
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12301-8
10.1016/j.heliyon.2024.e36270
e36270
Research Article
Application and valorization of novel indigenous Azadirachta indica leaf in leather processing
Shakil Saidur Rahman saidurrahman@du.ac.bd
a⁎
Zenith Fatema Tuz Johura a
Khan Mahabubur Rahman b
Tonay Waliur Rahman a
a Department of Leather Engineering, Institute of Leather Engineering and Technology, University of Dhaka, Dhaka, 1209, Bangladesh
b Institue of Leather Engineering and Technology, University of Dhaka, Dhaka 1209, Bangladesh
⁎ Corresponding author. saidurrahman@du.ac.bd
23 8 2024
15 9 2024
23 8 2024
10 17 e3627023 1 2024
12 8 2024
13 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
This study investigated the viability of locally available organic sources of Azadirachta indica leaf as vegetable tannin agent in leather processing to promote green leather manufacturing. Leather tanned with Azadirachta indica leaf powder (NPT), extract (NET) and conventional vegetable tanned leather (CVT) was characterized with FTIR, DLS, HPLC, DSC, TGA, and SEM. Total soluble solid, pH, tannin content, and tanning strength of Azadirachta indica leaf extract were found to be 24 %, 4.81, 12.34 % and 1.81 respectively. Moisture content, fat content, and water-soluble content of Azadirachta indica leaf extract tanned leather (NET) were 12.26 %, 10.8 % and 7.2 % respectively. The shrinkage temperature, tensile strength, stitch tear strength, grain crack load, and finished film bond strength of NET leather were 86 °C, 282.52 kg/cm2, 139.53 kg/cm, 24 kg, and 414 g/cm respectively. NET leather exhibited better anti-microbial sensitivity against E. coli, S. aureus, and P. aeruginosa than NPT and NET leather. Overall, the experimental results of this study indicate the A. indica leaf could serve as a prime renewable tanning agent, substituting hazardous chromium and imported conventional vegetable tannin chemicals in leather manufacturing. Thus, developed A. indica leaf tannin material from plant sources could provide sustainable leather production, contributing to eco-friendly and viable green leather processing options.

Keywords

Vegetable tannins
Azadirachta indica leaf
Extraction
Leather tanning
Physio-mechanical properties
==== Body
pmc1 Introduction

Throughout human history, leather manufacturing has held a highly esteemed position and continues to be significant in modern society. Crust leather, finished leather, footwear, and leather goods are mainly export-oriented, reflecting the increasing demand for high-quality leather [1]. There are several prominent methods available to transform putrescible raw hides and skins into non-putrescible stable leather by modifying their physical and chemical structure, including aldehyde tanning, aluminum tanning, combination tanning, chrome tanning, ion tanning, oil tanning, titanium tanning, and zirconium tanning [2].

Even though approximately 85 % of total leather production is achieved through the chrome tanning process, the bio-cumulative properties of chromium pose potential hazards, characterized by its typical chemical pungent odour among other concerns. When exposed to potent oxidizing agents, high temperatures, unsaturated organic compounds, or UV radiation, basic chromium sulphate forms the acute carcinogenic Cr6+ species [3]. Tannery effluent containing unreacted chromium and other salts from the conventional chrome tanning process causes detrimental effects on the environment and aquatic life [4].

Both chromium and vegetable tannins have their own merits and demerits; however, vegetable tanning, originating from various plant sources, is biodegradable and generates less toxic effluent as it does not involve any heavy metals [5]. The Body, fullness, compactness, and molding characteristics of vegetable tanning facilitate the production of embossed items, belts, bags, saddles, heavy boots, and industrial leather that are durable and resistant to any type of thermal, and biological degradations [6]. Tannins, plant-derived compounds with astringent properties, facilitate the transformation of putrescible raw hides and skins into leather [7]. Vegetable tanning utilizes natural tannins from sources such as quebracho, chestnuts, and mimosa trees, which contain tannic acid and bind with collagen protein turning into leather [8]. The content, structure, and qualities of tannins differ among various plant species, thereby influencing the desired properties of leather.

Commercial vegetable tanning agents are extracted from many plant sources, including oak trees, spruce, chestnuts, mangroves, pine, eucalyptus, quebracho, mimosa, gambler, sumac, valonia wattle, etc. Tannin is found in almost every part of the specific plant, with varying amounts, although some plant species contain tannin in bark, wood, pods, leaves, and roots [9]. The bark of douglas, oak, tan oak, beech, snouba, willow, birch, and chestnut are popular tannin sources. Likewise, wood from aka, mesquite, osage orange, wandoo eucalyptus, oak, chestnut, quebracho, and so on are traditionally used in leather processing [9]. Vegetable tannins from fruits and nuts such as acorns, myrobalan, and pomegranate are some of the most common sources with a long history in the leather industry. Leaves of sumac, manzanitas, gambier, tea, dhawa R. coggygria, R. glabra, and R. myrtifolia are still used to tan raw hides and skins in many parts of the world [10]. Additionally, mangrove, eucalyptus, and mango leaves have been reported to contain substantial amounts of complex tannin structure, making them potential tannin agents. However, no studies have been conducted to evaluate their application in leather making [11]. Raw skins treated with root and gall of avens, canaigre, ratanhy, sassafras, tormentil, and oak acquire desired leather properties [12]. Most of the vegetable tannin chemicals used in the Bangladeshi leather industry are imported from various countries. However, several researchers have reported that Azadirachta indica leaves contain negligible non-tanning content and can play an important role in the vegetable tanning system. Moreover, all the vegetable tanning agents from plant sources are considered renewable. Therefore, finding alternatives to imported chemicals and using locally available plant sources in leather manufacturing is indispensable. Azadirachta indica leaves are alternate and consist of several dark green leaflets with serrated edges, oblong shape, length of 20–40 cm, grow in opposite directions to each other at a single node are short-stalked broad compounds, dorsiventral, exstipulate, and have medicinal properties [13,11].

The prime objectives of this current research are to investigate the tanning potential of native Azadirachta indica tree leaves as a substitute for traditionally used vegetable tannin materials, thereby promoting sustainable practices in the leather industry. To the author's knowledge, no previous study has been done to apply Azadirachta indica leaf powder and extract in leather manufacturing. Thus, utilizing locally grown renewable plant sources could be an option for toxic chrome tanning and contribute to green leather manufacturing.

2 Materials and method

2.1 Materials

Around 5 kg of green Azadirachta indica leaves were collected from nearby plants, sundried for a few days, and then ground into powder form. Five green goat skins were purchased from the nearby slaughterhouse in Hazaribagh Bazar, Dhaka, Bangladesh. Ethanol (98.96 %), Methanol (99.45 %), FeCl3(99.97 %), Na2CO3, FeSO4 (96.98 %) (Merck Germany), Gelatin (98 %), Lead acetate (99 %), Potassium dichromate (98 %), Potassium hydroxide (98 %) (Research-Lab Fine Chem Industries, India), Folin-Ciocalteu reagent, Galic acid (Loba Chemie, India). Rutin hydrate, caffeic, p-coumaric acid, vanillic acid, catechin, catechol, and syringic acid, were brought from Sigma-Aldrich (St. Louis, MO, USA). Acetic acid (HPLC), methanol (HPLC), ethanol (HPLC), and acetonitrile (HPLC) were purchased from Merck (Darmstadt, Germany). TSA agar media (Thermo scientific, UK). Commercial grade chemicals were utilized at all stages during leather processing.

2.2 Preparation of neem powder and extract

Azadirachta indica leaves were collected from a green tree, dried in the sun, ground by a miller, and sieved to discard coarse particles. Acetone, water, ethanol, and methanol were used in the extraction process, with a materials-to-solvent ratio of 1:10. The mixture was vigorously shaken in magnetic stirrer for 2 h at 70 °C (Fig. S1). The solution was then and concentrated in vacuum rotary evaporator, and finally, the liquid extract was dried in the oven at 60 °C (Fig. 1).Fig. 1 Schematic diagram of Azadirachta indica tanned leather manufacturing.

Fig. 1

According to the following equation, the yield percentage was calculated based on the weight difference between the initial sample taken and the final dry sample[12].(1) Extractionpercentage=Weightoffinalextract(g)Weightofinitaldriedsample(g)×100%

2.3 Phytochemical tests of extract and powder

FeCl3, gelatin, and ferrous sulphate solutions were used to determine the presence of tannin. Furthermore, lead acetate tests were conducted to confirm the presence of polyphenol and tannin in Azadirachta indica leaf extract and powder samples [14,15]. Moreover, the existence of hydrolysable or condensed tannin was confirmed by potassium hydroxide test, and details of the experiments were provided in the supplementary materials.

2.4 Instrumental characterization of tannin

TGA and DSC: The denaturation and phase transition stages of leather samples were measured under nitrogen atmosphere with a 10 ml/min flow rate by DSC (DSC, 8000 PerkinElmer). DSC and TGA were employed to determine the leather's thermal stability and the presence of volatile components in a sample.

HPLC: Quantification and detection of Azadirachta indica leaf extract and mimosa were determined by HPLC-DAD analysis (Ahmed et al., 2021). The operating conditions included a Shimadzu system (LC-20A, Japan) with a binary delivery pump (LC-20AT), autosampler (SIL-20A HT), photodiode detector (SPD-M20A), and oven (CTO-20A) with LC solution software. The mobile phase consisted of A (1 % acetic acid in acetonitrile) and B (1 % acetic acid in water), with a gradient of 1–20 min (5–25 % A), 20–30 min (25–40%A), 30–35 min (40–60 % A), 35–40 min (60-30 % A), 40–45 min (30-5% A), and (5 % A). The flow rate was set at 0.5 ml/min, and the sample injection volume was 20 μL.

DLS: The hydrodynamic diameter of the prepared powder, extract and mimosa was measured using dynamic light scattering (Litesizer 500, Anton Paar) of six automatic processed runs after dissolving materials in Milli-Q water followed by several filtrations.

SEM: Morphological properties of leather were observed by JSM-7200 FLV FE-SEM(JEOL) after conditioning and coating with platinum at various magnifications.

2.5 Chemical tests of tannin

2.5.1 Moisture content

A 2 g sample of experimental leaf powder and the extract was balanced (W1), taken to moisture free crucible, pre-weighed, and the crucible was reweighed with the sample measured as W2. The samples were placed in an oven for 3 h at 105 °C, conditioned in desiccators, and the weight was recorded as (W3)[16].(2) Moisturecontentbyweight=(W2−W3)W1×100

Where W1 represents the weight of the sample (g), W2 represents the combined weight of the sample before drying and the crucible (g), and W3 denotes the weight of the crucible plus residue (g).

2.5.2 Total solid content

The total solid content was calculated using the values obtained from the moisture content[17].(3) Totalsolidcontent=W3W1×100

Where: W3 = Weight after drying, W1 = initial weight of the sample.

2.5.3 Total soluble solid in tannin

20 g (W1) of tannin agents were weighed accurately into a 100 ml beaker of double-distilled water (V1). After 15 min of heating, 50 ml of filtered solution was again heated in a water bath until a constant weight (W2) was obtained[18].(4) Totalsolublesolid=W2W1×V1V2×100

2.5.4 Ash content of leather

A fixed amount of sample was pre-weighed and heated in a furnace at 800 °C until all decomposed. Subsequently, the porcelain dish was placed in a desiccator to cool before being weighed to determine the amount of ash obtained. Chemical properties of leathers, such as fat content and water-soluble content, were provided in the supplementary information [18].(5) Ash%=Ashobtained(g)Originalamountofsampleused(g)×100

2.5.5 Tannin determination by hide-powder method

Tannin content was measured by shaking three flasks in an orbital shaker for 30 min at 50 rpm after adding 7 g of hide powder to 100 ml of filtered leaf extract, powder, and mimosa. The flask contents were stirred and allowed to penetrate through a sintered glass filter for 30 min. The non-tannin content was measured on 50 mL of each filtrate. The tannin contents (mg/g) of extract and powders were estimated by deducting total soluble tannins from total non-tannins[18].(6) Tannin (mg/g) = soluble solid (mg/g)- non-tannins (mg/g)

2.6 Tanning process

Goat skins were washed with mild alkali in the presence of wetting agent, followed by treatment with calcium oxide and sodium sulphide to ensure proper swelling and opening up of collagen fiber [19]. Lime residues remaining on the pelt were removed by treating it with ammonium sulphate and ammonium chloride, followed by acidification with formic acid and sulphuric acids in the presence of salt to create suitable tanning conditions [20]. The float pH was kept around 3.0–3.4 before commencing the tanning process illustrated in Table 1.Table 1 Recipe of full vegetable tanned leather from raw hides and skins using developed tannins.

Table 1Unit process	Quantity (%)a	Chemicals	Duration	Observation	
Tanning	80	Water		30 °C	
+8(In two installments)	Azadirachta indica leaf powder/extract/mimosa	60 min		
0.5	Magnesium oxide	20 × 3 min + 4 h	pH 3.8–4.2	
After few days of piling, the leather is shaved (thickness 0.8 mm), and then the retanning process commences as follows	
Retanning	100	Water		30 °C	
0.5	EDTA			
+4	Acrylic resin	30 min		
+20 (In two installments)	Azadirachta indica leaf extract/powder/mimosa	60 min	Penetration check	
6	TiO2-based replacement syntan (anti-oxidizing agent)		
4	Amphoteric syntan		
2	Dispersing syntan		
2	Lanoline oil		
1 %	Surface shinning agent (Prevent darkening)			
+5	Molasse	30 min	Proper drain and wash	
Fatliquoring	150	Water		45 °C	
+2	Synthetic oil	60 min		
2	Semi-synthetic oil		
1	Lecithin oil		
1	Neats foot oil		
Top fat and rinsing were carried out followed by mechanical operations.	
a Chemical dose was calculated based on pelt weight for tanning and shaved weight for retanning.

No additional dyes were added in the dyeing stage as Azadirachta indica leaf powder and extract have their own color, which imparts a characteristic light greenish-yellow shade.

2.7 Shrinkage temperature determination

The sample was fixed between two clamps (one fixed and one movable) and soaked in according to water IUP-16 method, illustrated in Fig. S2. The water was heated slowly at rate of 2 °C/min until the desired temperature was reached, without any contraction noticed by the deflection of the dial needle of the SATRA STD -114 shrinkage tester machine (Fig. S2). The average of the two leather samples was taken as the final value after they were measured in parallel and perpendicular directions [21].

2.8 Application of finished film

The finishing was applied to improve the aesthetic look, enhance resistance to detergent and soap, and provide fastness to acid, alkalis, and water spots of leather prepared from Azadirachta indica leaf powder, extract, and conventional vegetable tannin materials. The same quantity and recipe for finishing were applied to both experimental crust leather samples, and the finishing formulation is listed in Table S1. Images of the crust leather and finished leather are illustrated in Fig. S3.

2.9 Physical characteristic assessments of leathers

The crust and finished leather samples were examined to determine various physical-mechanical properties, indicating the quality of the crust and finished leather produced. These tests were also carried out for conventional mimosa-tanned leather to compare the physical properties of leather tanned with powder and extract. Before testing at a temperature of 27 ± 1 °C and relative humidity of 65 ± 3 %, the leather samples were conditioned for 48 h. Physical properties of leather such as tensile strength, stitch tear strength, Bauman tear strength, grain crack resistance test, scuff resistance test and adhesion test were performed, and experimental details were discussed in the supplementary information section [22].

2.10 Anti-microbial assessment

The anti-microbial properties of the developed leather samples (diameter 20 mm) were assessed against gram-negative E. coli (ATCC 25922), gram-positive S. aureus (ATCC 25923) and gram-negative P. aeruginosa (ATCC 27583). Broth media were prepared using the disk diffusion method by dissolving 30 gm of dehydrated Tryptone Soya agar (TSA) into 1 L distilled water. A broth culture of S. aureus strain was fused by inoculating it in TSB and incubated at 37 °C for 24 h [23]. A 0.5 McFarland solution was formulated by mixing 0.05 ml of 1 % BaCl2 and 9.95 ml of 1 % H2SO4 followed by swabbing bacteria in Tryptone Soy Agar (TSA) media and incubating the inoculated media at 37 °C for 24 h.

3 Result and discussion

3.1 Extraction yield rate

The percentage of tannin extraction depends on the type of solvent and its nature. The highest extraction yield was obtained when ethanol was used as the extraction solvent, with yield of 15.5 % achieved within 90 min of extraction time, whereas 10.24 % extraction was found with water, as presented in Fig. 2. Around 11.34 % of extract was obtained with acetone, making it compatible with other solvents. Ethanol can permeate the extraction environment and has significant contact with the polar group of tannin molecules due to its ability to extract lower polyphenolic groups, polarity, and minuscule size compared to other solvents [24,25]. In this study, ethanol has demonstrated superior extraction efficiency other solvents (Fig. 2).Fig. 2 Extraction percentage (Mean ± SD) with various solvents.

Fig. 2

3.2 Identification tests for tannin in Azadirachta indica leaf powder, extract and mimosa

The presence of tannin in experimental samples was revealed qualitatively by FeCl3, FeSO4, gelatin, Pb (CH3COO)2, and k2Cr2O7, which exhibited the characteristic blue-black, dark purple, white precipitate, and yellowish-brown color in the extract solution respectively, as shown in Fig. S2 [26,27]. FeCl3 reacts with phenolic groups and forms black color, indicating the presence of tannin compounds (Table 2).Table 2 Qualitative confirmation tests of tannin in Azadirachta indica leaf powder, extract and mimosa.

Table 2Test	Leaf powder	Leaf extract	Mimosa	
Ferric chloride test	Blue-black	Blue-black	Blackish green	
Ferrous sulphate test	No apparent color change	Dark purple	Dark purple	
Gelatin test	White sediment	White sediment	White sediment	
Lead acetate test	White precipitate	White precipitate	White precipitate	
Potassium dichromate test	Yellowish brown	Reddish brown	Dark red color development	
Hydrolysable or condenseda	
Potassium hydroxide test	+	+	+	
a (+) sign denotes the presence of condensed tannin.

The change in appearance of solutions indicates that both the leaf and extract contain tannin compounds in their structure, which is consistent with the previous study. Similar results were observed for widely used mimosa in leather production [28].

The tannin agents were tested with potassium hydroxide to distinguish the types of tannin, i.e., hydrolysable or condensed. Three tannin solutions changed color to red when potassium hydroxide was added dropwise, indicating the presence of condensed tannin, as shown in Table 2 [29].

3.3 Chemical analysis of tannin powder

The low moisture contents of A. indica leaf powder, extract, and mimosa i.e., 7.5 %, 8.5 %, and 8.05 % signify that tannin power will not aggregate easily in the packaging bag and will absorb low moisture during storage, while higher moisture content favors the growth of microorganism, thus lessening the storage life. The pH of vegetable tannin materials plays a vital role in determining tanning efficiency. Inadequate penetration, fixation, wrinkles, and loose grains of leather will result if the pH of tannin is too low, while too high pH may hinder the fixation of tannin materials onto the leather surface. pH around 4–6 of vegetable tanning float is considered suitable for achieving optimum vegetable tanning efficiency [30]. A. indica leaf powder, extract, and mimosa had pH values of 4.89, 4.81, and 4.87, respectively, within the range suitable for active tanning, as listed in Table 3.Table 3 Chemical analysis of different tannins.

Table 3Parameters	Leaf powder	Leaf extract	Mimosa	
Moisture content	7.5 %	8.5 %	8.05 %	
pH	4.89	4.81	4.87	
Total soluble solid (TSS)	20 %	24 %	23 %	
Total solid (TS)	92.5 %	91.5 %	91.95 %	
Tannin (T%)	9.63 %	12.34 %	64.52 %	
Non-tannins (NT%)	15.47 %	6.83 %	27.16 %	
Tanning strength (T/NT)	0.622	1.81	2.38	
Tannin purity (T/TSS)	0.48	0.51	2.80	

The total soluble solid (TSS) of A. indica leaf powder and extract and mimosa were 20 %, 24 % and 23 % respectively, suggesting that A. indica leaf extract would be suitable to use as vegetable tannin because soluble particles could easily penetrate the collagen matrix as well as produce less sludge in the bath liquor (Table 3). Leaf extract effluent leaves little or negligible residues in the environment because they degrade quickly. There were no significant differences between experimental tannin agents in the case of total solid, where maximum solid content was found in neem powder and the lowest total sloid content was found in A. indica leaves extract. The Percentage of tannin (T%) in A. indica leaf powder and extract was observed to be 9.63 % and 12.34 % respectively, implying the extract is more appropriate to use in leather tanning, which could convert putrescible raw hides and skins into durable leather (Table 3) [[31], [32]]. The body, fullness, and compactness of vegetable tanned leather depend on the quantity of non-tannins present in leather samples. The non-tannin content (NT%) of A. indica leaf powder, and extract was measured at 15.47 % and 6.83 %, however, mimosa has substantial amount of non-tannin compared to both powder and extract (Table 3). The tanning strength of A. indica leaf extract was better than powder but lower than mimosa, falling within the commercially acceptable range of 1.5–3. Since the tannin content of A. Indica leaf extract is lower than mimosa, thus it could be used combinedly with syntan to ensure the desired properties of leather. The purity of tannin influences the ability to bind tannin agents with collagen protein and reactivity. The tannin purity of A. indica extract (0.51) was higher than powder (0.48). It is reported that active tannin materials should have tanning purity between 0.5 and 1.0. Conversely, the control mimosa has high tannin purity this could be due to various factors such as soil type, pH of the soil, climate, plant age, and extraction methods [33,34].

3.4 DLS study

The hydrodynamic diameter of NE, NP and mimosa powder was found to be 86.71 nm, 182.7 nm and 188.75 nm respectively, while the polydispersity of those materials was 26.63 %, 23.43 % and 21.64 % respectively (Fig. 3).Fig. 3 DLS analysis of Azadirachta indica powder (NP), extract (NE) and mimosa.

Fig. 3

The particle size of NE was significantly smaller than that of NP and mimosa, indicating that the extract would have better penetration power than the powder. Moreover, the polydispersity values of both NP and NE materials indicate good distribution of particle sizes and low chance of aggregation in the bath liquor. The small particle size of NE would facilitate the filling of micro-porous leather surfaces during the tanning process [34].

3.5 FTIR analysis of tannins

FTIR was performed to study the functional groups present in A. indica leaves powder in comparison to mimosa and quebracho, which are commonly used in both retan and full vegetable tanning (Fig. 4). The polyphenolic tannin compound of NE and NP could be responsible for the intense broad bands at 3276 cm−1 and 3281 cm−1, assigned -OH stretching of phenolic groups (Fig. 4a and b). A. indica leaf extract and powder both exhibited bands at 2925 cm−1 and 2920 cm−1 bands for -C-H stretching, implying that condensation tannin might be present in these tannins. Signals at 1600 cm−1 and 1607 cm−1 may be associated with stretching the C=C aromatic ring of NE and NP, demonstrated in Fig. 4(a and b). Substantial bands at 1238 cm−1 and 1269 cm−1 respectively, are consistent with previous reports for aromatic stretching of -C-OH for NP and NE vegetable tannin agents.Fig. 4 FTIR of a) A. Indica extract, b) A. Indica powder, c) quebracho and d) mimosa.

Fig. 4

Condensed tannin is implied by the appearance of subtle signals emanating from asymmetric C-O-C cyclic ether at 1156 cm−1 and 1173 cm−1 for NP and NE (Fig. 4a and b). Bands were observed for NE, NP, quebracho, and mimosa at 1036 cm−1,1031 cm−1,1030 cm−1,1020 cm−1 -C-H deformation reported similar findings as shown in Fig. 4(c, d) [35]. A. indica leaf powder (NP) showed the bands at 1238 cm−1, and 1156 cm−1, suggesting the presence of flavonoid-based condensed tannin groups. The Peak found around 1035 cm−1 attributed to the aromatic ring for A. indica extract, quebracho, and mimosa powder. The analogous peaks of 1515 cm−1 and 1507 cm−1 for quebracho and mimosa tannins support the confirmation of C=C skeletal ring stretching illustrated in Fig. 4(c and d). Sharp peaks were observed for all four samples because of OH stretching, C=C stretching, and C-H bending [36]. Bands around 3276 cm−1,3281 cm−1,3236 cm−1, and 3232 cm−1 of NE, NP, quebracho, and mimosa are consistent with previous research, assigned the -OH stretching vibration in phenolic and aliphatic structure in the range of 3500–3000 cm−1. Unlike the sharp peak of A. indica leaf powder (NP), the Board -OH peak is attributed to A. indica leaf extract, mimosa, and quebracho presented in Fig. 4.

Just after the pickled pelt reacted with NET, there was a substantial distinction in bands, as present in Fig. 5 (a). Pickle pelt before tanning exhibits characteristic peaks at 1236 cm−1,1444 cm−1, 1549 cm−1, 1636 cm−1, 3291 cm−1, which shifted to 1242 cm−1, 1456 cm−1, 1551 cm−1, 1637 cm−1,3302 cm−1 when pickle pelt is treated with developed A. indica leaf extract, suggesting that there might have been some bonds formed between neem tannin molecules and functional groups of the leather surface (Fig. 5(a)) [36]. Typical characteristic bands between 1615 cm−1 -1030 cm−1 of NET indicate that leather has been vegetable tanned. Amide I and amide II bands were also seen in pickled pelt, consistent with previous researchers' findings.Fig. 5 FTIR of a) pickle pelt and NET tanned leather and b) CVT, NPT and NET tanned leather.

Fig. 5

Furthermore, CVT, NPT and NET tanned leather showed brands at 3297 cm−1, 3286 cm−1 and 3295 cm−1, attributing O-H stretching. Almost identical peaks were seen for C-N-H bending in CVT, NPT and NET tanned leather at 1640 cm−1, 1636 cm−1 and 1627 cm−1 respectively, shown in Fig. 5(b). Peaks were observed at 1450 cm−1, 1446 cm−1 and 1447 cm−1 attributed to C-H bending for CVT, NPT and NET leather. Bands around 1236 cm−1 and 1232 cm−1 for CVT and NPT leather attribute the C-O stretching from ester [37]. Bands at 1032 cm−1,1031 cm−1 and 1032 cm−1, owing to (C-O-C=O), confirmed the presence of ester from tannins in all three leather samples (Fig. 5(b)) [7].

3.6 TGA and DSC analysis of tanned leather

Thermogravimetric analysis (TGA) is a practical technique used to characterize leather's thermal behavior by censoring changes in weight as a function of temperature in an inert nitrogen atmosphere. It provides insight into moisture content (bound and unbound water), thermal decomposition, and the presence of organic or inorganic residuals within the collagen matrix. Correlation can be drawn between weight changes at different temperature ranges [38]. When tannin molecules occupy the position of water, displacing them during leather processing, thermal stability is elevated compared to raw hides and skins, where water remains between the interfibrillar spaces [39]. The main degradation occurs between 200 °C to 450 °C due to the loss of carbon dioxide from the organic materials of collagen and pyrrol from protein might have been decomposed in the range of 300–450 °C presented in Fig. 6(d). However, the conventional vegetable-tanned sample (CVT) showed better thermal resistance than the experimental samples in the range of 350oC–600 °C because full vegetable-tanned leather contains numerous phenolic and aromatic polymers that perhaps dissipate heat across the collagen matrix, unlike A. indica leaf powder-tanned leather (NPT) and extract (NET) tanned leather (Fig. 6 d). Leather samples underwent two major phases of thermal deterioration at 100 °C–500 °C and 500 °C–650 °C, the first of which is endothermic, and the latter phase is an exothermic process depicted in Fig. 6 (d). Lower molecular cellulosic materials, organic substances, and non-tannin residues might have reduced the thermal stability of NET and NPT leather samples illustrated in Fig. 6 (a &b) [40].Fig. 6 DSC of a) NET, b) NPT, c) CVT and TGA of d) NPT, CVT and NET leather respectively.

Fig. 6

Around 6.22 %, 7.59 %, and 10.07 % mass were lost during the first 50 to 150ᵒC for NET, CVT, and NPT indicating that A. indica leaf powder may contain higher amount of volatile organic matter and denatured collagen structure has occurred during this stage [41]. Between 150 °C to 450ᵒC temperature, NET, NPT, and CVT leather underwent weight loss of approximately 51.25 %, 57.06 % and 46.57 % signifying that powder-tanned leather is less stable than CVT leather (Fig. 6a–c). At elevated temperatures, for instance, 450ᵒC to 650ᵒC, NET leather mass loss was found to be 6.37 % resulting from the decomposition of the carbon backbone as well as organic substances. 36.31 %, 27.22 %, and 46.07 % residual mass remained at temperature of 650 °C of NET, NPT, and CVT leather, demonstrating that NET and CVT leather samples have better thermal stability, as presented in Fig. 6(a–c). Denaturation of temperatures of NET, NPT, and CVT leather was observed at 84.75 °C, 79.52 °C, and 80.18 °C respectively, which are consistent with the shrinkage temperature measured in the current study, as presented in Fig. 6.

3.7 HPLC study of extract

Specific polyphenolic compounds in A. indica leaves extract and mimosa were measured and identified by HPLC. A total of sixteen standard polyphenolic compounds were examined (Fig. 7(a)), where five polyphenolic compounds were recognized in both A. Indica (Fig. 7(b)) and mimosa extract (Fig. 7(c)).Fig. 7 HPLC chromatogram of a) Standard polyphenolic mixture (1. Gallic acid, 2. 3,4-Dihydroxybenzoic acid, 2. Catechin hydrate 3. Catechol, 4. (−) Epicatechin, 5. Caffeic acid, 6. Vanillic acid, 7. Syringic acid, 8. Rutin hydrate, 9. P-Coumaric acid, 10. Trans-ferulic acid, 11. Rosmarinic acid, 12. Myricetin, 13. Quercetin, 14. Trans-cinnamic acid, 15. Kaempferol b) HPLC chromatogram of A. indica leaf extract (1. Gallic acid, 2. Catechin hydrate, 3. Catechol, 4. Rutin hydrate, 5. P-Coumaric acid) c) HPLC chromatogram of mimosa (1. Catechin hydrate, 2. Catechol, 3. (−) Epicatechin, 4. Vanillic acid, 5. Syringic acid 6. Myricetin) d) Polyphenolic compounds in A. indica.

Fig. 7

The high content of catechin hydrate, catechol, and rutin hydrate in the ethanolic Azadirachta indica leaf extract indicates the condensed type of tannin, which aligns with previous research [42]. At least one or more phenolic groups present in gallic acid, catechin, catechol, rutin and p-coumaric acid, make A. indica leaf extract commercial tanning material displayed in Fig. 7 (d). High content of gallic acid (127 mg/100g dry extract), catechin (184 mg/100g dry extract), catechol (258 mg/100g dry extract), rutin (26 mg/100g dry extract), p-coumaric acid (12 mg/100g dry extract), and trace amount of quercetin were identified during the HPLC analysis of A. indica leaf extract (Fig. 7 b). Conversely, mimosa extract contained catechin hydrate (190 mg/100 dry extract), catechol (270 mg/100g dry extract), (−) epicatechin (25 mg/100g dry extract), vanillic acid (118 mg/100 dry extract), and myricetin (15 mg/100g dry extract), respectively demonstrated in Fig. 7(c). The hydroxy groups of gallic acid, catechin, catechol, rutin, and p-coumaric acid could be involved with the functional group of collagens during the tanning process, making it a suitable tannin agent because of its propensity to react with collagen [43].

3.8 Chemical analysis of leather

The moisture content of NPT, NET, and CVT tanned leather was 11.3 %, 12.26 %, and 11.28 % respectively, implying suitability for use, transport, and resistance to bacterial, mold and fungal attacks listed in Table 4. On the other hand, leather should contain an amount of moisture after drying, as this contributes to the degree of softness of final leather [44].Table 4 Chemical properties of tanned leather.

Table 4Chemical test	NPT leather	NET leather	CVT leather	
Moisture content	11.3 %	12.26 %	11.28 %	
Fat content	11.6 %	10.8 %	10.4 %	
Ash content	8.93 %	7.14 %	9.48 %	
Water soluble content	6 %	7.2 %	8 %	

The low ash content of NET leather alludes to minimum of inorganic substances, sulfur, and trace elements when compared to NPT, and CVT leather. The fat content of leather depends on the nature of tannage and the tanning agents applied and influences the elongation and tear strength of leather. The fat content of NPT, NET, and CVT leather was measured to be 11.6 %, 10.8 %, and 10.4 %, respectively, representing fiber that could slide over between them, enhancing the flexibility of leather without looseness (Table 4). On the other hand, if leather does not have sufficient quantity of fat in it, crankiness and stiffness could develop when the leather is flexed or bent [45]. Fibers tend to stick together during drying, which was prevented by the presence of fat content in leather. The water-soluble content of NPT, NET, and CVT leather was 6 %, 7.2 % and 8 % distinctively, whereas native hides and skins have water-soluble content of around 11 % (Table 4). The low water-soluble content of NPT and NET leather implies good water resistance properties as compared to CVT leather enumerated in Table 4 [46].

3.9 Physio mechanical strength

Resistance to damage, body, fullness, compactness, and mechanical properties of leather are influenced by the thickness of leather to a certain extent. Leather produced by the extract has more thickness than both NPT and CVT leather (Table 5).Table 5 Physio-mechanical strength of leather.

Table 5Test	NPT	NET	CVT	ISO standard [47]	Methods [47]	
Thickness (mm)	0.84	0.91	0.82	–	IUP-4	
Tensile strength(kg/cm2)	298.1	282.52	300	Min. 200	IUP-6	
Elongation	38.72 %	40.71 %	37.29 %	30–40 %	IUP-6	
Stitch tear strength (kg/cm)	138.31	139.53	136.34	100	SATRA PM-5	
Bauman tear strength (kg/cm)	148.65	146.9	143.86	Min. 30	IUP-8	
Grain crack load (kg)	26	24	29	Min. 16	IUP-9	
Distension (mm)	7.9	7.7	8.23	Min. 7	IUP-9	
Scuff resistance test (400 cycles) (mm2)	No scratch	No scratch	No scratch	<3	SATRA PM-140	
Flexing endurance (1,00,000 cycles)	No damage	No damaged	Cracked developed	No damage or crack	IUP-20		
Bond strength of finished film (gm/cm)	400	414	405	Min. 200	SATRA AM -8		

Tensile strength gives an overall impression of the strength of leather, more precisely the fiber strength of the collagen, which is crucial to ensure the quality of the final leather goods [48]. The degree of tensile strength is determined by the density of collagen fibers found in specific skin types along with the type of tanning it underwent. Leather tanned with the conventional vegetable tanning process (CVT) and A. indica leaves extract-tanned leather (NET) exhibited 300 kg/cm2 and 282.52 kg/cm2 respectively, while A. indica leaf powder-tanned leather (NPT) 298.1 kg/cm2 (Table 5). Vegetable tannins, when reacting with protein fiber, strengthen the bonding of skin's fibers and solidify their structure; subsequently, cross-linking of tannins with collagen fibers causes an increase in tensile strength [49].

The percentage of elongation of leather indicates endurance to stress when load is applied. A Certain percentage of elongation of leather is required for garments and shoe upper leather because those types of leather are subjected to significant stress during manufacturing and service life. Both NPT and NET showed fair percentage of elongation compared to CVT, although all the leather met the minimum ISO standard. According to ISO, good-quality upper leather should have percentage elongation not below 30 %. Pretanning, tanning, and post-tanning processes have an impact on leather elongation, which varies from tanner to tanner [50].

When leather was treated with the extract infusion of A. indica leaf powder i.e., NET, its stitch tear strength was found to be 139.53 kg/cm, whereas NPT and CVT stitch tear strength were 138.31 kg/cm and 136.34 kg/cm respectively, as listed in Table 5. NET exhibited high stitch tear strength, indicating that fibers are compact, which will retain thread firmly during the manufacturing of shoes or leather goods as well as in shelf life [51]. Fiber strength and seam strength of leather samples can be implied by stitch tear strength. Stitch tear strength gives an idea regarding the strength of the fibers bundle and the firmness of its fibers after the tanning process [52]. Grain crack resistance of NPT, NET, and CVT leathers was 26 kg, 24 kg, and 29 kg respectively, satisfying the minimum ISO standard of 16 kg (Table 5). High grain crack resistance ensures that leathers do not rapture in any mechanical operation during lasting operation in shoe production. The distension of the three experimental leathers acceded to 7 mm in the lastometer test. No visible scratches were seen for all three leathers after performing the scuff resistance test, fulfilling the minimum ISO standard of <3 mm2 after 400 cycles for shoe upper leather (Table 5). Scratch resistance is one of the vital properties of shoe upper, garment, upholstery leather and so on because if the leather is prone to scratches, suppleness would be largely impeded during daily use [47]. Resistance to flex is one of the vital properties that shoe upper, lining, and other heavy-duty leather must possess to prevent any damage or wrinkle development during use.

After a defined number of cycles, leather samples were visually assessed, with no damage or crack observed for NET and NPT; however, visible cracks were observed in the case of CVT, which failed to attain the minimum ISO standard when leather samples were subjected to the flexing endurance test. The retention of the finished film on crust leather depends on the type of leather, finishing chemicals, and tanning methods [53]. The finish film adhesion of CVT was 405 gm/cm, while NET leather showed an improved value of 414 gm/cm compared to other experimental leathers, though all leather samples met the minimum ISO standard of 200 gm/cm, as shown in Table 5 [47].

3.10 Shrinkage temperature analysis

The shrinkage temperature (Ts), which indicates the breakdown of the hydrogen bonds between collagen polypeptide chains as a result of wet heat and causes leather to shrink by one-third of its initial length, is a commonly used criterion for determining the completion of the tanning process and provides a full picture of tanning efficiency [54]. When collagen is treated with a wide range of tanning agents during the tanning process, it raises the resistance to the wet heat of raw hides and skins, making collagen more stable. Ts gives an indication of nature and the number of cross-linkages present in the leather structure [55]. Subjected experimental leathers to wet heat, amino acid structures are altered leading to the reduction of the structural integrity of the protein, resulting in cleavage of the peptide chains thus the collapse of collagen structure.

The shrinkage temperature of NPT, NET, and CVT leather was found to be 81 °C, 86 °C, and 84 °C respectively, which are comparable with previous studies as shown in Fig. 8. An increase in intermolecular cross-linking due to hydrogen bonding interactions between tannin complexes and polypeptide chains (-CO-NH-) of collagen protein leads to high Ts. Furthermore, synergistic interaction between the polyphenols of neem extract with collagen protein formed matrixes within the collagen matrixes, stabilized the collagen by the multiplicity of connected hydrogen bonds in the new macromolecule that resulted in the improved hydrothermal stability of the experimental leathers [56]. Tannin molecules of leaf extract cross-link with different functional groups of collagens, reduce the entropy, form new types of bonds between the adjacent polypeptide chains of collagen, and create irreversible structure hence Ts increases. Another reason for high Ts could be due to the long-range ordering of the matrix formed by establishing cross-links, which alters the skin tertiary structure of collagen protein [57]. Higher Ts of NPT and NET-tanned leather might be attained through crosslinking more collagen with the large high content of catechin, and rutin, in NET, which have several phenolic hydroxyl groups in their structure, unlike mimosa [29].Fig. 8 Shrinkage temperature of different leather.

Fig. 8

When neem extract was used in the retanning step after pre-tanning with chromium, the shrinkage temperature of chrome retan (CR) rose to 106 °C, as illustrated in Fig. 8. Furthermore, when neem extract was applied in the pre-tanning step and retanning was done with basic chromium sulphate, the shrinkage temperature of the semi chrome (SC) leather was recorded at 98 °C. NET leather exhibited higher Ts than standard vegetable-tanned leather, which is typically in the range of 75–84 °C, as reported by Covington [58].

3.11 Anti-microbial properties

Raw hides and skins are susceptible to microorganism attack, degrading not only the quality of raw hides and skins but also finished and crust leather. A. indica leaf powder and extract possess antimicrobial attributes that help preserve crust and finished leather for extended periods without any signs of putrefaction due to bacterial, fungal, and mold attacks [59]. The zone of inhibition of NET leather was visible against S. aureus, while NPT and CVT did not exhibit any signs of susceptibility, as depicted in Fig. 9. No clear inhibition zone was seen in CVT, whereas NET leather had clear zones around the surface compared to NPT leather against P. aeruginosa and E. coli. The bacterial growth of S. aureus was less than that of E. coli around the NET leather surface because of the different cell compositions of gram-positive and gram-negative bacteria.Fig. 9 Anti-microbial properties of NPT, NET and CVT leather against (a) Staphylococcus aureus (b) Pseudomonas aeruginosa (c) Escherichia coli.

Fig. 9

Passive diffusion of tannin molecules in NET leather penetrates the peptidoglycan layer of the protein structure of S. aureus and P. aeruginosa, preventing bacterial growth and hence exhibiting antimicrobial effects. A thicker peptidoglycan layer of gram-positive bacteria such as S. aureus and lipopolysaccharides outer membrane of gram-negative such as P. aeruginosa are also damaged by the tannin compounds present in NPT and NET leather. Generally, polyphenolic compounds of NPT and NET leather amalgamate with bacterial protein irreversibly and form complexing within bacterial cells, in doing so tannins neutralize bacterial activities. Moreover, tannin compounds present in the NEP and NPT not only interfered with and damaged the amino acids metabolism but also disrupted the enzymatic action of bacteria. As a result of deactivation of enzymatic action and breaking of the cross-link of peptidoglycan, thus NEP and NET leather greatly reduced microbial activity and inhibited growth. Color bleeding and sweating were noticed in CVT leather (Fig. 9) for S. aureus, P. aeruginosa, and E. coli, which may be due to the migration of color from the flesh surface of the leather [60].

3.12 SEM images of pickle pelt and tanned leather

The stereo microscope and SEM images were taken before and after the tanning process with extract to observe the instance changes on the pelt surfaces. The pickle pelt is white, flaccid, and acid saturated (Fig. 10 a), where hair pores could be visible under the SEM (Fig. 10 c). Subsequently, darker-tanned leather was depicted in Fig. 10 (b) while the final appearance was seen under electron microscopy (Fig. 10 d) after A. Indica tanning.Fig. 10 SEM images of pickle pelt before (a & c) and right after vegetable tanning (b &d) (100x) with prepared tannin extract.

Fig. 10

Fibers of NET leather are well opened up, as seen in Fig. 11(a–b) because A. indica leaf extract has been properly dispersed onto the leather surface. On the contrary, fibrils and fiber are visible in Fig. 11(a–b), which could be due to proper cross-linking and dispersion rate between the extract and the fiber. Cross-sectional SEM images of NPT in Fig. 11(c–d) show void spaces between collagen fibers filled with minuscule A. indica vegetable tanning agents, thus stabilizing the collagen structure. Fig. 11(e–f) represents the morphology of mimosa tanned leather (CVT), which appears compact, causing stiffness, and incomplete fiber opening is visible.Fig. 11 Cross-sectional SEM images of NET(a-b), NPT(c-d) and CVT (e–f) leather.

Fig. 11

3.13 Possible mechanism of extramental tannins with collagen

When A. indica leaf extract is added to pickle pelt, significant number of bands shift, as presented in Fig. 5(a), implying new bond formation between the polyphenolic compounds of extract and active sites of the collagen matrix. Vegetable tanning occurs in two simultaneous processes. During the first stage, tannins are dispersed into the collagen fiber and fill the inter-fibrillary spaces, while in the second stage, tannins combine with protein [58]. The existence of -OH groups in vegetable tannin promotes interaction with the nitrogen of amino groups (-NH2) and the oxygen of the peptide link (-CO-NH-) of the polypeptide side link of collagen, modifying the properties, and hence stabilizing the collagen structure (Fig. 12) [59].Fig. 12 Interaction between tannins and collagen.

Fig. 12

Furthermore, polyphenols can create bridges with (-NH2) of side chains, which is one of the reasons for the elevated shrinkage temperature compared to raw hides and skins [61]. Not only are hydrogen bonds of developed tannin agents from A. indica leaves involved, but also Van Dar Waals forces and other weak forces are entailed during cross-linking with collagen. In addition to bond formation, some of the non-tannin molecules of the developed tannin agents are physically held between void spaces on the leather surface, replacing water molecules. Both A. indica leaf powder and extract contain polyphenolic groups that facilitate bond formation between the functional groups of collagen and the prepared tannin molecules through weak hydrogen links and hydrophobic bonds [62]. In this study, both powder and extract absorbed and formed bonds within and between collagen fibers, subsequently improving the mechanical properties of the leather samples illustrated in Fig. 12.

4 Conclusion

This study investigated the availability of utilizing A. indica leaf as an alternative vegetable tanning material, intending to replace conventional import-oriented chemicals. The abundance of A. indica trees in Bangladesh makes them easily accessible for tanning purposes. However, it is important to note that while powder and extract may play a role in improving the chemical properties, physical strength, and shrinkage temperature of experimental leathers, other synthetic tanning agents (syntans) commonly used in leather processing could have also contributed to some extent. A. indica leaf extract exhibited superior performance compared to leaf powder-tanned leather in terms of physio-mechanical, shrinkage temperature, tannin content, and fastness properties, which are comparable to standard full vegetable leather. Nonetheless, leather tanned with leaf powder proved to be hard, lacking body, prone to fading of the base color, and exhibited inadequate flexibility, unlike A. indica leaf extract-tanned leather. However, while some of the chemical and physical characteristics of A. indica leaf extract-tanned leather were lower than those of traditional vegetable-tanned leather, they still met the minimum standard value. High-quality NET leather can be utilized as shoe upper leather, bag leather, medium leather goods, and garments leather, while NPT leather might find application in belt leather, lining leather, and the production of small leather goods. The powder form of A. indica leaf extract offers easy handling, ensures safety for tannery staff, and its simplified application process facilitates industrial-scale production. The application of organic-based tanning agents in leather processing from indigenous plant sources could make sustainable leather processing, maintaining mechanical and other desirable properties in the coming years. Utilizing Azadirachta indica leaf in leather processing will serve as a viable option that paves the way for economically and environmentally friendly leather production. Further investigations are needed to modify prepared tanning agents by incorporating suitable functional groups to enhance overall tanning performance.

Funding

This study was financed by the 10.13039/501100008804 Ministry of Science and Technology (Applied Science and Engineering Group, serial no. 11 and registration no. 19, R & D project 2022–2023) Bangladesh.

Availability of data and materials

This manuscript contains the raw images, and data generated or analyzed during this study.

CRediT authorship contribution statement

Saidur Rahman Shakil: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization. Fatema Tuz Johura Zenith: Resources, Methodology, Investigation, Conceptualization. Mahabubur Rahman Khan: Validation, Software, Resources. Waliur Rahman Tonay: Resources, Investigation, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A Supplementary data

The following is the Supplementary data to this article:Multimedia component 1

Multimedia component 1

Acknowledgment

Authors expressed their gratitude to the Institute of Leather Engineering and Technology and the Department of Microbiology, University of Dhaka for providing their lab facilities.

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.heliyon.2024.e36270.
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References

1 Sivakumar V. Towards environmental protection and process safety in leather processing – a comprehensive analysis and review, Process Safety and Environ Protect. Met. 163 2022 10.1016/J.PSEP.2022.05.062
2 Gao D. Cheng Y. Wang P. Li F. Wu Y. Lyu B. Ma J. Qin J. An eco-friendly approach for leather manufacture based on P(POSS-MAA)-aluminum tanning agent combination tannage J. Clean. Prod. 257 2020 10.1016/j.jclepro.2020.120546
3 Ahmed S. Tuj-Zohra Fatema Khan Md.S.H. Hashem Md.A. Chromium from tannery waste in poultry feed: a potential cradle to transport human food chain Cogent Environ. Sci. 3 1 2017 1312767 10.1080/23311843.2017.1312767
4 Humayra S. Hossain L. Hasan S.R. Khan M.S. Water footprint calculation, effluent characteristics and pollution impact assessment of leather industry in Bangladesh Water (Switzerland) 15 3 2023 10.3390/w15030378
5 Mohammed S.A. Naisini A. Madhan B. Demessie B.A. Rumex abyssinicus (mekmeko): a newer alternative for leather manufacture Environ. Prog. Sustain. Energy 39 6 2020 10.1002/EP.13453
6 Hashem M.A. Islam A. Mohsin S. Nur-A-Tomal M.S. Green environment suffers by discharging of high-chromium-containing wastewater from the tanneries at Hazaribagh, Bangladesh Sustain. Water Resour. Manage. 1 4 2015 343 347 10.1007/S40899-015-0033-4
7 Das A.K. Islam M.N. Faruk M.O. Ashaduzzaman M. Dungani R. Review on tannins: extraction processes, applications and possibilities, South Afr J. of Botany 135 2020 58 70 10.1016/j.sajb.2020.08.008
8 Fedorov V.S. Ryazanova T.V. Bark of Siberian conifers: composition, use, and processing to extract tannin Forests 12 8 2021 10.3390/f12081043
9 Nalyanya K.M. Rop R.K. Onyuka A.S. Birech Z. A review of natural plants as sources of substances for cleaner leather tanning technologies, Textile and Leather Rev 4 3 2021 137 148 10.31881/TLR.2021.03
10 Muralidharan V. Palanivel S. Balaraman M. Turning problem into possibility: a comprehensive review on leather solid waste intra-valorization attempts for leather processing J. Clean. Prod. 367 2022 10.1016/j.jclepro.2022.133021
11 Conde M. Combalia F. Baquero G. Ollé L. Bacardit A. Exploring the feasibility of substituting mimosa tannin for pine bark powder. ALCA perspective Clean. Eng. Technol. 7 2022 10.1016/j.clet.2022.100425
12 Kusuma S.B. Wulandari S. Nurfitriani R.A. Awaludin A. The potential solvent for tannin extraction as a feed additive made of coffee husk (Coffea canephora) using Soxhlet method IOP Conf. Ser. Earth Environ. Sci. 980 1 2022 012024 10.1088/1755-1315/980/1/012024
13 Bhowmik S. Chowdhury S.D. Kabir M.H. Ali M.A. Chemical composition of some medicinal plant products of indigenous origin Bangladesh Vet. 25 1 2008 32 39 10.3329/bvet.v25i1.4616
14 Ishaq R. Chand N. Khan R.U. Saeed M. Laudadio V. Tufarelli V. Methanolic extract of neem (Azadirachta indica) leaves mitigates experimentally induced coccidiosis challenge in Japanese quails J. Appl. Anim. Res. 50 1 2022 498 503 10.1080/09712119.2022.2096037
15 Kim W.H. The methanol extract of Azadirachta indica A. juss leaf protects mice against lethal endotoxemia and sepsis Biomol. Ther. 20 1 2012 96 103 10.4062/BIOMOLTHER.2012.20.1.096
16 Antwis-Boasiatao C. Animapauh S.O. Tannin Extraction from the barks of three tropical hardwoods for the production of adhesives J. Appl. Sci. Res. 8 6 2012 2959 2965 http://www.aensiweb.com/jasr/jasr/2012/2959-2965.pdf
17 Sousa T.B. Souza S.G. Franco T.B.B. Jesus M.S. Mori F.A. Quantification of tannins from curupay bark Floresta e Ambiente 26 1 2019 10.1590/2179-8087.008216
18 Unango J.F. Duraisamy R. Ramasamy K.M. Birhanu T. Shukla S.K. Characteristics and tanning potential of Hagenia abyssinica tannin extracts and its possible use in clean leather production Cogent Eng. 8 1 2021 10.1080/23311916.2021.1993520
19 Cassano A. Leather Industry, Unhairing-Liming vols. 095–1096 2016 Encyclopedia of Membranes 10.1007/978-3-662-44324-8-1349
20 Biškauskaitė R. Valeikienė V. Valeika V. Enzymes for leather processing: effect on pickling and chroming Materials 17;14 6 2021 1480 10.3390/ma14061480
21 Nasr A.I. El Shaer M.A. Abd-Elraheem M.A. Potential application of used coffee grounds in leather tanning J. of Ecology Eng. 24 3 2023 10.12911/22998993/157388
22 China C.R. Nyandoro S.S. Munissi J.J.E. Maguta M.M. Meyer M. Schroepfer M. Tanning capacity of Tessmannia burttii extracts: the potential eco-friendly tanning agents for the leather industry J. of Leather Sci. and Eng. 3 1 2021 10.1186/s42825-021-00055-2
23 Elgailani I.E.H. Ishak C.Y. Determination of tannins of three common Acacia species of Sudan, advan Chemistry 2014 1 5 10.1155/2014/192708
24 El-Monem F.A. Hussain A.I. Nashy E.S.H.A. El-Wahhab H.A. Naser A.E.R.M. Nano-emulsion based on acrylic acid ester co-polymer derivatives as an efficient pre-tanning agent for buffalo hide Arab. J. Chem. 10 2017 10.1016/j.arabjc.2014.05.024
25 Cheloti M. Evaluation of mechanical properties of goat leather tanned using Acacia xanthophloea Textile and Leather Rev 6 2023 10.31881/TLR.2023.053
26 Nalankilli G. Tadesse K. Antimicrobial cotton textiles by finishing with extracts of an Ethiopian plant (Solanum incanum) fruit Afr. Rese. Rev. 12 2 2018 143 10.4314/AFRREV.V12I2.14
27 Santos K.S. Antiproliferative activity of neem leaf extracts obtained by a sequential pressurized liquid extraction Pharmaceuticals 11 3 2018 10.3390/PH11030076
28 Hashim N. Abdullah S. Hassan L.S. Ghazali S.R. Jalil R. A study of neem leaves: identification of method and solvent in extraction Mater. Today Proc. 42 2021 217 221 10.1016/J.MATPR.2020.11.726
29 Das R.K. Mizan A. Zohra F.T. Ahmed S. Ahmed K.S. Hossain H. Extraction of a novel tanning agent from indigenous plant bark and its application in leather processing J. of Leather Sci. and Eng. 4 1 2022 10.1186/s42825-022-00092-5
30 Abdulla-Al-Mamun M. Sarker B. Islam D. Rahman A. Novel vegetable tanning techniques by notholithocarpus densiflorus extract and fatliquoring with indigenous bovine fat Textile and Leather Rev 6 2023 10.31881/TLR.2023.006
31 Ayalew A.A. Wodag A.F. Extraction and chromatographic analysis of Ethiopian oak bark plant for leather tanning applications Chem. Afr 6 3 2023 10.1007/s42250-022-00580-5
32 Chowdury M.J. Razzaq Md.A. Biswas Md.I. Quadery A.H. Uddin Md.T. Extract of trema orientalis (L.) stem bark: a potential source of environmentally friendly tanning agent for leather industry J. Am. Leather Chemists Assoc. 117 1 2022 10.34314/jalca.v117i1.4696
33 Bhargavi N.R.G. Jayakumar G.C. Sreeram K.J. Rao J.R. Nair B.U. Towards sustainable leather production: vegetable tanning in non-aqueous medium J. of America. Leather Chem. Assoc. 110 4 2015 97 102
34 John Unango F. Duraisamy R. Ramasamy K.M. Birhanu T. Characteristics and tanning potential of Hagenia abyssinica tannin extracts and its possible use in clean leather production Cogent Eng. 8 1 2021 10.1080/23311916.2021.1993520
35 Unango F.J. Duraisamy R. Ramasamy K.M. Birhanu T. Characteristics and tanning potential of Hagenia abyssinica tannin extracts and its possible use in clean leather production Cogent Engineering 8 1 2021 1993520 10.1080/23311916.2021.1993520
36 Dayanidhi P.D. Anithabanu P. Vaidyanathan V.G. Studies on stabilization of collagen using Cr-doped polydopamine complex Biophys. Chem. 292 2023 10.1016/j.bpc.2022.106917
37 Falcão L. Araújo M.E.M. Application of ATR-FTIR spectroscopy to the analysis of tannins in historic leathers: the case study of the upholstery from the 19th century Portuguese Royal Train Vib. Spectrosc. 74 2014 10.1016/j.vibspec.2014.08.001
38 Grasel F.D.S. Ferrão M.F. Wolf C.R. Development of methodology for identification the nature of the polyphenolic extracts by FTIR associated with multivariate analysis Spectrochim. Acta Mol. Biomol. Spectrosc. 153 2016 94 101 10.1016/J.SAA.2015.08.020
39 Koochakzaei A. Sabaghian M. Tannin characterization and sourcing in historical leathers through FTIR spectroscopy and PCA analysis Collagen and leather 5 1 2023 10.1186/s42825-023-00128-4
40 Hu Y. Liu J. Luo L. Li X. Wang F. Tang K. Kinetics and mechanism of thermal degradation of aldehyde tanned leather Thermochim. Acta 691 2020 178717 10.1016/J.TCA.2020.178717
41 Zhang C. Lin J. Jia X. Peng B. A salt-free and chromium discharge minimizing tanning technology: the novel cleaner integrated chrome tanning process J. Clean. Prod. 112 2016 1055 1063 10.1016/J.JCLEPRO.2015.07.155
42 Sabatini F. Corsi I. Ceccarini A. Brillanti M. Colombini M.P. Bonaduce I. Pyrolysis gas chromatography mass spectrometry: a promising tool for disclosing metal-free tanning agents used in leather industry J. Anal. Appl. Pyrolysis 169 2023 10.1016/j.jaap.2022.105803
43 El Moujahed S. Extraction of insoluble fibrous collagen for characterization and crosslinking with phenolic compounds from pomegranate byproducts for leather tanning applications RSC Adv. 12 7 2022 10.1039/d1ra08059h
44 Cesprini E. Iseppi A.D. Giovando S. Chemical characterization of cherry (Prunus avium) extract in comparison with commercial mimosa and chestnut tannins Wood Sci. Technol. 56 5 2022 10.1007/s00226-022-01401-1
45 Vergallo C. Panzarini E. Dini L. High performance liquid chromatographic profiling of antioxidant and antidiabetic flavonoids purified from Azadirachta indica (neem) leaf ethanolic extract Pure Appl. Chem. 2019 10.1515/pac-2018-1221
46 China C.R. Nyandoro S.S. Munissi J.J.E. Maguta M.M. Meyer M. Schroepfer M. Tanning capacity of Tessmannia burttii extracts: the potential eco-friendly tanning agents for the leather industry J. of Leather Sci. and Eng. 3 1 2021 10.1186/s42825-021-00055-2
47 Seda Badessa T. Hailemariam M.T. Ahmed S.M. Greener approach for goat skin tanning Cogent Eng 9 1 2022 10.1080/23311916.2021.2018959
48 Hussein S.A. Utilization of tannins extract of Acacia seyal bark (Taleh) in tannage of leather J. Chem. Eng. Process Technol. 8 3 2017 10.4172/2157-7048.1000334
49 Hossain M.D. Bin Azam F.A. Chowdhury M. Quality assessment of shoe leather based on the properties of strength and comfort, collected from different footwear and leather industries in Bangladesh Textile and Leather Rev 4 1 2021 10.31881/TLR.2020.20
50 Nasr A.I. Influence of some mechanical finishing processes on manufactured leather properties Majalah. Kulit. Karet. dan Plastik. 33 2 2017 10.20543/mkkp.v33i2.3139
51 Biškauskaitė R. Valeikienė V. Valeika V. Enzymes for leather processing: effect on pickling and chroming Materials 14 6 2021 10.3390/ma14061480
52 Ding W. Bridging-induced densification strategy based on biomass-derived aldehyde tanning integrated with terminal Al (III) crosslinking towards high-performance chrome-free leather production J. Environ. Manag. 307 2022 10.1016/j.jenvman.2022.114554
53 Parisi M. Nanni A. Colonna M. Recycling of chrome-tanned leather and its utilization as polymeric materials and in polymer-based composites: a review Polymers 13 3 2021 10.3390/polym13030429
54 Roh E.K. Mechanical properties and preferences of natural and artificial leathers, and their classification with a focus on leather for bags J. of Eng. Fibers and Fabrics 15 2020 10.1177/1558925020968825
55 China C.R. Maguta M.M. Nyandoro S.S. Hilonga A. Kanth S.V. Njau K.N. Alternative tanning technologies and their suitability in curbing environmental pollution from the leather industry: a comprehensive review Chemosphere 254 2020 10.1016/j.chemosphere.2020.126804
56 Yorgancioglu A. Onem E. Yilmaz O. Karavana H.A. Interactions between collagen and alternative leather tanning systems to chromium salts by comparative thermal analysis methods Johnson Matthey Technol. Rev. 66 2 2022 10.1595/205651322X16225583463559
57 Conde M. Combalia F. Baquero G. Ollé L. Bacardit A. Exploring the feasibility of substituting mimosa tannin for pine bark powder. A LCA perspective Clean Eng. Technol. 7 2022 10.1016/j.clet.2022.100425
58 Covington A.D. Wise W.R. Tanning Chemistry: the Science of Leather 2019 10.1039/9781839168826
59 Sartika D. Astuti S. Iswandari R. Inhibitory study of cassava leather ethanol extract as natural antimicrobial in reducing Salmonella Sp. And Escherichia coli on contamination chicken meat (Gallus Domesticus) J. of Phys. Confer. Series. IOP Publishing Ltd 2021 10.1088/1742-6596/1751/1/012048
60 Wang L. Preparation and application of tremella polysaccharide-based chrome free tanning agent for sheepskin processing Int. J. Biol. Macromol. 241 2023 10.1016/j.ijbiomac.2023.124493
61 Schröpfer M. Meyer M. Investigations towards the binding mechanisms of vegetable tanning agents to collagen Res. J. Phytochem. 10 2 2016 58 66 10.3923/RJPHYTO.2016.58.66
62 Teshome Z. Agazhi T. Gashew T. Solomon B. Extraction and optimization of tanning material from osyris lanceolata barks: cleaner leather tanning processing J. Chem. 2023 10.1155/2023/6610247
