Eco-Friendly Alternatives in Leather Production: Performance of Biodegradable Alginate-Based Retanned Leather Compared to Conventional Leathers and Plant-Based Materials

Abstract

This study explores the development and characterization of biodegradable leather using alginate derivatives as sustainable tanning agents, aiming to reduce the environmental impact associated with traditional leather tanning processes. Alginate, a natural polysaccharide derived from brown algae, was modified through ultrasound treatment to reduce viscosity and improve its application in leather tanning. This study investigated the use of sodium alginates as bio-based retanning agents, comparing their performance against that of conventional chromium-tanned and vegetable-tanned leathers, as well as synthetic alternatives such as leatherette, Piñatex^®, and Desserto^®. The physical, chemical, and thermal properties of the resulting leathers were assessed. The results demonstrated that alginate-based tanning agents could produce leather with comparable or superior properties to conventional and synthetic leathers, meeting the quality standards required for high-end footwear and leather goods. This research highlights the potential of alginate derivatives to serve as eco-friendly alternatives in the leather industry. The findings underscore the feasibility of integrating bio-based materials into industrial applications, promoting environmental conservation and resource efficiency.

1. Introduction

The process of tanning leather involves the chemical modification of collagen, the primary structural protein of animal hide, to transform it into a durable and pliable material. This process can be broadly divided into three distinct phases: (i) Preparation. The raw hide undergoes a series of preparatory steps to remove unwanted components such as fat, hair, and the outer layer of hides (epidermis and subcutaneous tissue). (ii) Tanning. The core of the tanning process entails the stabilization and cross-linking of collagen fibers, rendering the hide resistant to decomposition and imparting desirable characteristics such as strength, flexibility, and water resistance. Various tanning agents, typically derived from plant, mineral, or synthetic sources, are employed to achieve this transformation. (iii) Finishing. The final stage involves the application of specialized chemicals and treatments to enhance the appearance, texture, and performance of the leather. This may include mechanical operations and coatings [1,2].

Leather is regarded as a sustainable material due to its origins as a byproduct of the meat industry. It effectively utilizes a waste product, minimizing environmental impact and resource consumption. Moreover, leather exhibits inherent biodegradability, unlike synthetic alternatives derived from petroleum [3].

Despite its sustainability advantages, the tanning process itself presents environmental concerns. The use of chemical tanning agents, often of mineral or vegetable origin, can generate hazardous byproducts and pose risks to human health and the environment. Ongoing research aims to develop eco-friendly tanning methods that minimize environmental impact [4].

Global leather production stands at an estimated 10 million tons annually, addressing a significant demand for durable and versatile materials. This production contributes significantly to the economy, generating revenue of approximately USD 40 trillion. Moreover, the tanning process generates valuable byproducts, such as gelatin, which are utilized in various industries, including pharmaceutical and food [5].

In conclusion, leather presents a unique balance between sustainability and functionality. Its origins as a byproduct, biodegradability, and versatility make it an attractive material. However, the use of environmentally harmful chemicals in the tanning process necessitates ongoing efforts to develop sustainable alternatives [6].

The incorporation of bio-based alternatives in the leather industry aligns with the broader transition towards sustainable and circular practices [7]. These innovations offer promising solutions for mitigating environmental impacts, promoting resource conservation, and enhancing the overall sustainability of the leather industry [8].

Bio-based alternatives, encompassing polysaccharides, lipids, and proteins derived from animal or plant sources, have gained prominence due to their biodegradability, biocompatibility, and abundance in nature [9]. One option that follows this trend is the use of alginate derivatives.

Algin is a carbohydrate that is abundant in the cell wall of brown algae belonging to the phylogenetic class Phaeophyceae. It has many applications in drug delivery, textile printing, and the food industry (as a thickener and emulsifier) [10]. Alginates are natural linear anionic polysaccharides. They are formed by different proportions of β-D-mannuronate (M) and α-L-guluronate (G) residues linked by C1-C4 bonds, as depicted in Figure 1 [11]. In alginate, M and G units are linked through 1 → 4 glycosidic bonds. These bonds create linear dimers within the polymer. The polymer contains homodimers (GG and MM) and heterodimers (MG/GM).

A limitation of the use of this material in the textile field is its viscosity [12]. An efficient ecological alternative to reduce viscosity and, consequently, the molecular weight of alginates is represented by the use of ultrasound. In 2017, Cao et al. [13] showed that ultrasonic treatment of alginates at different frequencies caused degradation, reordering, and changes in their properties, such as molecular weight and viscosity. They demonstrated that ultrasonic treatment of alginate could also increase its hydrophobic interaction and interfacial activity. In addition, Dodero and Castellano successfully employed ultrasonic treatment to reduce the molecular weight of two sodium alginates in saline solutions in 2020 [14].

The use of oxidized sodium alginate in tanning was first reported by Ding et al. in 2018 [15]. To obtain oxidized sodium alginate (OSA), sodium alginate (SA) was selectively oxidized using NaIO₄. This oxidation increased the thermal stability of leather powder. During the reaction, the C2-C3 bond between two adjacent hydroxyl groups in the glucuronic or mannuronic units is cleaved. The 1,2-diol group converts into a dialdehyde. As a result, OSA becomes an open-chain biopolymer containing aldehyde groups. See Figure 2.

However, the industrial use of NaIO₄ is limited by its relatively high cost. In addition, the industrial use of NaIO₄ requires great caution because it can cause severe skin burns, eye injuries, and organ damage in case of prolonged exposure. It is highly toxic to aquatic organisms and can cause a fire or explosion. This substance is registered under REACH and cannot be used/imported in the European Economic Area in quantities greater than 1000 tons per year.

This study introduces a new sustainable route for obtaining an oxidized alginate, developing a biodegradable SA-based retanning agent that is easy to adopt in the tanning industry without the need for changes in the current technology.

The present study aims to develop a biodegradable, chromium-, and aldehyde-free leather using alginate derivatives as a retanning agent to significantly reduce potentially toxic substances, petroleum-based products, and the environmental impact of leather production. At the same time, it will meet the quality requirements of the high-end footwear and leather goods industries. The characteristics of the leather obtained with the new tanning system using alginate derivatives will be compared with those of three types of conventional leather, as well as alternative materials to genuine leather: leatherette, Piñatex^® by Ananas Anam online store, and Desserto^® by Desserto Mexico in Zapopan, Jalisco, Mexico. Leatherette is an artificial leather commonly used in furniture upholstery, clothing, and leather goods. Piñatex^® and Desserto^® are derived from the recovery of plant-origin fibers and structured with various polymers to attain the required resistances.

2. Materials and Methods

To conduct this study, a modification of commercial sodium alginate was performed. Commercial sodium alginate (SA) was procured from Kemia Tau in La Cassa (Torino), Italy and employed without any additional purification procedures. The SA solution utilized for the ultrasound treatment was prepared by blending deionized water and sodium alginate under stirring for a prolonged duration of 12 h to achieve a uniform solution. All the ultrasound-mediated syntheses were carried out employing a SA solution at a concentration of 2% w/w.

This study was carried out using raw hides which were processed from the beamhouse, tanning, post-tanning, and drying stages of the crust leather prior to finishing. The entire global tanning process was carried out with the latest clean technologies, and by means of wet–white chrome-free tanning, which reduce the environmental impact of leather compared to conventional processes. Wet–white chrome-free tanning is a leather processing method that avoids using chromium salts, producing leather with a light or “white” color. In wet–white tanning, chrome salts are replaced with alternative agents like glutaraldehyde, syntans (synthetic tannins), aldehydes, or vegetable tannins. In this study, the tannage was performed by using synthetic tannins. The new oxidized alginate product was applied in the retanning formulation, as a bio-based retanning agent, designed to carry out the complete post-tanning process without interactions between the products applied. The retanning formulas used are shown in

Table 1. SA and SAD application formula.

Step	°C	%	Product	Time	Remarks
Washing	30	200	Water	10′
					Drain
Neutralization	30	200	Water
		0.4	Sodium Formate	90′	pH = 4–5
Retanning	35	32	SA/SAD + ZnO	120′
Dyeing		3	Dyeing	240′
		3	Formic acid (1:10)	60′	pH = 3.87
					Drain
Washing	50	200	Water	15′
					Drain
Fatliquoring	50	200	Water
		4	Sulphited oil
		8	Sulphated oil	60′
		1.5	Formic acid (1:10)	30′	pH = 3.10
					Drain
Washing	40	200	Water	10′
					Drain
Horsing 24 h
Sammying, drying, conditioning, staking, and milling

and

Table 2. SA and SAD application formula using Tara.

Step	°C	%	Product	Time	Remarks
Washing	30	200	Water	10′
					Drain
Neutralization	30	200	Water
		0.4	Sodium Formate
		0.4	Sodium Bicarbonate	120′	pH = 5
Retanning		10	SA/SAD + ZnO	60′
		5	Tara
		10	SA/SAD + ZnO	60′
		5	Tara
Dyeing		3	Dyeing	240′
		3	Formic acid (1:10)	60′	pH = 3.87
					Drain
Washing	50	200	Water	15′
					Drain
Fatliquoring	50	200	Water
		4	Sulphited oil
		8	Sulphated oil	60′
		1.5	Formic acid (1:10)	30′	pH = 3.10
					Drain
Washing	40	200	Water	10′
					Drain
Horsing 24 h
Sammying, drying, conditioning, staking, and milling

.

As mentioned above, biodegradable leather, chromium-, and aldehyde-free leather using sodium alginate derivatives (SAD) were compared with three types of conventional leather, as well as alternative materials to genuine leather: leatherette, Piñatex^®, and Desserto^®. The 10 study samples are identified as follows:

• Sample #1: Finished ovine leather, chromium-tanned;
• Sample #2: Finished bovine leather, chromium-tanned;
• Sample #3: Finished bovine leather, vegetable-tanned;
• Sample #4: Leatherette;
• Sample #5: Piñatex^®;
• Sample #6: Desserto^®;
• Sample #7: Bovine leather tanned with SAD;
• Sample #8: Bio-based finished bovine leather tanned with SAD and ZnO nanoparticles;
• Sample #9: Bovine leather tanned with SAD and Tara;
• Sample #10: Bovine leather tanned with SAD, ZnO nanoparticles and Tara.

The 10 indicated samples were characterized for their acceptance in the footwear and leather goods markets, using the following tests:

-

Thickness determination according to the standard EN ISO 2589 (mm) [16].

-

Tensile strength according to the standard EN ISO 3376 (N/mm²) [17].

-

Tear strength according to the standard EN ISO 3377-2 (N/mm) [18].

-

Determination of distension (mm) and strength (kg) of surface according to the standard EN ISO 3379 [19].

-

Water vapor permeability according to the standard EN ISO 14268 (mg/h·cm²) [20].

-

Shrinkage temperature according to the standard EN ISO 3380 (°C) [21].

-

Determination of flex resistance according to the standard EN ISO 5402-1 [22].

-

Light fastness according to the standard ISO 105:B02 [23].

-

Accelerated aging color change according to the standard ISO 17228 [24].

-

Color fastness to rubbing according to the standard ISO 11640 [25].

-

Finishing adhesion according to the standard ISO 11644 (N/cm) [26].

-

Determination of substances soluble in dichloromethane according to the standard EN ISO 4048 (%) [27].

-

Water and volatile content according to the standard EN ISO 4684 (%) [28].

-

Extractable organic and inorganic matter according to the standard EN ISO 4098 (%) [29].

-

Ash and insoluble mineral matter according to the standard EN ISO 4047 (%) [30].

-

Nitrogen and leather substance according to the standard ISO 5397 (%) [31].

-

pH and difference index according to the standard EN ISO 4045 [32].

-

Determination of metal content: total metal content according to the standard EN ISO 17072-2 (mg/kg) [33].

-

Determination of formaldehyde content in leather: HPLC quantification, according to the standard EN ISO 17226-1 (mg/kg) [34].

An investigation of the materials was also carried out using thermometric techniques. The following analyses were performed: (i) the determination of the differential scanning calorimetry (DSC) curve and (ii) thermogravimetric analysis (TGA). The differential scanning curve is carried out in a nitrogen atmosphere without pre-humidifying the samples. The DSC technique is one of the thermal techniques used to measure the energy absorption or release of materials. In this study, dry samples were weighed to 5 mg, put into aluminum crucibles, and heated at 10 °C/min increments from 20 °C to 280 °C under nitrogen gas (20 mL/min flow rate), which was applied for DSC analysis.

Thermogravimetric analysis (TGA) is used to determine the thermal stability of a material and its fraction of volatile components by tracking the change in weight that occurs when the sample under study is heated at a constant rate. TGA is a thermal analysis technique that involves determining the weight of samples at the specified initial temperature in a controlled manner–depending on mass, time, and temperature–on a sensitive scale that can weigh with an accuracy of 0.0001 mg, in a temperature program that can change in a controlled manner. In the present study, the samples were weighed by ultra-micro balance and heated at 20 °C/min increments from 30 °C to 700 °C under nitrogen gas (20 mL/min flow rate), which was applied for TGA analysis.

3. Results

3.1. Physicochemical Characterization

The results of the physicochemical characterization of the 10 samples that were studied are shown in

Table 3. Characterization of the 10 samples under study.

TEST	1	2	3	4	5	6	7	8	9	10
Substances soluble in dichloromethane	3.8	4.0	7.1	23.0	29.1	2.4	10.2	8.1	8.7	11.7
Water and volatile content	11.3	12.8	9.8	6.1	3.0	1.3	10.7	9.9	10.5	10.4
Extractable organic and inorganic matter
Inorganic matter	1.8	0.08	0.06	-	-	-	0.08	0.09	0.2	0.3
Organic matter	0.1	0.06	0.5	-	-	-	0.08	0.08	0.2	0.2
Ash and insoluble mineral matter	12.8	5.5	5.5	15.1	15.1	2.4	0.5	0.6	1.3	1.5
Nitrogen and leather substance
Nitrogen	11.3	13.0	13.0	5.3	5.3	1.9	12.8	13.2	12.1	11.8
Leather substance	63.5	72.8	72.81	29.9	29.9	10.4	71.9	74.2	68.06	66.1
pH and difference index
pH	4.5	3.7	3.7	4.2	4.2	5.8	3.0	3.0	3.2	3.3
Difference index	-	-	-	-	-	-	-	-	-	-
Formaldehyde	10.5	<10	<10	<10	<10	<10	9.0	<5	<10	<10
Titanium (Ti)	<12	<12	12.2	449	<12	133	<12	<12	<12	<12
Aluminum (Al)	102	339	247	3948	244	153	102	38.0	66.9	77.1
Zirconium (Zr)	<12	<12	<12	<12	<12	<12	<12	<12	<12	<12
Chromium (Cr)	25,079	18,034	215	9179	<3	<3	11.3	11.1	1127	1121
Zinc (Zn)	<3	<3	<3	<3	<3	<3	<3	<3	<3	<3
Iron (Fe)	38.6	60.7	217	26,377	48.7	10.8	38.3	31.3	57.4	71.5
Tensile strength
Strength	25.4	25.39	32.32	5.50	5.10	9.48	38.37	35.86	18.35	21.04
Elongation	79.8	79.75	45.45	4.63	33.47	15.77	70.35	62.0	64.50	68.20
Tear load	41.1	41.11	67.50	7.12	68.12	47.74	145.67	140.83	77.36	78.65
Distension and strength of surface
Thickness	1.14	1.27	2.20	0.34	1.59	1.28	1.75	1.83	1.81	1.86
Break
Strength	34.8	20.5	55.0	1.1	23.5	17.6	62.9	51.9	25.1	26.9
Distension	12.0	6.2	8.5	4.1	10.6	10.9	11.0	9.6	9.0	9.5
Burst
Strength	45.9	52.3	102.1	1.1	23.5	17.6	105.6	97.5	49.8	64.8
Distension	14.3	11.1	13.2	5.7	10.6	11.6	14.3	13.3	12.8	14.1
Water vapor permeability	4.9	4.0	1.1	0.4	6.8	0	5.9	7.7	10.1	9.9
Shrinkage temperature	106	107	81	>140	>140	>140	68	69	72	71
Flex resistance
Dry, 100,000 cycles	Small wrinkles	Wrinkles	Wrinkles	Breaks	No changes	Wrinkles	Wrinkles	Wrinkles	Wrinkles	Wrinkles
Wet, 50,000 cycles	Wrinkles	Wrinkles	Wrinkles	Breaks	No changes	Wrinkles	Breaks	Big wrinkles	Wrinkles	Wrinkles
Light fastness	7	>7	>7	>7	>7	>7	3–2	2–3	4–5	4–5
Accelerated aging color change
50 °C, 90%HR for 96 h	5	5	4–5	5	5	5	4	3–4	4–5	4
Rub fastness
Dry, 100 cycles	4–5	1–2	3–4	5	-	5	4	4	4	4–5
Wet, 20 cycles	4	3	3–4	4–5	-	4–5	3	2–3	4–5	4–5
Finishing adhesion
Dry	18.3	6.9	14.2	Leather breaks	Leather breaks	41.8	51.1	64.7	37.3	39.5
Wet	17.5	6.5	14.2	Leather breaks	46.1	42.7	44.4	50.9	23.4	27.4

.

As shown in Table 3, leatherette (#4) and Piñatex (#5) materials have a high content of substances extractable in dichloromethane.

The water content remains similar in all collagenic leather substrates (#1, #2, #3, #7, #8, #9, and #10), and is lower for the three alternative materials.

The ash content is quite high in the alternative materials leatherette (#4) and Piñatex (#5), which indicates the presence of inorganic substances in the composition of the materials. Sheepskin also has a high ash content, due to its capacity to absorb chrome salt, in comparison to the collagen content; it is a more porous skin than bovine hide.

The nitrogen contents of the leathers were found to be significantly higher compared to the alternative materials since the leathers consist of protein which is a huge source of nitrogen. The nitrogen content is comparable for all analyzed leather substrates (#1, #2, #3, #7, #8, #9, and #10). Conversely, for the alternative materials, the nitrogen content is lower, especially for the Desserto material (#6). The pH of all the samples remains similar, except for the Desserto material (#6), which has a higher value of 5.8.

Comparing the chromium content of the samples, the samples of sheepskin (#1) and bovine wet–blue (#2) as well as the leatherette (#4) have a high chromium content, owing to the use of chromium (III) salts. Leather samples #9 and #10 have some chromium content due to the use of a dye that contains chromium.

The leatherette (#4) has a high iron content (2.6%), as well as high aluminum (3948 ppm) and titanium (500 ppm) contents.

In the Piñatex (#5) and Desserto (#6) materials, the total metal content remains below 1000 mg/kg.

The formaldehyde content is less than 10 mg/kg in most samples; only sample #1 contains a value of 10.5 mg/kg. All samples fall below the regulation of 75 mg/kg in articles that are in contact with human skin and 150 mg/kg for articles that do not have contact with the user’s skin.

The leather samples (#1, #2, and #3) show comparable values, the burst strength value being higher for the vegetable-tanned sample.

The leather samples from #7 to #10 are also comparable. The tear resistance of articles #7 and #8 is notable.

The alternative material samples, especially the leatherette (#4), show much lower resistance values in all categories tested. The tensile strength is very low compared to collagen or natural leather samples. Only the Piñatex material (#5) exhibits a tear resistance similar to the leather samples.

The water vapor permeability values, associated with the breathability of the materials, are similar for the leather substrates, with the lowest value corresponding to the vegetable-tanned leather, due to the thickness and market of the article. Of the alternative materials, only the Piñatex material exhibits water vapor permeability.

The shrinkage temperature of the chromium-tanned leathers (#1 and #2) reaches values above 100 °C; the vegetable-tanned leather (#3) remains at 81 °C. The wet–white leather substrates retanned with sodium alginate (#7, #8, #9, and #10) reach values between 68 and 72 °C, typical for this type of tanning.

With respect to the shrinkage temperature of the alternative materials, the leatherette (#4), the Piñatex material (#5), and the Desserto material (#6) reach values above 140 °C since they do not contain collagen.

Differences can also be observed among the alginate-based leathers. Samples #9 and #10, which contain Tara, show lower values in tensile strength, burst strength, and tear load compared to sample #8, which does not contain Tara, as Tara creates a more rigid and less flexible structure.

3.2. Thermometric Characterization

DSC is a common method used to analyze the effects of tanning and retanning components on the collagen matrix by calculating the energy consumption (ΔH). Increasing the denaturation temperature and enthalpy indicates better cross-linking action through the collagen fiber network [35]. Variation in denaturation enthalpy areas indicates differences in material structure which are directly related to the reagent used during the leather manufacturing process [36]. The DSC curves of the specimens are given in Figure 3; chromium-tanned bovine leather gained the highest denaturation temperature and enthalpy areas of 106.83 °C and −813.27 J g⁻¹, respectively. Additionally, the leathers retanned with alginate (except sample #9) show better thermal performance than vegetable-tanned bovine and chromium-tanned ovine leathers, which indicates the compatible synergy of alginate with wet–white tanning reagent. Alginate, a polysaccharide derived from seaweed, may introduce different functional groups and bonding capabilities compared to the polyphenolic compounds typically found in vegetable tannins. These additional functional groups can participate in stronger or more numerous intermolecular interactions with collagen, resulting in greater thermal stability and higher enthalpy. Additionally, the linear structure of alginate allows for more uniform and consistent interactions with collagen fibers. In contrast, the complex and variable nature of vegetable tannins can lead to less effective binding and cross-linking, resulting in lower thermal performance and enthalpy values. The higher thermal decomposition values among the alginate-based retanned leathers belong to samples #7 (98.78 °C, −585.77 J g⁻¹), #10 (87.08 °C, −323.47 J g⁻¹), #8 (87.07 °C, −334.04 J g⁻¹), and #9 (78.5 °C, −183.19 J g⁻¹). The results showed that the use of Tara with alginate solution did not affect the thermal performance of the leathers positively, as was expected. Since the denaturation temperature of sheep collagen is lower than that of bovine [37,38], the DSC performances of chromium-tanned ovine (#1) leather were determined to be 87.3 °C and −214.90 J g⁻¹. Due to the difference in characteristic structure of collagen-based leathers and leather alternatives (#4, #5, and #6), the thermal behavior of the specimens was interpreted separately. In the DSC thermograms, there were two exothermic peaks belonging to sample #5 and sample #6. The DSC curves of sample #5 (first peak: 43.94 °C, −16.15 J g⁻¹; second peak: 165.55 °C, 29.91 J g⁻¹), and sample #6 (first peak: 58.24 °C, −43.70 J g⁻¹; second peak: 265.13 °C, −6.77 J g⁻¹) were found to be similar to those of the composite containing polylactic acid (PLA) [39,40]. It is known that PLA containing hydroxyl and carboxyl groups is popular in crystalline biomaterials used in the production of biodegradable and biocompatible substances for packaging and drug applications. The DSC thermogram of sample #4 showed that there were two main peaks at 186 °C (ΔH = 605.80 J g⁻¹) and 367 °C (ΔH = 88.14 J g⁻¹) belonging to PVC and PU. It is known that PU, with its high toughness, is modified by PVC to obtain more flexible materials [41].

The literature showed that collagen-based leathers have two main mass loss areas including the dehydration of the biopolymeric structure (50–150°C), the evaporation of the volatile compound and, finally, protein carbonization (150–600 °C) [42]. The TGA curves of the specimens are given in Figure 4. Vegetable-tanned materials are linked to collagen by hydrogen bonds which are not a stable mechanism [43]; therefore, sample #3 has the lowest moisture loss peak at 75.38 °C, losing 8.30% of weight. One of the important characteristics that makes the chromium irreplaceable is the higher thermal stability that it gives to the collagen [44]. Chromium-tanned bovine leather (#2) reached 83.91 °C in the first dehydration peak with 3.77% mass loss. Surprisingly, the highest peak in the first step belongs to sample #10 at 88.35 °C with 8.09% mass loss and sample #8 at 87.36 °C with 9.36% mass loss, respectively. The second peaks, which are attributed to the collagen thermal decomposition of leather [45], are given in

Table 4. DSC and TGA values of the samples ¹.

	Td °C	ΔH J g−1	T1 °C	W1 %	T2 °C	Residue Mass %
1	87.3	−214.90	77.93	−8.63	314.40	12.91
2	106.83	−813.27	83.91	−3.77	352.64	4.56
3	85.19	−270.24	75.38	−8.30	329.41	0.80
4	186	605.80	-	-	391.39	10.66
5	43.94	−16.15	-	-	355.75	1.16
6	58.24	−43.70	-	-	372.04	2.51
7	98.78	−585.77	73.84	−9.70	330.29	1.86
8	87.07	−334.04	87.36	−9.36	331.15	1.84
9	78.5	−183.19	80.62	−11.03	315.99	2.74
10	87.08	−323.47	88.35	−8.09	341.20	2.57

¹ Denaturation temperature of collagen-based samples and the first peak of alternative materials (Td°C), denaturation enthalpy (ΔH Jg−1), moisture loss peak (T1 °C), weight loss at T1 (%), main decomposition peak (T2 °C), and residual ash (%).

as T2 (°C).

Chromium-tanned bovine leather (#2) decomposed at 352.64 °C and samples #10, #8, and #7 followed at 341.20, 331.15, and 330.29 °C, respectively. Eventually, sample #3, with 0.80% of residual ash weight, almost pyrolyzed completely and, afterwards, alginate retanned leathers #8, #7, #10, and #9 followed with 1.84%, 1.86%, 2.57%, and 2.74% decomposition, respectively, being the most thermally decomposed samples. The results in Table 4 show that leather alternatives contain much less moisture (#4 = 6%, #5 = 3%, and #6 = 1.3%); therefore, the moisture evaporation peaks in the TG curves are not as evident as those for the leathers and they are not presented in Table 4. The main thermal decomposition of sample #5 occurred between 240 and 400 °C and T2 was detected at 355.75 °C; this is similar to results obtained by Barba et al. [46]. Sample #6 showed a wider thermal decomposition range of 220–480 °C with T2 = 372.04 °C. Due to the different structure of sample #4 compared to the others, it shows a unique TG performance which is unlike other bio-based samples. Thermal decomposition of sample #4 took place between 200 and 500 °C and included three different stages. The first peak was at 344.70 °C, the second peak was at 391.39 °C, and the last one was at 412.62 °C; these correspond to the primary decomposition of the conveyor textile layer and polymer structure, respectively [47,48]. Among these synthetic and bio-synthetic samples, higher residual carbon content was observed in sample #4 at 10.66%, followed by samples #6 and #5 at 2.51% and 1.16%, respectively.

Another significant observation is that samples #1 and #4 display markedly higher residual mass values than the other samples. The ovine leather (#1) utilized in this study underwent chromium tanning and was subsequently retanned with inorganic mineral products. Conversely, leatherette (#4) is often made from synthetic polymers, such as polyvinyl chloride (PVC) or polyurethane (PU), which can include inorganic fillers or additives to enhance properties like durability, flexibility, or fire resistance. These inorganic materials contribute to the overall inorganic content. Additionally, to achieve specific characteristics in leatherette, manufacturers often incorporate various additives, including stabilizers, plasticizers, and pigments. Many of these additives are inorganic compounds, leading to a higher overall inorganic matter content compared to genuine leather, which primarily consists of organic collagen fibers.

Implementing alginate-based tanning agents in industrial leather production presents several challenges including cost, resource availability, scalability, and market acceptance.

While alginate is bio-based, it may be costlier than traditional tanning agents like chromium. The commercial viability of alginate-based tanning may also be limited by resource availability, especially if seaweed production cannot meet high demand. Although lab-scale results indicate promising performance, scaling up for industrial production could face challenges in ensuring both efficiency and batch-to-batch consistency. Rigorous assessment of these factors is essential to maintain quality at a larger scale.

Moreover, successful market adoption will require acceptance from manufacturers, brands, and consumers. Despite growing interest in sustainable materials, widespread adoption of alginate-tanned leather will depend on its ability to match the cost and durability of conventional tanning methods.

4. Conclusions

This study conducted an in-depth analysis of the use of alginate-based tanning agents in the production of leather. The experimental results and their interpretation led to several conclusions. The thermometric characterization demonstrated that leathers retanned with alginate show better thermal performance than vegetable-tanned bovine and chromium-tanned ovine leathers. This indicates the compatible synergy of alginate with wet–white tanning reagent. Our research demonstrated that alginate-based tanning agents could produce leather with comparable or superior properties to conventional and synthetic leathers. These leathers meet the quality standards required for high-end footwear and leather goods. This research underscores the potential of alginate derivatives to serve as eco-friendly alternatives in the leather industry. It highlights the feasibility of integrating bio-based materials into industrial applications, thereby promoting environmental conservation and resource efficiency. In conclusion, the findings of this study suggest that alginate-based tanning agents hold significant promise for the future of sustainable leather production. They offer a viable and environmentally friendly alternative to traditional tanning methods.