Evaluation of Mechanical Properties of Goat Leather Tanned using Acacia xanthophloea

Acacia xanthophloea is a tree in the Fabaceae family with wide distribution mainly around Laikipia in Central Kenya and major parts of the Rift Valley town of Naivasha, Kenya. A number of trees under the Fabaceae family are renowned as sources of vegetable tannins for leather processing. Despite recent advances in research, locally available vegetable tanning materials have not been widely exploited in Kenya for commercial tanning purposes. This study aimed to evaluate the mechanical properties of goat leather tanned with crude extracts of Acacia xanthophloea from Naivasha, Kenya. Crude extracts of Acacia xanthophloea combined with pre-tanning and tanning procedures were used to produce leather. The commercial mimosa was used as a control. The leather tanned with crude extracts of Acacia xanthophloea had a thickness of 0.81 ± 0.11 mm, tearing strength of 37.87 ± 2.09 N, tensile strength of 27.50 ± 7.51 N/mm 2 , percentage elongation of 18.00 ± 6.67, grain crack of 6.19 ± 0.20 mm and grain burst of 7.10 ± 0.27 mm. The crude extracts of Acacia xanthophloea confer good tanning and give the leather a reddish tinge, whereas some mechanical properties attenuated, compare favourably with the control (mimosa). Acacia xanthophloea which is abundantly available in Kenya with scarce use can potentially be cultivated and refined as a commercial source of tannins .


INTRODUCTION
Processing of hides and skins into leather is one of the key agro-processing industries in Kenya. Its potential towards commodity development addresses pertinent issues of socioeconomic importance and positively impacts rural development, creation of wealth and employment [1]. Tanning is a very important process in the leather industry as it brings out the required characteristics in the raw materials, which are essential for the quality of the finished product [2]. It is a crucial part of leather 334 https://doi.org/10.31881/TLR.2023.053 manufacturing operations that stabilizes the proteins in raw skin or hides through either the use of minerals or vegetable tanning agents and largely determines the characteristics of the finished product [3]. Tannins can be defined as water-soluble polyphenolic compounds with molecular weights ranging from 500 to 3,000 Da (gallic acid esters) and up to 20,000 Da (proanthocyanidins), that can form reversible and irreversible complexes with proteins, polysaccharides, alkaloids, nucleic acids, and minerals [4]. About 90% of the tannins produced globally are used in the production of leather [5].
Further, vegetable tannin can be of three different categories such as complex tannin, condensed tannin, and hydrolyzable tannin [3]. Condensed tannins are flavan-3-ol biopolymers that generate anthocyanidins and catechins as end groups when heated in alcohol solutions of a strong mineral acid.
Gallotannins and ellagitannins are examples of hydrolysable tannins. Gallotannins are galloyl esters of glucose or quinic acid whereas ellagitannins are generated from hexahydroxydiphenic acid [6].
In leather processing, different kinds of tanning methods, materials and chemicals may be used.
Chrome tanning is the most prominent in leather production globally [7,8]. Other forms of tanning include; vegetable tanning, aldehyde tanning, oil tanning, mineral tanning and combination tanning.
Vegetable tanning materials continue to draw interest in leather processing because of their non-toxic form and the perceived environmental credentials because of the less environmental pollution associated with their use. Moreover, plant tannins are readily abundant in nature and are a renewable source. In addition, vegetable tanning agents contain tannin, non-tannin, and gum. Based on their origin, vegetable tannins are known to confer unique attributes that produce compact, full and easily embossable leather [2]. Vegetable-tanned leather possesses better water permeability, stability, strength and moulding properties [9]. The growing global demand for vegetable-tanned leather continues to put much pressure on the conventional sources of commercial tannins such as mimosa, divi-divi and quebracho. Hence there is a need to look for alternative sources of tannins to diversify production. Some indigenous African tree species have been found to contain appreciable quantities of tannins, which could be exploited commercially [10]. A study by Cheloti et al. evaluated the tanning potential of Acacia xanthophloea and found its viability as a good source of tannins for leather tanning [11]. However, there is limited study on the evaluation of mechanical properties of leather tanned using Acacia xanthophloea extracts. A study by China et al. showed the viability of A. xanthophloea bark extract as a tanning agent with properties similar to A. mearnsii (a commercialized source of tannins) [12]. Sources of vegetable tannin and the difference in functional groups present in the tannin could affect the tanning efficiency and properties of tanned leather [13,14].
The importance of the mechanical properties of leather is in the evaluation of the performance characteristics for the specific end-use. This study aimed to evaluate the mechanical properties of goat leather tanned with crude extracts of Acacia xanthophloea as a tanning material and explore its suitability for commercial sources. The properties such as thickness, tensile strength, tear strength, https://doi.org/10.31881/TLR.2023.053 percentage elongation and distention at crack and ball burst were conducted on the goat leather and verified with the recommended standard values of shoe upper leather [15][16][17][18].

Samples collection and preparation
The goat skins for the tanning process were obtained from the Dagoretti slaughterhouse, Nairobi County, Kenya and cured by wet salting. Acacia xanthophloea barks sample was collected from Naivasha, Nakuru County, Kenya and air-dried under a shade for 7 days at room temperature (25 °C).
The Acacia barks were considered to be dry when there was no further variation in the moisture content over a period of 24 h. Further size reduction was done on the barks which were cut into small chips with overall dimensions not exceeding 5 cm in length and 0.6 cm in diameter. The small chips were then ground using a milling machine and sieved by 1 mm size mesh. The prepared sample was stored in a sealable polyethylene bag and kept at room temperature for the extraction process. About 100 g of the milled barks were then soaked in 1 L of distilled water in a conical flask overnight before commencing the extraction process. The mixture was transferred to a water bath initially at 30 °C while constantly stirred using an overhead stirrer. The residue was subjected to an extraction process using 1 L of distilled water that led to the collection of the first batch of the filtrate after 4 h. The temperature was adjusted to 50 °C and a second batch of the filtrate was collected after 4 h. Finally, the temperature was adjusted to about 80 °C and the remaining quantity was extracted. Thereafter the filtrates were mixed and set aside for the tanning/skin treatment step.

Pre-tanning of the skins
Two goat skins were prepared for tanning using crude extracts of Acacia xanthophloea with mimosa as a control. The Pre-tanning step was conducted as enumerated in Table 1 below.

Treatment step
Tanning of the skins was done according to the procedures shown in the recipe for process steps in Table 2 below.  [17] and ball crack/ball burst test was measured using a lastometer according to the official method (IUP/9, 2015) [18]. All the tests were performed in triplicates for both parallel and perpendicular to the backbone and reported the mean with standard deviation in this study.

Thickness
From between Acacia-tanned leather and commercial mimosa could be attributed to the presence of a higher molar mass of non-tans in mimosa as compared to the Acacia-tanned leather that led to plumping of the mimosa-tanned leather [3]. and 22.62 ± 1.00 N/mm 2 (leather samples cut across/perpendicular to the backbone) as shown in Table   3 above. The recorded measurements of tensile strength in this study were all way above the expected minimum of 10 N/mm 2 for shoe upper leather as per the Kenya Bureau of Standards (KEBS) specifications for tensile strength of upper leather and compares favourably with the previous research [21]. Tensile strength determines the structural resistance of upper leather to tensile forces hence its state and usability [3].

Percentage elongation test
The percentage elongation of leather is another physical property measured when assessing the leather quality and this has a relationship with the elasticity. Elongation refers to the ability of a leather product to lengthen/stretch when stress is applied to it and represents the maximum extent to which the leather can stretch without breaking. As shown in Table 3 above, leather tanned with Acacia xanthophloea had a lower percentage elongation of 12.64 ± 0.87 (leather samples cut across/perpendicular to the backbone), 18.00 ± 6.67 (leather samples cut along/parallel to the backbone). The low performance in percentage elongation for Acacia-tanned leather emphasizes that the leather has not had enough elasticity required for making shoe uppers. Elongation is an important property to be considered when choosing garment leather because a low elongation value results in easy tear while a high elongation value causes leather goods to become deformed very quickly or even lose usability [21]. Good quality leather should have a percentage elongation of 30-80 as per the Kenya Bureau of Standards (KEBS) specifications for elongation of the upper leather. Elongation of leather is affected by pre-tanning, tanning and post tanning process which always differs from one tanner to another [3].

Tear strength test
The tear strength of the leather products in use is indicated by the quality standard relating to the tearing load. This study found the tearing strength of the Acacia-tanned leather to be lower than the recommended standard tear strength as shown in Table 4

Grain crack and ball burst tests
The grain crack and ball burst tests are other physical properties for testing the quality of leather. In this study, as shown in Table 4

Conflicts of Interest
The authors declare no conflict of interest.