STUDY OF THE CHARACTERISTICS OF NATURAL LEATHER TREATED WITH CHROMIUM BY THE ADSORPTION METHOD

ABSTRACT

In this article, using a vacuum McBean-Bakr device for measuring adsorption isotherms, the indices of microporosity, mesoporosity, pore radius and porosity of shoe upper leathers in technological processes of raw leather processing, as well as the change in the physicochemical properties of shoe upper leathers depending on the raw leather processing technology, were studied. The structure of the treated semi-finished product has large pores of 4.88 nm, but due to the relatively small surface area, its sorption properties are low. The micropores of size ref<1.0 nm and mesopores of size ref<2.5 nm in the sample are small. As a result of tannery raw material treatment, depending on the nature of leather and type of treatment, the radius of pores in the samples decreases from 2.16 nm and their specific surface area increases to 222.73 m2/g. It is found that the samples contain a small number of mesopores, while the number of micropores increases by 2-2.5 times.

АННОТАЦИЯ

В данной статье на вакуумной установке для измерения изотерм адсорбции «Мак-Бен-Бакра» исследованы показатели микропористости, мезопористости, радиуса пор и пористости кож верха обуви, а также изменение физико-химических свойств кож верха обуви в зависимости от технологии обработки кожевенного сырья. Структура обработанного полуфабриката имеет крупные поры размером 4,88 нм, но из-за относительно небольшой площади поверхности его сорбционные свойства низкие. Микропоры размером ref<1,0 нм и мезопоры размером ref<2,5 нм в образце мелкие. В результате обработки кожевенного сырья в зависимости от природы кожи и вида обработки радиус пор в образцах уменьшается от 2,16 нм, а их удельная поверхность увеличивается до 222,73 м2/г. Установлено, что в образцах содержится небольшое количество мезопор, тогда как количество микропор увеличивается в 2-2,5 раза. 

Keywords: skin raw materials, preparation process, tanning picked processes, microgovac, mesogovac, pore radius and porosity indicators, technology, physical properties, chemical properties, leather.

Ключевые слова: кожевенное сырье, процесс подготовки, пикированные процессы дубления, микроговак, мезоговак, показатели радиуса пор и пористости, технология, физические свойства, химические свойства, кожа.

Introduction. The issues of improving the quality of leather, expanding its assortment, and digitizing production technologies through innovations remain topical in leather production. The lack of sufficient data on leather porosity and its structural composition is still a problem, including in solving such technical issues as improving the quality of chrome leather for shoe tops and determining its quality parameters [1].

In the production of shoe uppers, product quality control and management are of great importance, and this issue is constantly in the field of attention of production management. The issues of leather quality management cover the whole period from its production to waste processing and operation and require a good understanding of the properties that determine the quality of leather, accurate measurement and objective evaluation of important indicators, the ability to reliably predict the quantitative characteristics of leather properties [2-4].

It is known that various types of leather for shoe tops, clothing leather, furniture leather, horse wash leather and other types of leather are produced from raw materials of black cattle. The technology of production of the above-mentioned types of leathers consists of a complex set of physical and chemical processes, and their structure and porosity are formed depending on the type of leather obtained in these processes. Processing of tannery raw materials begins with preparatory processes, where raw leather is first heated, then the wool layer is removed from the leather in the Sangop and shedding processes. These processes are carried out in an alkaline environment, often using lime, sodium sulfide, sodium hydrosulfide, sodium hydroxide and other substances [5-8]. During these preparatory processes, soluble albumin, globulin proteins and polysaccharides are washed out of the skin tissue, spaces and pores are formed between collagen protein fibers in the skin tissue, and the skin tissue is prepared for the growth process. During the aging process, the formation of skin structural elements continues, skin porosity, pore shape and size are enhanced by the increasing effects of enhancing agents. Many studies have been conducted on the changes occurring in skin microstructure and the relationship between the sorption properties of the skin and porosity and microstructural changes. Leather microstructure and its physical and mechanical properties begin to form during the technological processes of preparation, pre-tanning and tanning in tanning technology [9].

Comfortable conditions for wearing shoes are largely determined by the sorption and hygienic properties of leather, which in turn depend on the development of its inner surface, the size, number and configuration of pores, as well as the degree of blocking in the skin of active groups of collagen capable of interacting with vapors. Studies of the sorption properties of collagen and gelatin have been carried out by many authors for various purposes. Among them are Likov, Kavkazov, S. Greg, K. Singh and A. Zeldina. It is described in detail in the monographs by E. et al [10,11].

Studies have shown that the chrome tanning process reduces the maximum sorption capacity of collagen without changing the forward and reverse directions of the isotherm and does not affect the uniformity of the sorption properties of the dermis of different topographic areas of the skin.

While recognizing the usefulness of the data obtained from these studies, which have helped to expand our understanding of structural changes in collagen, it should be noted that the instrumentation and methods used do not allow us to detect a number of subtle changes in collagen. The bulk of the studies were carried out using the excicator method. Studies on the formation of porosity of shoe upper leathers show that the structure is mainly formed during the growing process [12].

Objects and methods of research. In the research of technological processes of processing, such types of raw materials as black cattle hides, calf skins and goatskins were taken as objects of research. Standard norms of leather raw materials from black cattle hides, goatskins and calf skins used in the study ISO 2418:2017 (E) Leather-Chemical, physical and mechanical and fastness tests Sampling location. meets the requirements. Experimental and practical tests were carried out in the experimental testing laboratory of Angren CHARM INVEST LLC in the city of Angren, Tashkent region. Technologies of research on the processes of preparation, pre-tanning and tanning, defects of the process and operations, numerous scientific sources were studied and analyzed. At the same time, the technology of processing hides of cattle, calves and goats realized at the enterprise was taken as a basis. It is important for us to expand our knowledge of the physical parameters of leather structure related to porosity. We investigated the porosity of leather using the adsorption method, one of the modern methods of porosity research. To study the adsorption process on the skin, we used a Mak Ben-Bakr spring scale apparatus. The sorption of water vapor was controlled by independent weighing, which allowed us to significantly improve the accuracy of the experiment.

Results. For this study, in order to investigate the porous structure of leather, samples were selected from the following semi-finished products and leathers dressed using chrome tanning technology for the upper of Navvos leather shoes:

Figure 1. Dynamics of water vapor sorption processes by experimental leather samples:

A) Option 1 crust leather; B) Option 2 – semi-finished product in the process of coating painting; C) Option 3 finished split leather; D) Option 4 – semi-finished product; E) Option 5: Finished upper leather made of goat skin, tanned with chrome; F) Option 6 – upper part made of processed leather.

Despite the difference in the magnification and structure of the samples, the appearance of all sorption isotherms is similar (except for variant 4). At the same time, adsorption is characterized by a linear curve at low water vapor pressures. According to Brunauer's classification, these are type 2 isotherms, and the isotherm of variant 4 belongs to type 3. It is known from the literature on this issue that if the convexity at the beginning of the isotherm is directed toward the ordinate axis (as in samples 1-3,5-6), this indicates that the interaction energy between the adsorbate molecules is less than the interaction energy between the adsorbate and adsorbent molecules. This indicates rapid diffusion of the adsorbent (water vapor) into the skin structure. We observe this picture in all experimental samples, except for sample 4.

Three stages can be observed in the sorption isotherms: sections of mono- and polymolecular adsorption and sections of capillary condensation. In all isotherms, the Langmuir law is observed at the first stage, i.e., intensive sorption at low pressures. The slope of the curve for this section has a high value. The angular coefficient in the centers of polymolecular adsorption is significantly lower, and for most samples, the pressure isotherms begin with values ​​of  R/Rs = 0.12-0.15.  The pressure range of isotherms for the second sections is R/Rs = 0.20-0.75. Assumptions in the available analyses suggest that such changes in the sorption value are associated with changes in the dermis during the tanning process. In our opinion, the place of polymolecular adsorption is characterized by technological changes in the dermis. The third section of the isotherm is associated with capillary condensation, and it is this section that reflects the structure of collagen protofibrils and fibrils, and by the sorption of this section, it is possible to calculate the radii of capillaries in the range of 50-400 nm. The relative surface area (S) of the adsorbent structure was determined using the Brunauer, Emmett, Teller (BET) theory equation. In this case, if the ordinate is R/Ps/a(1-P/Ps) and the abscissa is R/Ps, then straight-line coordinates are obtained. The relative surface area of ​​the adsorbents was calculated using the following formula:

С = a _м  Н  ₀

Here: S is the relative support surface   (m²/g);

а_м- monomolecular layer (mol/kg);

Н_А- Avogadro's number;

ω- Surface area occupied by one molecule (nm²) .

The results obtained from the adsorption experiments and calculated using the BET formulas are presented in Table 1.

Depending on the increasing magnitude of sorption, the samples can be arranged in the following rows:

The structure of the treated semi-finished product has large pores of 4.88 nm, but due to the relatively small surface area, its sorption properties are low. The micropores of size ref<1.0 nm and mesopores of size ref<2.5 nm in the sample are small. As a result of tannery raw material treatment, depending on the nature of leather and type of treatment, the radius of pores in the samples decreases from 2.16 nm and their specific surface area increases to 222.73 m²/g.

Table 1.

Sorption characteristics of porous skin structure

It is found that the samples contain a small number of mesopores, while the number of micropores increases by 2-2.5 times. The increase in the value of water vapor sorption indicates the value of skin porosity, the hydrophilicity of the balance of hydrophilic-lyophilic groups in the leather material and the value of the effective radii of the available pores in the structure. From the information in the sorption column, it can be seen that the highest sorption value belongs to the crust layer. So, at this stage, our semi-finished product (crust) has the largest porous structure. At the same time, the structure of finished goatskin and finished leather, which occupy the second place, is characterized by large porosity. The fact that the value of skin sorption in the coating process (variant 2) is lower than that of the previous samples of the series is explained by the saturation of the semi-finished product with hydrophilic solutions at the beginning of the coating process. The sorption value of the semi-finished product that has undergone the aging process shows that the porosity of the structure is not fully manifested in it. That is, it would be more correct to explain that there is potential porosity, but the fibrils have stuck together. Analysis of the effective radii shows that the leather with the largest relative surface area has an effective radius of 2.33 nm, while the sample with the smallest relative surface area (tanned semi-finished product) has an effective radius of 4.88 nm. The logic here is that the porosity of the semi-finished product is not fully formed and as it forms and the fibrillar assembly separates into individual fibrils, the effective radius decreases and this is clearly seen in the experimental results. The pores (meso- and micro), depending on their relative size, are arranged in the following row of samples:

Conclusion. Thus, in the conducted experiments on determination of adsorption isotherms in the Mac-Ben-Bacre apparatus, the results obtained by the BET method showed that leather and leather semi-finished products tanned with chromium salts have high sorption properties in experiments with water vapor. Structural characteristics characteristic of porous materials, including natural leather, have been determined on a group of leather samples. The specific surface area of leather is in the range of 175-225 m2/g. The micropore range of the samples was 0.18-0.09, and the mesopore range was 0.08-0.04. The effective radii of skin pores were 2.25-4.88 nm. Determination of the structure of leather porosity by the Mac-Ben-Bacre method allows obtaining highly accurate results that influence the character of leather semi-finished products, since porosity during processing leads to changes in micro- and meso-dimensions, as well as on the formation of physical and mechanical properties of semi-finished products.

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