Experimental determination of the non-homogeneity of the physical parameters of a leather semi-finished product

. The non-homogeneity in mass and geometrical parameters of leather of one type and one manufacturing method is experimentally investigated and analyzed. Experimental studies were conducted on semi-finished cattle products of a combined batch and chrome kips from one technological batch. The results of the experiments showed that with a combined batch, between the mass and thickness of the leather semi-finished product, there is a moderate correlation between the studied physical parameters and the average error of the correlation coefficient of the leather. It is determined that high correlations are expressed between the mass and area of the leather, and between the thickness and area of the leather, respectively. It was revealed that the samples of leathers selected for experiments (kips), depending on their type, weight and area, have a higher correlation. So, between the mass, thickness and area at the standard point of the kip, there is a higher correlation between the studied physical parameters of the semi-finished leather samples. The results obtained provide an opportunity to influence the properties of leather for further creation of methods of controlling the relevant devices and equipment at various stages of production processes.


Introduction
After processing, the natural fibrous structure and differences characteristic of the hides of different animals and topographical sections of these hides are maintained in the finished leather.
It is known that the characteristics of each animal species are reflected not only in the general structure of the hide but also in the grain pattern, as a result of which the hide of each animal species corresponds to a certain grain pattern, which in its most essential features does not change depending on the breed, sex or age of the animals. This is the basis for the possibility of recognizing the origin of the hide based on its grain pattern. Some types of leather have an extremely peculiar pattern and coloring, giving an original look to shoes and other products made from them. Appropriate leather finishing (grinding or sawing off the surface layer, embossing and printing of the grain face, applying top coats, including varnish, etc.) can largely mask the natural grain pattern of the hide.
The equilibrium moisture content of the skin, in addition to atmospheric conditions, depends on the type of tanning, the amount of tanning, lubricants, load matter, and other substances introduced into the skin. According to current standards and specifications, the normal moisture content in the skin is 16%.
Generally, raw hides are measured by weight, while finished bovine leathers are measured by surface area [1]. In addition, along with other indices, live weight and sex affect the weight of the skin [2]. When finishing the skin, balanced control over the equipment mechanisms is required. For the production of high-quality leather, the characteristics of processing machines must correspond to those of the leather [3]. Therefore, due to the increased demand of consumers in the leather industry, the issue of developing and improving technological machines, taking into account the parameters of processed leather, is relevant [4]. The studies of the authors [5][6][7][8][9] are devoted to improving the design of roller technological machines for mechanical processing of leather.
A distinctive part of this mechanism is that in the processing, namely, when the skin contacts the working rollers, the latter move along a certain arcuate trajectory [10]. This greatly simplifies the process of feeding the skin between the rotating working rollers under their constant pressure.
In [11], the authors developed a method for determining the rheological parameter -the coefficient of strain inertness of the material. An improved rheological model of the skin was developed on the basis of this method, which makes it possible to more accurately describe the strain of the material, depending on its loading.
The factors that characterize the gripping conditions and the deformed state of the sheet material (using leather as an example) between rotating rolls were studied in [12].
So, let us consider the results of research and analysis of various properties of leather. Methods were proposed to make leather and leather products flame retardant to make them more compliant with requirements [13].
In [14], the authors compared the properties of vegetable tanned leathers of four goat genotypes using a batch of ninety-six skins of one-year-old goats. The properties of leather were compared in terms of elongation, thickness, and tear load.
The new method of retanning the skin does not have a harmful effect on humans. The developed method showed the best effect of leather retanning, which contributes to the clean production of leather and the development of environmental protection [15].
In [16], a new tanning process was studied, using valonea tannin as a natural tannin, with different process durations from 2 to 8 hours.
The authors of [17] studied the physical characteristics of the quality of the skin of various breeds of sheep. Skin samples of one-year-old sheep of various breeds were considered. After tanning, skins were selected to determine their tensile strength and stretch properties. The properties of wool sheepskins are better suited to production requirements.

Material and method of research
Therefore, it is relevant to study the non-homogeneity in weight and geometric parameters of leathers of the same type and of the same manufacturing method. The authors of the article conducted research on semi-finished cattle products of a combined batch and chrome kips from one technological batch.
To assess the non-homogeneity of the leather batches, physical parameters were selected for the semi-finished cattle products of the combined batch (Table 1) and for the side leather of cattle from the technological batch (Table 2): mass m, thickness at the standard point t and area S. ( ) where tх is the deviation of certain indices of the x series from their average value; tу, is the same value for the y series. In our case, tm are the deviations of individual indices of mass m from their average value; tt, ts are the same values for thickness t and area S, respectively. The average error of the correlation coefficient ρ, for n analyses, is calculated by the following formula [18][19][20][21].
where r is the correlation coefficient; n is the number of measurement repetitions. The values of the indices are calculated to determine the correlation coefficients for mass m and thickness t leather (Table 3).
The average error of the correlation coefficient calculated from ten analyses is  (Table 4). The values of the indices are calculated to determine the correlation coefficients for thickness t and area S leather (Table 5). The values of the indices are calculated to determine the correlation coefficients for mass m and thickness t leather (Table 6).
The average error of the correlation coefficient calculated from ten analyses is The values of the indices are calculated to determine the correlation coefficients for mass m and area S leather (Table 7).
we determine the average error of the correlation coefficient and get the following: The values of the indices are calculated to determine the correlation coefficients for thickness t and area S leather (Table 8).
We determine the average error of the correlation coefficient and get the following: Thus, calculations were carried out according to experimental data obtained in semiproduction conditions.

Results
The experiment show that in a combined batch, there is a moderate correlation between the studied physical parameters between the mass and thickness of the leather, i.e., Leather and kips sorted by the type, weight, and area of products have a higher correlation. There is a higher correlation between the mass, thickness, and area at the standard point of the kip between the studied physical parameters of the semi-finished product, i.e., , and the average error of the correlation coefficient, according to ten analyses, is 1311 . respectively.
To determine the correlation dependencies between the physical parameters of the semifinished cattle leather product, graphical dependencies were plotted based on Tables 1 and  2.  The analysis of graphic dependencies shows that the physical parameters of the semifinished tanning product of cattle have a certain correlation. With an increase in the thickness and area of the leather, its mass increases. At the same time, as the area increases, the thickness at the standard point increases too.

Conclusion
It was experimentally revealed that the samples of leather semi-finished products selected for experiments (kips), depending on their type, weight, and area, have a higher correlation. There is a higher correlation between the studied physical parameters of the leather between the mass, thickness and area at the standard point of kips. The results obtained provide an opportunity to influence the physical and mechanical properties of the finished leathers for further creation of methods for controlling relevant devices and equipment at various stages of production processes.
The considered factors can serve as preconditions for the use of physical parameters as one of the criteria for the control of a semi-finished product in the production of chrometanned leather.