Investigation of the Effect of a Natural Extract From Oak Bark on the Properties of the Leather

For the study, undyed chrome-tanned bovine leather was used, processed by the «Turan-Skin» factory, located in Kazakhstan in the city of Shymkent.

In the finishing process of the leather, natural extract from oak bark was used. The process of preparation of the extract is given in another study [24-25]. For the control sample, a basic finishing formulation using a chemical pigment and samples without finishing were used. Characteristics of the dye solution are shown in Table 1.

The finishing was carried out by spraying, the process of which consisted of two stages. The first stage was painting the coating, and the second was fixing the coating with lacs and dyes. The finishing recipe is shown in Table 2.

The finishing formulation used in this research belongs to the factory itself. The proportions of the materials used in this research may vary according to the desired properties. For this reason, it may be appropriate to try plant extracts in different formulations.

Unfinished, and finished leather samples using chemical pigment and oak bark are shown in Figure 1. The leather sample with the oak bark extract turned a pale pink colour, while that treated with the chemical turned a beige colour.

Fig. 1.

Prior to the tests all finished and unfinished leathers were conditioned according to Standard EN ISO 2419:2012 Leather - Physical and mechanical tests -Sample preparation and conditioning.

The alternative specific standard atmosphere has a temperature of 20.0±2°C and a relative humidity of 65.0± 2%.

A Shimadzu AG-IS tensile testing device was used for all tests. The measurement of sample thickness was performed in accordance with EN ISO 2589:2002. Leather - Physical and mechanical tests - Determination of thickness.

The tensile strength and percentage extension was determined with EN ISO3376:2011. Leather - Physical and mechanical tests - Determination of tensile strength and percentage extension, single edge tear load with EN ISO 3377-1:2011. Leather - Physical and mechanical tests - Determination of tear load - Part 1: Single edge tear, double edge tear load with EN ISO 3377-2.:2002. Leather - Physical and mechanical tests - Determination of tear load - Part 2: Double edge tear.

During the sampling, half of test pieces were cut parallel to the backbone and the other half perpendicular to the backbone.

The percentage water absorption is determined according to ISO 5403-1:2011. Leather - Determination of water resistance of flexible leather - Part 1: Repeated linear compression.

The percentage water absorption, W₀, is calculated using the formula: 1W0=(m1−m0)⋅100m0$${W_0} = {{\left( {{m_1} - {m_0}} \right) \cdot 100} \over {{m_0}}}$$ Where, m₁-the mass of the test piece after any time period, in grams; m₀ - the initial conditioned mass of the test piece, in grams.

The water transmission is determined according to ISO 5403-1:2011. Leather - Determination of water resistance of flexible leather - Part 1: Repeated linear compression.

The water transmission, m_wt, in grams, is calculated using the formula: 2mwt=mam1−mam0$${m_{wt}} = {m_{am1}} - {m_{am0}}$$ Where, m_am1 - the mass of the absorbent material after the test, in grams; m_am0 - the initial conditioned mass of the absorbent material, in grams.

To control the colour stability of the dye extract contained in the finished leather, studies were conducted on the colour fastness to water spotting in accordance with the standard TS EN ISO ISO 15700:1998 - Leather - Tests for colour fastness.

In this study, the absorption of water by the leather was controlled by instilling drops of distilled water over two time periods, i.e. 30 minutes and 16 hours. The experiment was repeated 3 times.

Two methods were used to monitor the results of the study. The first was evaluation by visual inspection using an organoleptic method in accordance with the ISO 105-A02 standard, and the second method was instrumental evaluation according to the ISO 105-A05 standard on a gray scale.

In a single tear test a specimen which is partially slit from one edge is pulled so that a tear is propagated from the end of the slit. In the double edge tear load test, the specimen is pulled from the both edges of a specifically shaped hole in the sample. In both tests the highest force exerted during tearing is measured. Both tests gave information about the mechanical strength of the leathers in the case of an applied force on a created tear on the leather.

The single edge tear load in the samples without finishing is 90.1 N, with a chemical pigment - 88.9N, and in the samples finished with oak bark it is on average 101.3 N. The average double edge tear load in the samples without finishing is 157.1 N, with a chemical pigment - 138.8 N, and in the samples finished with oak bark it is 164.7 N. These results clearly show that the tearing strength of the materials can be affected by process variation, as similarly reported by Mukhopadhyay et al. for tearing behavior [26-27].

For the experiment, samples without finishing and samples finished with the use of chemical pigment and oak bark extract in the parallel and perpendicular directions were prepared. A graph of the effect of the plant extract on mechanical properties of the leather is shown in Figure 2.

Fig. 2.

According to the results of the study, the tensile strength of the sample in the parallel direction using oak bark is 4.8 N/mm² and in the perpendicular direction - 9.2 N/mm² more than in samples without finishing, and 1.3 N/mm², more than in the sample with a parallel section using a chemical pigment (Figure 2a).

The percentage extension of the sample using oak bark is also higher than that of control samples. For example, the elongation of the sample along the parallel section using oak bark is 94.8%, without finishing 62.9%, and with the use of chemical pigment, the percentage extension of the sample is 87.3% (Figure 2b).

The results of the study showed that leather samples finished by natural extract from oak bark have the best mechanical properties.The partial penetration of extract from oak bark through the pores of the leather and its interaction with the surface layers leads to an improvement in mechanical and hygienic properties of the leather.

The difference in mechanical properties of the leather samples regarding the parallel and perpendicular directions of the sections is related to the structure and fibers of the leather [28].

For the samples cut along the backline, the majority of their fibres are already aligned in the same direction as the strain applied, hence they have little scope to orientate towards the strain axis. This therefore means that the fibres themselves are directly strained at low levels of nominal strain and that the process of fibre orientation is limited.

For samples cut perpendicularly, many fibres are aligned normal to the direction of the strain applied; therefore, the fibres orientate towards the strain axis. This orientation minimizes levels of nominal strain and deformation occurs by straining themselves. Further deformation is associated with straining of the fibres themselves. Since tensile strength is proportional to the number of fibres aligned in that direction, its value is higher in the parallel orientation because the strain is applied to a more orientated network of fibres than in the of perpendicular sampling. Fibre orientation will cause frictional damage to the fibres. Since tensile strength is proportional to the number of fibres aligned in that direction, its value is higher in the parallel orientation because the strain is applied to a more orientated network of fibres than in the case of perpendicular sampling.

This shows a significant difference between samples cut perpendicularly and those cut along the backline regarding physical and mechanical properties of the leather.

Determination of water absorption and water penetration are very important indicators for leather used in the shoe industry. Increased moisture capacity leads to a weakening of the fastening of the upper and lower parts together, faster wear due to abrasion, and a decrease in the comfort of shoes. The results of determining water absorption are shown in Table 4, displaying that the water absorption of leathers finished with oak bark extract is less than in control samples.

According to the results of the study, the water absorption of the leather sample with a parallel section using oak bark is 8.7%, and with perpendicular - 17.8%, and in samples without treatment with a parallel section it is 40.1%, and with a perpendicular - 32.3%.

With the use of a standard chemical pigment, water absorption decreases by an average of 21%.

It is known that the main amount of moisture is absorbed by the leather as a result of molecular and capillary wetting by filling the interstructural voids of the skin tissue with water. Water absorption occurs in two stages [2].

At the beginning - rapid absorption of water, lasting 10-15 minutes. At this time, the largest pores are filled. During the second stage, water absorption slows down, water-soluble substances are washed out and some swelling of the skin fibers occurs.

A decrease in water absorption, apparently, can occur due to a decrease in porosity andhydrophobization of the surface of the leather.

The results of determining water penetration are shown in Table 5, displaying that as a result of finishing using oak bark extract, water penetration in the leather samples is significantly reduced. For example, for leather samples without finishing, water penetration averages 1.93 g, and when finished with a standard chemical pigment, it averages 1.29 g. In leather samples using oak bark extract, water penetration averages 0.46 g, which is on average 1.15 g less than in control samples.

The reduction of water penetration in leather samples using oak bark extract is their positive difference.

Reduction of water absorption by reducing porosity, hydrophobization of the surface of leather elements (fiber and bundle), as well as limited swelling of leather fibers were achieved.

The results of the study of leather samples for colour fastness to water spotting are shown in Table 6.

From the above data, it can be seen that leather samples finished using oak bark extract show better results of colour fastness to water spotting compared with other samples. For example, the indicators of colour fastness to water spotting in leather samples without finishing after 30 minutes of testing averaged 3, after 16 hours 2.5, in leather samples with oak bark extract finishing after 30 minutes averaged 4, and showed the same results after 16 hours of testing. And leather samples finished with chemical pigment have an average colour fastness of 4.

During the finishing process, the appearance and functionality of the skin improved, and a protective film was created on its surface. This made this a delicate and important stage when we decided to invest in the development of increasingly environmentally sustainable production. The use of a plant extract of oak bark for leather finishing also solves a very important environmental problem. When finishing, binders, waxes, pigments and dyes were applied to the leather texture to impart colour, as well as improve organoleptic and mechanical characteristics.