APPLICATION OF PISTACHIO LEAF TANNINS IN THE FILLING PROCESS OF LEATHER SEMI-FINISHED PRODUCTS

ABSTRACT

The article analyses the leather manufacturing process, in particular the role and importance of the filling process, which is considered the stage following tanning. The share of filling materials of different nature—polymer- and resin-based fillers, vegetable tannins, synthetic syntans, biopolymer and mineral fillers—in filling the empty areas of the leather structure, increasing fullness and improving the appearance is presented. On this basis, the possibility of using a tanning agent obtained from a local raw material, pistachio leaves, instead of imported vegetable tanning agents (mimosa and quebracho) in the filling process of lining leather produced from sheep skins has been investigated.

АННОТАЦИЯ

В статье анализируется процесс производства кожи, в частности, роль и значение процесса наполнения, который рассматривается как этап, следующий за дублением. Показана роль наполнителей различной природы – полимерных и смоляных наполнителей, растительных дубителей, синтетических синтанов, биополимерных и минеральных наполнителей – в заполнении пустот в структуре кожи, повышении ее полноты и улучшении внешнего вида. На этой основе исследована возможность использования дубителя, полученного из местного сырья – листьев фисташки, вместо импортных растительных дубителей (мимозы и квебрахо) в процессе наполнения подкладочных кож, вырабатываемых из овечьих шкур.

Keywords: leather manufacturing, filling process, vegetable tanning agents, mimosa, quebracho, pistachio leaves, tannins, lining leather.

Ключевые слова: производство кожи, процесс наполнения, растительные дубители, мимоза, квебрахо, листья фисташки, дубители, подкладочная кожа.

Introduction. Leather manufacturing processes are long-lasting, multistage and complex, and as raw material they mainly use animal hides obtained from the meat industry.[1] Through the tanning process the hide is converted into leather; that is, in this process the raw hide, which is prone to putrefaction, is transformed into a non-putrescible material resistant to bacterial attack, chemical degradation and mechanical deformation. As a result of tanning, the leather acquires hydrothermal stability, good air permeability, strength and a number of other advantageous properties.[2]

The stability of leather mainly depends on the strong interactions formed between collagen fibers and the tanning agent, which may be organic or inorganic in nature. Inorganic tanning agents usually include chromium salts; chromium salts form coordination complexes with collagen in the hide and thereby ensure its biochemical stability.[3] Other inorganic salts used include those of aluminium, zirconium, silicon and iron.[4] In general, leather processing operations [5,6] can be divided into three main stages (Table).

Table 1.

 Main stages of leather processing

As can be seen from this table, pretanning operations prepare the hides for tanning; the tanning stage converts the hide into stable leather by chemically modifying collagen; and the post-tanning operations form the appearance and service properties of the leather. All processes in leather processing are systemic, showing their interdependence and significance for the quality of the final product.

One of the post-tanning operations is the filling process, which has particular importance. In practice, in order to reduce common defects of natural hides—such as areas where the vein pattern is visible and regions of low density or “looseness”—it is often necessary to use special filling materials. Therefore, the filling process is an important operation aimed at introducing substances into the leather structure and filling the spaces between collagen fibres. During filling, materials of various natures are used (Figure 1).

Figure 1. Approximate share of materials used for leather filling worldwide

The diagram shows the approximate share of materials used in leather filling. The largest portion is occupied by polymer- and resin-based fillers (60%), followed by vegetable tannins (20%) and conventional synthetic syntans (10%); biopolymer and inorganic-mineral fillers each account for about 5%.

The chemical composition of filling materials and their proportions determine the tensile strength, flexibility, thickness and degree of fullness of the treated leather. When selecting a filling material, it must correspond to the end-use of the leather being produced, dissolve well in water, penetrate deeply into the leather structure and fill it uniformly over the entire surface.

Although the use of vegetable tanning agents—classified as organic materials—as fillers is economically more expensive, they are considered environmentally efficient. From an economic point of view, there is also the possibility of reducing the cost price of vegetable tanning agents containing tannins; one option is the use of pistachio leaves as a filling material. In this research, pistachio leaves, a local raw material, were used in the filling process of obtaining lining leather from sheep skins instead of imported vegetable tanning agents such as mimosa and quebracho (Figure 2).

Figure 2. Leather semi-finished products after the filling process

Initial research results showed that, according to organoleptic evaluation, leather semi-finished products filled in the experimental variants did not lag behind leather semi-finished products filled with imported materials. In terms of appearance and fullness there was almost no difference. This confirms the high technological efficiency of local vegetable tannins and the possibility of substituting imports.

In conclusion, the filling process in leather manufacturing is one of the most important stages after tanning, determining the fullness, appearance and service properties of the leather. The nature, amount and processing conditions of the filling material directly affect the quality of the final product. Along with widely used polymer- and resin-based fillers, vegetable tannins are of particular importance as an environmentally safe and sustainable resource. From this point of view, the study of local vegetable raw materials, especially pistachio leaves, is of strategic importance for the leather industry.

References:

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3. Focking, D., M. Simoes, and P. Piana, Resistance of the Nile Tilapia (Orechromis niloticus) Tanned with Vegetable Tannin. Journal of the Society of Leather Technologists and Chemists, 2013. 97(2): p. 56-61.
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