Development of aldehyde and similar-to-aldehyde tanning agents

Abstract

Aldehyde and similar-to-aldehyde tanning agents were used in oil tanning and smoke tanning in ancient times. The aldehyde group reacts with the protein amino group in electrophilic form to obtain the preservation stability and practical function of fur and leather. The results showed that aldehyde-tanned leather can promote the hydrothermal stability of skin collagen and has better water and sweat resistance properties than chrome tanning. However, aldehyde tanning agents always lack sufficient coulombic attraction to penetrate well into skin collagen, and some aldehydes have biotoxicity and color, are not easy to dissolve and have high manufacturing costs. In this paper, the structure and application characteristics of some aldehydes are reviewed. According to prior theory and practice, the development of new aldehyde tanning agents and additives has good prospects and significance for providing substitutes for chrome tanning agents.

Keywords

Aldehyde leather tanning agent chrome-free hydrothermal stability

Animal skin is composed of natural collagen, which is braided into a three-dimensional network of multiple fiber bundles. This pure natural collagen lacks commercial value because it becomes highly hardened when dry due to the polar bonding of the fiber bundles, and it is susceptible to acid, alkali, and salt action and microbial erosion and degrades in the wet state because of its strong hydrophilicity and low biological toxicity.¹ To date, tanning is the main method of maximizing the use of natural animal skin resources. In leather manufacturing, collagen fiber bundles are separated and fixed by a tanning agent to obtain porous leather goods that are resistant to heat and moisture, chemicals and microbes,^2,3 and meet use requirements.

The history of tanning began more than 4000 years ago.⁴ Until 1850, chromium salts were used in Europe as preservatives, and they laid the foundation for chrome tanning. After German professor F Kinapp published a report on the excellent tanning power of chromium salts in 1858, K Schultz invented the industrial two-bath tanning method in 1884 and M Dennis invented the one-bath tanning method in 1893. German Chakavotr (1959) explored leather tanning using metallic salts from 49 elements of the periodic table and confirmed that chromium salts have the strongest tanning power. After more than a century of development, chromium salt has been recognized as one of the main tanning agents with the best comprehensive properties in leather formation, and approximately 90% of leather production is chrome-tanned. However, since the 1970s, environmental protection laws and regulations have been established in various countries, and chrome tanning has become a root pollution problem because its remediation is difficult. Thus, leather manufacturing must consider the exploration of chrome-free tanning. Vegetable tanning was used in Egypt in approximately 1450 BC, and tobacco tanning and mirabilite tanning technology was used in China in approximately 1029 BC. However, the processing techniques and quality requirements of leather in the 19th century before the advent of chrome tanning are no longer acceptable because of the physical and chemical functional requirements of modern leathers. Therefore, nonchrome tanning agents that were previously should be investigated before exploring new tanning agents and tanning methods.⁵ According to Gustavson,⁶ the humidity and thermal denaturation temperature of collagen can be increased by strengthening the conformational stability of three-strand helical fibers. The humidity and thermal shrinkage temperature (T_s) of the leather can be increased by increasing the activation entropy and enthalpy of collagen helical rotation by crosslinking between the tanning agent and collagen fibers. The strength of the crosslinking bond between the various tanning agents and collagen fibers may determine the T_s, which is called the tanning power of the tanning agent.⁷ However, the binding strength of the tanning agent in collagen is dependent not only on the type of chemical reaction but also on the number of reaction points and the density of crosslinking in collagen; moreover, the T_s of collagen of animal bone can reach 155 degrees.⁸ If the space and channels of the collagen three-dimensional network are kept unchanged, then the tanning effect is determined by the tanning agent's structure, size, and penetration locus in the collagen.

Aldehyde and similar-to-aldehyde tanning agents are among the most important chrome-free tanning agents. The oldest oil tanning and tobacco tanning may be representative of an aldehyde tanning agent for tanning leather.^9,10 The amino nucleophile group is the most important reactive group of aldehyde tanning agents for collagen. For example, collagen is tanned by formaldehyde to obtain aldehyde tanning leather. The aldehyde tanning mechanism is relatively simple, and the tanning effect is achieved through covalent crosslinking of the aldehyde group with amino or imino groups on collagen molecules.^11–13 Among the modern aldehyde tanning agents, only formaldehyde, glutaraldehyde, and oxazolidine have sufficient tanning power (T_s ≥ 85°C) for use in the tanning of leather and fur. Aldehyde-tanned leather is white, has good resistance to sweat and washing and good antiseptic effects, and can be preserved for a long time.^14–16 This aldehyde reactivity is also a sign of toxicity to active organisms. The presence of free aldehydes, especially free formaldehyde, can not only prevent the growth of microorganisms but also cause great harm to the human body.¹⁷ To use aldehydes that have both advantages and disadvantages as tanning agents, it is necessary to understand the tanning principle of aldehydes. In this paper, the aldehydes that have been used as tanning agents are reviewed.

Aldehyde and similar-to-aldehyde tanning agents

A typical method of aldehyde tanning occurs by electrophilic reaction between positive carbon and collagenic amino groups. The main functional groups representing aldehyde tanning agents are the aldehyde group, nitrogen hydroxymethyl, phosphonium hydroxymethyl, etc. These groups can combine with nucleophilic amino groups, imino groups, phenolic hydroxyl groups, imidazole groups, peptide bonds, etc., to stabilize raw skin collagen or improve the collagen T_s.

The reaction of aldehydes with electrophilic groups can be carried out under acidic or alkaline conditions, although the reaction with collagen is usually carried out under alkaline conditions because the electrophilic groups on collagen are usually enclosed by hydrogen bonds or hydrogen ions (H⁺). Therefore, to achieve a balance of penetration and binding, the aldehyde tanning agent penetrates collagen in an acidic environment first and then binding is accomplished by alkalization.

Formaldehyde tanning agent

In 1875, Basch discovered that formaldehyde can harden collagen, and in 1898, E Pullmann declared the first patent for formaldehyde leather tanning; thus, formaldehyde was the first confirmed aldehyde tanning agent, and these discoveries marked the start of formaldehyde tannage.

Formaldehyde can produce crosslinking in the form of single or polyethers, which is shown in Figure 1 for several formaldehyde tanning products. The ¹³C tracer method showed that the formaldehyde monomer rather than polyformaldehyde was involved in formaldehyde tanning; however, the distance between reactive groups in collagen microfibers was not sufficient for crosslinking by formaldehyde alone, at least in terms of the number of crosslinks. Therefore, it is easier to understand the role of the formation of ether bonds and polyformaldehyde.^18–20 As a tanning agent, formaldehyde has the highest tanning power among aldehydes thus far, and the T_s of the tanned leather can reach 87°C. Formaldehyde-tanned leather is white and has a good ability to withstand the action of water, sweat, and light and can be plasticized and softened with oil or grease; however, it lacks fullness. The isoelectric point of formaldehyde-tanned leather is below 4.0, and the absorption of anionic materials is low in the subsequent processing. Due to the reversible reaction to form N-hydroxymethyl (—NHCH₂OH—) before converting to a Schiff base and the decomposition of polyether, the content of free formaldehyde in the tanned leather is relatively high. Thus, formaldehyde tannage alone was eventually abandoned by the tanning industry.

Figure 1.

Crosslinking of formaldehyde and collagen.

Glutaraldehyde tanning agent

In 1908, German C Harries et al. synthesized glutaraldehyde, and 30 years later, American researchers found that glutaraldehyde had good tanning effects on animal skin collagen and could be used alone for tanning leather and fur. At present, glutaraldehyde is still considered a low-biotoxicity substance that is used in the crosslinking modification of biomedically engineered materials and for tissue repair.^21,22

Glutaraldehyde is a double reactive group molecule with an aldehyde group at each end that can participate in covalent crosslinking. Similar to formaldehyde, the crosslinking reaction produced by glutaraldehyde mainly takes place between the aldehyde group and the basic amino acid of collagen. Although both aldehyde groups are capable of interacting with proteins, steric hindrance reduces the crosslinking of glutaraldehyde relative to that of formaldehyde, and the tanned leather has a slightly lower shrinkage temperature (≤85°C). The glutaraldehyde-tanned and finished leather becomes soft and plumps easily after finishing, and its physical and chemical properties are superior to those of formaldehyde-tanned leather. The use of a glutaraldehyde tanning agent can enhance the effects of chromium salt tanning leather, and the torsion resistance and sweat resistance of finished leather are stronger than those of chromium-tanned leather.^23–27 The theoretical tanning process of glutaraldehyde is shown in Figure 2.

Figure 2.

Mechanism of glutaraldehyde tanning.

In fact, although the glutaraldehyde tanning agent exists in the form of aqueous solution, it easily forms polymers in water at room temperature. This form of polymerization can be partially degraded by hot water but cannot exist completely in linear form.²⁷ Therefore, when glutaraldehyde is used as a tanning agent, the tanning speed is slower than that of formaldehyde. The main structural forms and tanning of glutaraldehyde are shown in Figure 3.

Figure 3.

The tanning mechanism of glutaraldehyde aqueous solution.

This structure of glutaraldehyde produces conjugate discoloration with the removal of hydrogen in alkaline environments or under light treatment.²⁸ Due to the lack of coulombic affinity of glutaraldehyde to collagen, especially the increase in molecular volume after polymerization, it is difficult to achieve the desired penetration when directly used in tanning. Therefore, improving the anti-discoloration and penetration effects have become important issues for using glutaraldehyde for tanning,^29–32 and modified glutaraldehyde has become one of the most common tanning agents. Ganesan K-tanned leather with D-lysine glutaraldehyde has better penetration and absorption than traditional glutaraldehyde tanning leather, and the COD (chemical oxygen demand) in the waste liquid of tanning leather is much lower than that of glutaraldehyde tanning leather.³³ Figure 4 shows a schematic diagram of the modification of glutaraldehyde with formaldehyde.

Figure 4.

Preparation of modified glutaraldehyde.

Glyoxal tanning agent

Glyoxal is the simplest aliphatic binary aldehyde in structure. In addition to the common features of aliphatic aldehydes, glyoxal also has special chemical properties due to the presence of a juxtaposed aldehyde group at each end of the molecule, which can crosslink with collagen and cellulose.³⁴ As a kind of chemical raw material, glyoxal is widely used in pharmaceuticals, paper making, textiles, and other fields. Because the molecular weight and steric hindrance of glyoxal are smaller than those of glutaraldehyde, its ability to crosslink with collagen in tanning is better than that of formaldehyde but lower than that of glutaraldehyde.³⁵ Glyoxal-tanned leather is white in color and has good shrinkage resistance and tensile properties, and the leather shows enhanced mechanical strength. The comprehensive properties of sheepskin garment leathers produced by chrome tanning were greatly improved after glyoxal pretanning treatment.³⁶ Due to its biotoxicity to the environment and human body and its high price, glyoxal is rarely used in leather tanning.^37–39

Dialdehyde polysaccharide tanning agent

The most familiar natural macromolecule representatives from the condensation of sugar units are starch and cellulose. In 1937, Jackson and Hudson⁴⁰ reported the oxidation of starch and cellulose with periodic acid, which specifically oxidized two secondary carbon hydroxyl groups on the chain of starch or cellulose to aldehyde groups, while the primary hydroxyl group did not react, as shown in Figure 5.

Figure 5.

Preparation of bisaldehyde tanning agent.

Dialdehyde glycan can be used as an aldehyde tanning agent to improve the T_s of collagen. The double aldehyde groups present in dialdehyde starch and cellulose can crosslink with the amino and imino groups in collagen, as shown in Figure 6.

Figure 6.

Dialdehyde tanning agent with collagen.⁴¹

Dual-aldehyde starch and cellulose-tanned leathers are washable, white in color, compact on the grain surface, and have a T_s of 80°C; the tanning power of these polysaccharides improves with increases in the aldehyde group content. However, there are some difficult problems in the preparation of these polysaccharides. (1) The preparation of these dialdehyde groups occurs on the surface of the macromolecules or particles; thus, the volume increases to avoid conducive penetration. Moreover, lower absorption rates result in a large amount of retanning agent in the waste liquor after tanning. (2) Low molecular weight or high dialdehyde group content in polysaccharides can be soluble in water, which increases the difficulty of separating oxidants. (3) Dialdehyde polysaccharides with high aldehyde group content have high activity, resulting in serious intermolecular or intramolecular condensation in storage. Therefore, it is still difficult to obtain industrial applications of dialdehyde polysaccharide tanning agents. The TEMPO-series nitro-oxy oxidation system has good selectivity for the primary hydroxyl group (C₆) of polyglycans. However, TEMPO is expensive and few studies have been conducted on the tanning of polyglycans with C₆ aldehyde groups.⁴²

Furfural tanning agent

Furfural is also known as furfuraldehyde, which is a colorless or yellowish oily liquid that mainly originates from pentose reacting with dilute acid, hydrolysis, dehydration, and distillation. The chemical properties of furfural are similar to those of formaldehyde and benzaldehyde and it has the dual properties of aldehyde and unsaturated furan heterocycles. Furfural can react quickly with collagen and strongly stabilize collagen to increase its thermal denaturation temperature.⁴³ However, forming crosslinks with collagen in solution is difficult due to the action of a single functional group (shown in Figure 7); thus, the T_s of furfural-tanned leather is only 7–8°C. At the beginning of the 20th century, furfural leather tanning was patented in the USA (patent No. 2976111), and a preprocessing method in which the leather was soaked in acid was proposed. Furfural-tanned leather has good water resistance and a light brown to dark brown appearance. The physical and mechanical properties of the leather are comparable to those of chrome-tanned leather. However, because furfural is slightly soluble in water, it cannot easily penetrate deep into the raw skin in an aqueous solution, resulting in a poor leather feeling. Generally, furfural is not used alone for tanning leathers except as a surface treatment, which is called pretanning. Notably, under the action of strong acid, furfural can be polymerized to form a polymer, and further hydrophilic modification is required to obtain an ideal tanning agent.

Figure 7.

Furfuraldehyde from dehydrated pentose.

5-Hydroxymethyl furfural (5-HMF) is formed by the dehydration of glucose or fructose and contains hydroxyl methyl, aldehyde, double bonds, and ether structures in the furan ring, and these active groups (as shown in Figure 8) can be crosslinked with collagen under certain conditions, thus achieving a certain tanning effect.⁴⁴ 5-HMF is expensive, and few reports of its use as a tanning agent have been found in Chinese journals. 5-HMF has similar physical and chemical properties and tanning power to furfural, although the good solubility and easy dehydration condensation of 5-HMF make it more promising as an excellent leather tanning agent.⁴⁵

Figure 8.

Dehydration condensation of 5-hydroxymethyl furfural.

Hydroxymethyl/methoxy group tanning agents

N-methoxy group tanning agents

Oxazolidine is also known as oxazole cyclopentane, which has one or two functional groups and is a kind of N-methoxy tanning agent. In the early 20th century, oxazolidine began to be used in medicine. Due to its ability to crosslink phenols and proteins in raw skin collagen, oxazolidine was patented by S Das Gupta as a tanning agent in 1973,⁴⁶ and it has been extensively studied and applied in leather tanning as a chrome-free tanning agent.^47,48

Oxazolidine tanning agents used for tanning are mainly monocyclic oxazolidine and bicyclic oxazolidine, and their common structures are shown in Figure 9.

Figure 9.

Structure of oxazolidine.

Deb CS et al.⁴⁹ showed that oxazolidine could be reduced in the form of methylene to combine with the amino group of collagen and generate a Schiff base, such as CH₂=N—P, and then electrophilic addition is performed to obtain recombination or crosslinking with collagen, as shown in Figure 10.

Figure 10.

Oxazolidine tanning agent.⁴⁹

Notably, the structure of oxazolidine indicates that there is a balance between the synthesis and hydrolysis of molecules, and free formaldehyde exists both in the product and in the tanning process. The free formaldehyde in the leather tanned by oxazolidine increases with increasing oxazolidine content. Therefore, oxazolidine concentrations greater than 2% are not suitable for use. The T_s of single-ring oxazolidine tanning is higher than that of double-ring oxazolidine tanning and close to that of formaldehyde tanning, and it reaches 86–87°C. Due to the high pH value of oxazolidine solution, oxazolidine reacts quickly with collagen and has strong surface tanning characteristics. Oxazolidine-tanned leather can be finished to obtain a product that is supple and has good physical and mechanical properties, especially sweat resistance and water resistance. The tear strength of oxazolidine-tanned leather is better than that of glutaraldehyde-tanned leather. Because oxazolidine reacts strongly with catechuic tannins and becomes a gel, the combination of oxazolidine tanning with catechuic tannins is expected to achieve a breakthrough in high T_s chrome-free tanning.^50–52

N-hydroxymethyl resin tanning agent

Urea, urea rings, dicyandiamide, and melamine resin have been used to improve the thermal stability of collagen for several decades.^53–56 Condensation of urea, urea rings, dicyandiamide, and melamine with formaldehyde to form N-methyl hydroxy groups is shown in Figure 11. These resins can be crosslinked with skin collagen and have good tanning power.^57–59 However, because the end group N-methyl hydroxy has very good activity, once coupling occurs between the molecules, these resins precipitate quickly. Thus, urea, urea rings, dicyandiamide, and melamine resin cannot be stored for a long time.

Figure 11.

N-hydroxymethyl resin tanning agent.

Ester methoxy group tanning agents

Genipin is an extract from gardenia fruit cycloalkene ether terpenoids and represents an active ingredient of the extraction by β-glycosidase enzyme hydrolysis products. Genipin has an oxygen methyl ester structure and low cytotoxicity. At the end of the 20th century, genipin was commonly used to replace formaldehyde and glutaraldehyde as a crosslinking agent.⁶⁰ In 2008, at the annual meeting of the leather chemistry in Greensboro (USA), Taylor et al.⁶¹ proposed the potential application of genipin in leather tanning through genipin-modified gelatin. In 2015, Xing et al.⁶² compared tanning bovine pericardium with glutaraldehyde with that by genipin, and the results showed that leathers tanned with the two materials were not significantly different. The shrinkage temperature of genipin-tanned leather can reach 81°C, which indicates that genipin is an available tanning agent. The tanning mechanism is shown in Figure 12. In fact, using genipin as a tanning agent is associated with higher prices. However, a small amount of genipin used in combination with other tanning agents has practical significance.⁶³

Figure 12.

Genipin tanning agent.

Phosphonium hydroxymethyl tanning agent

Tetrakis hydroxymethyl phosphonium salts can be dissolved in the form of hydrochloride (THPC) and sulfate (THPS), which are collectively known as THP salts. In 1921, Hoffman synthesized THPC, which was used in the flame retardant treatment of cellulose fibers in the 1950s. The phosphonium hydroxyl methyl in P⁺(CH₂OH)₄ was found to condense well with nitrogen compounds, and in 1966, Filachione⁶⁴ applied for a patent for its use. Research on the reaction of THP salt with collagen found that the group that binds to collagen is the phosphine trimethylol oxide (THPO) of THP salt because the hydroxymethyl group is easily removed and oxidized in the process of alkalization.⁶⁵ Although THP salt is cationic salt, the tanning principle is also the same as aldehyde tanning, and it interacts with the collagen amino to form a crosslink through dehydration; however, the hydrogen bond crosslinking is not ruled out and the T_s of normal THP salt-tanned leather is approximately 80°C.^66,67 Its mechanism of action is shown in Figure 13. Due to the presence of cationic P⁺, THP salt also has good bactericidal and precipitation anion flocculation functions.

Figure 13.

THP salt tanning agent.

THP salt is easily oxidized when the pH increases over 6.0, and it releases free formaldehyde.⁶⁸ In the tanning process, peroxides are needed to reduce free formaldehyde. There are two situations that limit the application of THP salts: (1) if THP salt is oxidized into O=P⁺(CH₂OH)₃ in the early stages of the tanning process, then THP permeates poorly without molecular charge and the T_s of the tanned leather is reduced; (2) because THP salt tanning is accomplished by raising the pH, formaldehyde release is unavoidable, which limits the use range of THP salt.

Cyanuric chloride tanning agent

The carbocation tanning agent is a new chrome-free tanning agent introduced by the Clariant Company in 2010. In 2012, Z Feng used F-90 in tanning sheep leather (CN102586504-A) and indicated that the tanning method is simple and environmentally friendly and consumes less energy. This tanning agent is included in a class of tanning agents derived from cyanuric chloride. Cyanuric chloride is the main monomer of reactive dyes. The tanning mechanism of the carbocation tanning agent is illustrated in Figure 14. The tanning reactive site of cyanuric chloride is based on the formation of carbocations after the departure of chloride ions, which have a strong affinity for amino groups.⁶⁹ To meet the requirements of leather tanning in aqueous solution, hydrophilic group grafting modification of cyanuric chloride is needed.

Figure 14.

Cyanuric chloride tanning agent.

To date, the cyanuric chloride tanning agent has good yellowing resistance and is considered to be formaldehyde-free and environmentally friendly. Thus, it is an ideal tanning agent for white or light-colored leathers. However, when used alone for tanning, cyanuric chloride tanning can only reach approximately 75°C T_s and the leather has poor physical properties that cannot meet the requirements for leather use. It is only used as a pretanning treatment agent for chrome and vegetable tanning.⁷⁰ For the chrome tanning pretreatment of clean production, the cyanuric chloride tanning agent can perform salt-free pickling and white wet leather manufacturing, which reduces the emission of sodium chloride and recovers waste collagen without chromium. For vegetable tannin-tanned leather after pretreatment with the cyanuric chloride tanning agent, a strong lyotropic inflation effect is caused by cyanuric chloride tanning agents on collagen; thus, subsequent machining processes are limited by adverse effects.

Quinone tanning agent

Benzoquinone is produced by oxidation of catechol. Under alkaline conditions, some vegetable tanning components cannot be washed out and the T_s of skin collagen is higher than that before tanning. European researchers in the mid-to-late 19th century showed that vegetable tanning included a quinone tanning effect on collagen.⁵⁸ In 1906, L Meunier and A Seyewetz applied for a patent for benzoquinone tanning, which made benzoquinone a tanning agent for leather. The tanning mechanism of common benzoquinone is shown in Figure 15. The carbon at the 2,5 position of benzoquinone can strongly electrophilically combine with the amino group to achieve the crosslinking of collagen.⁷¹ Due to the presence of dimers or polymers, the binding points between benzoquinone and collagen are increased and the crosslinking is enhanced, making the T_s of tanned leather reach 85°C. However, under alkaline conditions, colored benzoquinone easily condenses to a dimer, resulting in a darker leather color.^72,73 Moreover, simple benzoquinone also has strong toxicity and a pungent smell; therefore, it is not commonly used as a tanning agent.

Figure 15.

Benzoquinone tanning agent.

Conclusions

From 8000 BC, the ancient Indians used animal fat and smoke to make anti-corrosion oil tanning leather, which is thought to be the oxidation of oil and decomposition into smelly oil aldehyde and small molecules of acrolein.⁷⁴ This use of aldehydes and aldehydes tanning agents in the manufacture of leather and furs is now recognized as a stabilizing effect on collagen. To date, some aldehydes, such as formaldehyde, glutaraldehyde, and nitrocarboxymethyl compounds, are still used to improve the stability of leather and protein, or to protect biomaterials, especially as an alternative to the chrome tanning method in which it is difficult to overcome waste contamination.

In summary, it can be seen that although aldehyde materials and collagen can form stable covalent binding, the conversion rate of the final covalent binding is the key. The biotoxicity and stability of the product are affected by the activity of free aldehyde reactants, nitrohydroxyl in the product after the aldehyde tanning, and the acid decomposition of Schiff base. Therefore, it is very important to understand the main influencing factors in the reaction of aldehydes, such as the possibility of in situ contact, electrophilic and nucleophilic mechanisms, and the number of final irreversible products produced. If the product of the formaldehyde reaction is completed in the form of methylene, the hidden danger of the existence of free formaldehyde and the decomposition of nitrogen hydroxyl methyl can be eliminated completely. It is also suggested that innovative theories and practices are needed for the development of new aldehyde tanning agents or additives that are helpful for the completion of aldehyde tanning reactions.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the North Jiangsu Science and Technology Special Project (SZ-XZ 2017014).

ORCID iD

Chen Wenlong

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