*112327*

D.t.s. Abdullin I.S., d.t.s. Rakhmatullina G.R., c.t.s. Akhverdiev R.F., c.t.s.Tikhonova

Kazan National Research Technological University, Russia

Nanostructural modification of capillary-porous materials, light industry

Natural capillary-porous materials such as leather are fibrous high-molecular materials of protein origin. Leather has a number of advantages over artificial and synthetic materials - good hygienic properties, providing a comfortable microclimate in the exploitation of products. However, one of the shortcomings of leather is its insufficient durability. Technological chain of chamoising includes several processes: preparatory processes, tanning and trimming. To improve the strengthening and aesthetic qualities of the material it is necessary to improve the technology.

The capabilities of the traditional methods of modification have almost been exhausted. At present, different electrophysical methods of modification are becoming more common: the impact of the electromagnetic field, gas-discharge plasma, etc. The advantage of these methods is that they almost do not lead to changes in the chemical composition of the material.

The peculiarity of leather is a multi-level structure of its constituent protein - collagen. Table 1 shows the diameters of the main structural elements of collagen.

Table 1 - Diameters of the structural elements of collagen

The structural elements of collagen	Diameter, nm
α-chain	0,7
Tropocollagen particle	1,5
Microfibrille	4,5
Subfibrille	21
Fibrille	100
Primary fibre (micrilevel)	5000 (5 mcm)
Secondary fibre (macrolevel)	10^5 (100 mcm)

The porous structure of leather is a special case of a series of regular porous systems. The underpressure radiofrequency (RF) plasma discharge processing is used as an effective tool for the modification of such a structure.

The underpressure RF plasma processing was carried with the following parameters: discharge capacity Pp = 0.5-5 kW, generator frequency f = 1,76-13,56 MHz, gas consumption G=0-0,2 g/sec, pressure р=13,3-133 Pa, plasma-forming argon. In these modes, the RF plasma in argon is characterized by the following parameters: the degree of ionization 10-4-10-7, the electron density ne = 1015-1019 m-3, the electron temperature Te = 1-4 eV, the temperature of the atoms and ions in the discharge Tai = (3-4) × 103 K., the plasma jet Tai = 30-900 K.

The advantages of this method: first, the heating of the material does not exceed 100 0C due to the high level of thermal nonequilibrium of the discharge of this type. This factor is very important because this biological material has a high level of sensitivity to the thermal stress.

The second important advantage of the usage of the RF plasma in the modification of the structure of leather is the possibility of the volumetric processing.

In the works of some authors [1-3] a significant improvement of the strengthening and exploitation properties of leather were established. Because of the fact that the properties of leather are largely determined by the structure of the material, it is necessary to study the structural changes of leather under the influence of the nonequilibrium low-temperature plasma.

On the account of the experimental data it is established that the basic structural element of the volumetric modification is a nanopore. Due to this the modeling of the origination of a discharge in the nanopores of the material is produced.

Nanopores in the material under study exist at the level of the supermolecular structure – these are interfibrillar and intrafibrillar voids. Collagen fibrils are characterized by regular alternation of polar and nonpolar parts along the entire length of its constituent polypeptide chains. The length of each part is 15 - 30 Å.

Figure 1 shows ultrathin sections of leather at a magnification of 40,000 times, showing the nanostructure of collagen fibers before and after modification in the flow of non-equilibrium low-temperature plasma.

a

b

Figure 1 - The ultra-thin sections of leather at a magnification of 40,000 times: a – before plasma modification; b – after plasma modification

When leather is modified in the flow of the nonequilibrium low-temperature plasma the conditions for the appearance of the non-self-maintained discharge inside the natural capillary - porous body are created, so long as:

a) free charge carriers appear in the gas i. e. conductivity appears;

b) within the micropore exists an electric field which communicates the directional movement to these carriers.

The main source of free electrons and ions within a nanopore is the gas that fills it.

Initially the gas is ionized due to the ion flux coming from the plasma, and the secondary electrons dislodged from the walls of nanopores. There are no conditions for the maintaining of the stationary self-sustained discharge because the pore sizes are smaller than the free length of electrons and ions. Gas breakdown in nanopores and capillaries occurs only when the electric field strength exceeds the value that corresponds to the breakdown voltage; this is the moment of the discharge occurrence. The discharge stops when the field strength becomes smaller than the value corresponding to the discharge maintaining voltage.

During the processing of the capillary - porous bodies the non-self-maintained pulse-periodic discharge is maintained within the high-frequency discharge plasma. Ions generated by this discharge recombine on the walls of nanopores with the release of the recombination energy. This leads to the modification of the inner surface of the walls of nanopores. This means that during the processing of the capillary - porous materials in the RF discharge plasma it is possible to carry out the processing both of the surface and the interior space, unlike other types of gas discharges.

The main process responsible for the modification of the capillary - porous materials is the recombination of ions on the material and the bombarding of its surface by the low-energy ions. The impact of ions is transferred to the to protein macromolecules which leads to the rotations of links of the polymeric chain, to the rupture of hydrogen and van der Waals bonds through which the degree of order of the supramolecular structure of the capillary - porous materials increases. Owing to these changes, the intrafibrillar structures are reoriented, the sizes, the shape and the properties of the fibrils of the materials are changed, that is, the nanostructure of the capillary - porous materials is modified. With the help of the RF plasma treatment it is possible to achieve two effects of the modification of the nanostructures of the capillary - porous bodies: separation of the fibrils and ordering of their location, or building of the compact structure. The modification of the nanostructure of the capillary - porous bodies affects the microstructure of the materials. The sizes of the primary (Fig. 2) and secondary fibers (Fig. 3) are changed in the modified leather as well as the sizes of macro-and micropores, allowing to regulate the strengthening and exploitation properties of genuine leather.

a

b

Figure 2 - Micrographs of the microstructure of leather at a magnification of 500 times: a – before plasma modification; b – after plasma modification

a

b

Figure 3 - Micrographs of the macrostructure of leather at a magnification of 50 times: a – before plasma modification; b – after plasma modification

Thus, the processing of capillary-porous materials by high-frequency plasma leads to the change of their nanostructure, as a result of which leather of the new generation is created, hence, electrophysical modification discovers new opportunities of creation of natural materials with the set properties.

Literature

1. Shaehov, M. F., Modification of leather by underpressure high-frequency discharge / Shaehov M. F. / / Physical Electronics: Sat Proceedings of the III All-Russian conference. - Makhachkala. Publishing House CPI DSU, 2003.-p. 80-83.

2. Abutalipova, L. N., Experimental studies of modification of fancy leather in a nonequilibrium low-temperature plasma: preprint / L. Abutalipova, I. Krasina, Kazan State Tekhnol. Univ. - Kazan, 2003. – 32p.

3. Krasina, I. Effect of low-temperature plasma on the physico-mechanical and physico-chemical properties of leather / I. V. Krasina / / Math. institutions of higher education, Ser. Chemistry and chemical technology. - 2003. - № 6. - P.143-145.