins was determined by analyzing the Zeta potential of their solutions across a pH range of 3-9.

Retanning performances investigation of LTSL
The retanning efficacy of LTSL, FMR, and CMR was evaluated using wet-blue leather, following the procedure summarized in Additional file 1: Table S1.Wet-blue leather without retanning treatment served as a control.The initial crust leather was shaved to a thickness of 1.2 mm for these tests.

Absorption rate of retanning and fatliquoring agents
The stepwise absorption rates of retanning agents were quantified by analyzing the total bound nitrogen (TNb) in the bath at various times, in which 4 points was chosen and the stepwise absorption rate of retanning agent was calculated using the following equation: where TNb 0 and TNb 1 represent the TNb values of the bath solution at adjacent sample points of the operation, respectively.
The absorption rates of fatliquoring agents were quantified by analyzing the total organic carbon (TOC) in the bath solution at the beginning and end of the process.
(1) Absorption rate The absorption rate was calculated using the following equation: where TOC 0 and TOC 1 represent the TOC values of the bath solution at the beginning and end of the operation, respectively.

Thickness rates of leather
The initial and final thicknesses of the leather were measured using a dial constant weight thickness gauge (MingYu, China).The thickness rate was determined as follows: where b 0 and b 1 denote the initial and final thicknesses of the samples, respectively.

Time of static water absorption
The time of static water absorption of leather samples was measured by a contact angle goniometer (DSA30, Krüss, Germany).

Scanning electron microscopy (SEM)
SEM images of the retanned leathers were captured using an Apreo S HiVoc scanning electron microscope (2) Absorption rate (Thermo Scientific, USA) to examine the dispersion of the fibers.

Differential scanning calorimetry (DSC)
The DSC of different layers (splitting retanned leather into three layers averagely) was determined by differential scanning calorimeter (NETZSCH DSC214, Germany).

Physical and mechanical strength measurement of leather
The leather samples were conditioned at 20 °C and a relative humidity of 65% for 48 h according to IUP 3 standard methods [24].The softness of the samples was then evaluated using a standardized leather softness tester, in accordance with IUP 36 [25].Tensile strength, elongation, and tearing strength were measured using a tensile tester as per IUP 6 & 8 standards [26][27][28].The compression-resilience performance was assessed following a method described in the literature [29].

FTIR analysis results
The FTIR spectra of TCT, TS, TSL, and LTSL are shown in Fig. 2a.In the TCT spectrum, the peaks at 1496 and 789 cm −1 are attributed to the stretching vibration and in-plane bending vibration of the triazine ring, respectively [30].In addition, a prominent peak at 850 cm −1 , corresponding to the C-Cl bond, signifies the main reaction.In the spectra of TS, TSL, and LTSL, the strong and broad peak at 3415 cm −1 , representing the stretching vibrations of -NH 2 and -NH-, indicates the presence of amino groups [31].Specifically, in the TS spectrum, peaks at 1128 and 1198 cm −1 are associated with the sulfonic acid group, confirming the conjugation between SE and TCT [32].In the TSL spectrum, the peak at 1599 cm −1 corresponds to the asymmetric stretching of COO-and asymmetric bending of the α-amino group (NH 3

+
), while the peak at 1406 cm −1 corresponds to the symmetric stretching of the carboxylic group [33].This confirms the introduction of Lys in the product.The gradual weakening and eventual disappearance of the C-Cl peak in the TS, TSL, and LTSL spectra suggest the progressive consumption of C-Cl and polymerization of LTSL [34]. Figure 2b shows the main reactions of key involved groups.

13 C NMR results
The molecular structures of the resins were further analyzed via 13 C NMR, as shown in Fig. 3.The TCT spectrum (Fig. 3a) shows a single peak at 171.95 ppm, which is attributable to the carbon atoms in the triazine ring [35].In the TS spectrum (Fig. 3b), the peak at 169.52 ppm (peak 1) signifies the reaction between a triazine carbon and SE.In addition, peaks at 139.53, 138.53, 126.67, and 122.17 ppm (peaks 3-6) correspond to carbon atoms in aromatic rings, suggesting a reaction between TCT and SE [36].The TSL spectrum (Fig. 3c

XPS analysis
The XPS spectra are depicted in Fig. 4a.The absorption peaks at approximately 531, 399, and 285 eV correspond to the O 1 s, N 1 s, and C 1 s orbitals, respectively.As shown in Fig. 4b, the N 1 s spectra of TS display two distinct peaks at 398.9 and 400.3 eV, associated with the -NH-and triazine nitrogen functionalities [39], aligning with the FTIR analysis (Fig. 2a). Figure 4c shows the N 1 s spectra of TSL, revealing three distinct peaks at 400.2, 399.3, and 398.3 eV, which correspond to -NH + /-NH 3 + , -NH-/-NH 2 , and triazine nitrogen, respectively [40,41].The presence of the -NH + /-NH 3 + peak indicates the occurrence of a reaction between TS and Lys.As shown in Fig. 4d, the N 1 s spectrum of LTSL exhibits peaks at 400.9, 399.2, and 398.1 eV, representing NH + /NH 3 + , -NH-/-NH 2 , and triazine nitrogen, respectively.A comparison of Fig. 4c, d shows a decrease in the -NH + /-NH 3 + area ratio in Fig. 4d, suggesting the condensation of TSL.Overall, the FTIR, 13 C NMR, and XPS analyses confirm the successful synthesis of LTSL.

Optical rotation results
The optical rotation was determined as 0.062, a positive number, which indicated the rotation of LTSL was dextrorotatory.

EA results
Elemental analysis was further conducted to verify the reaction between Lys, TCT and SE.As shown in Additional file 1: Table S2, compared to Lys, the contents of carbon (C), hydrogen (H), and oxygen (O) in the LTSL were reduced, whereas the nitrogen (N) content exhibited an increase.This augmentation in nitrogen contents can be ascribed to the substantial nitrogen presence in cyanuric chloride, thereby corroborating its contribution to the chemical reaction.Furthermore, an elevation in the sulfur content within the LTSL was observed, substantiating the participation of SE in the substitution reaction.

Storability
The storability of a retanning agent is a key indicator of its performance in MR applications.Traditional MR often exhibits turbidity due to the condensation of hydroxymethyl groups during storage, limiting its applicability.The storability of LTSL and FMR is shown in Fig. 5. Initially, LTSL and FMR maintain clear state; however, LTSL demonstrates consistent clarity during long-term storage, whereas FMR, which is produced using traditional methods, becomes turbid.This turbidity in FMR can be attributed to the continuous condensation of excess hydroxymethyl groups.The superior storability of LTSL is attributed to the higher concentration of hydrophilic groups and absence of hydroxymethyl groups.

Particle size and pI
The charge on retanning agents is pivotal in the retanning process, particularly for chrome-tanned leather, which carries a positive charge.Therefore, anionic retanning agents are typically used to effectively bind with collagen fibers in the leather.However, these agents may disrupt charge distribution in the leather, potentially impeding the absorption of commonly used anionic fatliquoring agents.Therefore, an amphoteric retanning agent that can acquire a cationic character during the fatliquoring process with a suitable isoelectric point (pI) is desirable [42].The Zeta potentials of LTSL, FMR, and CMR over a pH range of 3-9 are shown in Fig. 6a.LTSL, with a pI of 4.37, is an amphoteric retanning agent, which enhances its absorption rate as well as that of the fatliquoring agent.In the retanning process, LTSL functions as an anionic agent to combine with cationic chrome-tanned leather, while in the fatliquoring process, it becomes cationic to aid the absorption of anionic fatliquoring agents.Conversely, CMR and FMR retain an anionic charge across the pH range of 3-9, potentially reducing the absorption of the fatliquoring agent.
In addition to the Zeta potential, the average particle size is critical.The average distance between collagen fibers is ~ 892.42 nm [43].Retanning agents with large particle sizes may fail to effectively penetrate the leather, depositing on the flesh side, while very small particles are easily washed away.As shown in Fig. 6b, the average particle size of LTSL is 254.17 nm, which falls between those of CMR (612.76 nm) and FMR (150.39 nm).The suitable particle size distribution of LTSL, primarily within 200-500 nm, suggests potential for superior performance, whereas CMR and FMR exhibit poor performance due to a higher proportion of particles exceeding sizes of 1000 nm.Additionally, the particle sizes of LTSL As for those coexisting with NaCl, it has almost no effect on that of LTSL, but enlarged that of FMR.This indicated the outstanding electrolyte resistance of LTSL.

Retanning performances of LTSL
Figure 7 shows the stepwise absorption of retanning agents, thickness rates, cmpressed and resilient thichness of retanned leather.Figure 7a shows that the absorption rate of LTSL exceeds that of CMR and FMR, which is attributable to its amphoteric charge and favorable particle size.In the retanning process, LTSL possesses a negative charge at a pH of approximately 5.5, while wet-blue leather possesses a positive charge, the electrostatic attraction between LTSL and wet-blue leather facilitating improved absorption of LTSL.Regarding fatliquoring-agent absorption (Additional file 1: Fig. S1a), the amphoteric character of LTSL, which turns cationic during the fatliquoring process (pH ~ 3.4), facilitates the absorption of conventional anionic fatliquoring agents.Consequently, as shown in Fig. 7b, leather retanned using LTSL achieves an excellent thickness rate compared to that achieved in the case of CMR and FMR, owing to the enhanced absorption rate of LTSL.In addition, the presence of more rigid aromatic rings in LTSL contributes to higher thickness rates.
Compression and resilience, which reflect leather fullness, were evaluated, as shown in Fig. 7c, d, respectively.Leather retanned using LTSL exhibited superior fullness compared to that retanned using CMR due to more effective dispersion and fixation effects on the collagen fiber network resulting from increased amino and carboxyl groups in LTSL.However, leather retanned using FMR, containing more aldehyde groups, showed higher compression thickness than that retanned using LTSL.

Time of static water absorption
Additionally, the static water absorption of leather samples was assessed as depicted in Additional file 1: Fig. S2.Obviously, leather retanned by LTSL gained the longest time in all samples for water to infiltrate completely.It looks like the hydrophilic groups in LTSL, such as -COOH and -SO 3 H, may increase the water absorption and exhibit poor water repellency of the leather, but these groups could participate the coordination with Cr 3+ in chrome-tanned leather after retanning, thereby diminishing its hydrophilicity.

SEM images
The dispersion of collagen fibers, a key indicator of leather quality, is significantly influenced during the fatliquoring process.As the fatliquoring agent penetrates and demulsifies, it forms a coating of oil on collagen fibers, enhancing the leather's softness and hydrophobicity and reducing friction between the fibers [44].The SEM images of leathers retanned using different retanning agents are shown in Fig. 8.As shown in Fig. 8b, the collagen fibers in leather retanned using LTSL are distinctly separated, indicating a more effective retanning and fatliquoring process.The higher absorption of the fatliquoring agent increased the sepadistance between the collagen fibers, facilitating relative sliding between the fibers.Conversely, Fig. 8c,  d show that the collagen fibers in leather retanned using CMR and FMR are weakly bonded, likely due to the weaker absorption of the fatliquoring agent.The improved dispersion observed in LTSL-treated leather is attributed to the significant absorption of the longchain molecules of LTSL, aiding the lubrication of the collagen fibers.

DSC results
The DSC of different samples can be seen in Additional file 1: Fig. S3.The peak temperatures for the middle and top layer of blank registered at 80.03 °C and 82.52 °C, respectively, while those for LTSL retanned were recorded at 88.04 °C and 89.53 °C, respectively, noticeably higher than those for the blank.This is due to the crosslinking between LTSL and chromed leather, suggesting that LTSL has effectively penetrated into leather.Compared to blank, the values of the peak temperatures for the top and middle layers of leather retanned with LTSL becomes closer, further corroborate LTSL's effective penetration.

Physical and mechanical strengths of leather retanned using LTSL
For commercial shoe and leather products, the minimum tensile strength and tearing strength of approximately 10 N/mm 2 and 30 N/mm, respectively, are often required [45,46].To assess commercial viability, the physical and mechanical strengths of leathers retanned using different agents were measured, as presented in Fig. 9a-c, which demonstrate that leather retanned using LTSL exhibits superior softness, tensile strength, and elongation compared to those retanned using CMR and FMR.The abundance of amino and carboxyl groups in LTSL facilitates both electrostatic and coordination interactions with chrome-tanned leather, enhancing the crosslinking within the collagen fiber network [47].Moreover, the increased absorption rate of the fatliquoring agent reduces friction among fibers, contributing to the leather flexibility.Regarding tearing strength, shown in Fig. 9d, leather retanned using LTSL did not show a clear advantage over CMR or FMR, but it still met the standards for commercial applications.The interaction schematic of leather retanned using LTSL is depicted in Fig. 9e.The carboxyl groups in LTSL can coordinate with Cr 3+ in wet-blue leather, while its amino and carboxyl groups establish electrostatic interactions with those in collagen fibers [48,49].Consequently, the multifunctional groups in LTSL enhance

Conclusions
This paper introduces a new method for synthesizing an aldehyde-free MR retanning agent, LTSL, characterized by its amphoteric and waterborne properties.TCT, a triazine compound with three active chlorine atoms, was identified as an effective alternative to aldehydes in MR preparation.The inclusion of Lys endowed LTSL with an amphoteric nature, evident from its pI of 4.37, which significantly improved the absorption of retanning and fatliquoring agents.The remarkable hydrophilicity of LTSL ensured its long-term storability, preventing precipitation or turbidity.The presence of abundant amino and carboxyl groups in LTSL considerably enhanced the crosslinking within the collagen fiber network, resulting in superior physical and mechanical properties of the retanned leather compared to those treated with CMR and FMR.

Fig. 1
Fig. 1 Schematic of the LTSL synthesis process ) shows a splitting pattern of triazine carbon signals, resulting in peaks at ~ 166.83, 165.38, and 163.88 ppm (peaks 2-4), indicating the combination of TS and Lys.The carbon signal of the carboxyl group at 179.84 ppm (peak 1) [37] is consistent with the FTIR results, while signals of other carbon atoms in Lys are observed at 55.32, 39.39, 32.41, 27.00, and 21.87 ppm (peaks 9-13) [38], indicating the introduction of Lys.In the LTSL spectrum, the bifurcation of triazine carbon signals into peaks at 165.40 and 163.98 ppm (peaks 2 and 3) is observed, suggesting the interaction between the remaining C-Cl in TCT and -NH 2 in Lys, thereby indicating successful polycondensation.

Fig. 2 a
Fig. 2 a FTIR spectra of TCT, TS, TSL, and LTSL.b Schematic of the main reactions of key involved groups

Fig. 5 a
Fig. 5 a Storability of LTSL at a pH of 5, b FMR at a pH of 5, c LTSL at a pH of 6, d FMR at a pH of 6, e LTSL at a pH of 7, and f FMR at a pH of 7

Fig. 6 a
Fig. 6 a Zeta potentials of LTSL, CMR, and FMR.b Average particle size of LTSL, CMR, and FMR.Distribution of particle sizes of c LTSL, d CMR, and e FMR

Fig. 7 a
Fig. 7 a Stepwise absorption rates of retanning agents.b Thickness rates, c compressed thickness, and d resilient thickness of leather retanned using different retanning agents

Fig. 8 Fig. 9 a
Fig. 8 SEM images of leather retanned using a blank, b LTSL, c CMR, and d FMR