Abstract
Recovery of fat-and-oil production wastes will reduce the technogenic impact on the environment, as well as involve them in a new production cycle as a secondary material resource. As part of solving this problem, the possibilities of using fat-and-oil production wastes in the production of a tyre reclaim and a modified tyre reclaim are considered. In the course of the studies, the fat-and-oil industry wastes’ sorption characteristics are determined, and in relation to oils in static and dynamic conditions, the spent reagent reclamation ways are determined. The authors obtained a tyre reclaim and a modified tyre reclaim using the fat-and-oil industry wastes (soap stock, diatomite, bleaching clay, and fatty acids isolated from soap stock). In this work, the authors studied the possibility of using the fat-and-oil industry wastes in the formulations of a tyre reclaim and a modified tyre reclaim. Extended physical and mechanical tests of experimental rubbers led to the conclusion that it is most expedient to use the fat-and-oil industry wastes in the formulation of rubber compounds for production of sleeper pads for railroad tracks, since when using a tyre reclaim and a modified tyre reclaim, the indicators of rubber properties practically do not change and comply with control standards.
1. Introduction
Currently, in our country, the amount of polymer wastes is more than one million tons per year, and the percentage of their use is still small. Taking into account specific properties of polymer materials, they are not subject to decay and corrosion; the problem of their recovery is, first of all, of an environmental nature. The total volume of solid domestic waste disposal in Moscow alone is about 4 million tons per year. Only 5,7% of the mass of wastes is recycled from the total level of wastes [1–3].
According to experts from Professional Integrated Solutions (PIS), the volume of the Russian tyre market in 2010 amounted to 35,2668 million units, which is 27% more than in 2009. In developed countries, the percentage of recovery of used tyres is approaching 100% and in Finland approaching 101%; according to the press service, more than 90 thousand tons of used car tyres are produced annually in Kazakhstan [4]. This means that the country disposes all tyres that have become unusable and has begun processing the accumulated stocks. Europe realized the need for used tyres: similarly, Finland plans to import 30 thousand tons of used tyres from Germany for further processing, since the country’s demand for this material exceeds the generated amount by 40-45 thousand tons [5].
It is known that worn tyres can be a source of cheap polymer raw materials in the production of reclaim from them, which is currently used in sports facilities, for the production of artificial turf [6]. A reclaim is a rubber waste product characterized by the ability to mix with raw rubber and ingredients and be repeatedly vulcanized. In terms of structure, composition, and properties, the reclaim is similar to the rubber compounds used for manufacturing of new products. During reclamation, thermal destruction of sulfur bonds occurs, as a result of which their content in the reclaim decreases. Many of the newly formed bonds in the reclaim are carbon-carbon. Rubber reclamation accelerators provide a decrease in the process’ duration or temperature, a decrease in the softener consumption, and an improvement in the technical qualities of the reclaim and rubber with its additives. Technological properties of rubber compounds containing the reclaim are improved. Therefore, when dividing the reclaim into technical grades, both of these factors are taken into account [7].
Issues related to the reclamation and further use of spent tyres and rubber products [8] are of current importance.
Currently, the main areas of their processing [9] are production of a tyre reclaim, which is used mainly as a filler in rubber compounds and as an additive in unsaturated products. Due to the improvement of the tyre reclaim formulations using the fat-and-oil industry wastes, it is possible to solve the problem of increasing the service life of rubber products and technological methods for their manufacture [10].
The main process of reclaim production is devulcanization. The devulcanization is a process in which vulcanized rubber wastes are converted by mechanical, thermal, and/or chemical energy to a state in which they can be mixed, processed, and repeatedly vulcanized [7].
When obtaining a tyre reclaim on a roller equipment, rubber crumb is premixed with chemical activators and softeners. Soap stock, a waste of fat-and-oil industry, is used as a softener.
The process of obtaining a tyre reclaim from tyre crumb includes operations of mixing ingredients in a mixer of bulk components and devulcanization of rubber in a cam extruder and on rollers at a temperature not exceeding 100°C.
To obtain a tyre reclaim, tyre crumb up to 2.0 mm in size is used, obtained by the bar-destructive grinding method [11]. A distinctive feature of such a crumb is its developed surface, which increases the susceptibility of the crumb to the effects of devulcanizing factors.
Operating temperature in the extruder is 70-100°C and on the rollers is 30-60°C. Technological modes for obtaining the reclaim and chemical reagents are selected in such a way as to ensure the devulcanization of rubber, i.e., maximally destroy transverse, most often polysulfide bonds, while preserving the raw rubber molecule from thermal destruction as much as possible. This allows to obtain a high-molecular rubber compound with plasticity and, after repeated vulcanization, rubber with a high level of mechanical properties. The measure of destruction is the amount of extractable impurities. The extract contains softeners. The content of softeners is estimated by acetone extract. The content of sol fraction is estimated, as a rule, by chloroform extract.
Carrying out the reclamation process at moderate temperatures allows it to be carried out without cooling with water and water vapor; therefore, the reclaim obtained by this method does not contain water. All this ensures the high quality of the obtained reclaim and creates prospects for its use in rubber technical goods.
The undoubted advantage of this method of obtaining a tyre reclaim is the low cost of the latter, which is achieved by low energy consumption and the use of soap stock—a waste of fat-and-oil industry.
In this work, the authors studied the possibility of using the fat-and-oil industry wastes (soap stock, diatomite, bleaching clay, and fatty acids isolated from soap stock) in the formulations of a tyre reclaim and a modified tyre reclaim.
Taking into account the potential of fat-and-oil enterprises in Kazakhstan, the issue of processing secondary resources is important in order to create domestic competitive products based on the fat-and-oil industry wastes after the neutralization of oils and fats.
Recovery of these wastes will reduce the technogenic impact on the environment, as well as involve them in a new production cycle as a secondary material resource. The creation of reclaim ingredients on their basis will allow replacing scarce and expensive chemical additives, including imported ones, based on petroleum products, will improve the technological properties of the reclaim while maintaining or increasing the required level of physical and mechanical parameters, and will also reduce the load on the environment. Therefore, the search for ways to use plant oil production wastes in the production of rubber technical goods is an urgent task, both from economic and environmental points of view.
The purpose of this work is to study the possibility of using the fat-and-oil industry wastes (soap stock, diatomite, bleaching clay, and fatty acids isolated from soap stock) in the formulations of a tyre reclaim and a modified tyre reclaim and their use in rubber compound formulations.
2. Research Materials and Methods
The research objects are the following: (i)Fat-and-oil industry wastes (soap stock, diatomite, bleaching clay, and fatty acids isolated from soap stock (Arai LLP, Shymkent))(ii)Tyre reclaim (Table 1)(iii)Modified tyre reclaim (Table 2)(iv)Rubber compound for sleeper pad OP 356 and CP 328 of ECO-Tyre LLP (Table 3)
IR spectral analysis of the fat-and-oil industry wastes (soap stock, diatomite, bleaching clay, and fatty acids isolated from soap stock) was carried out on Shimadzu IR Prestige-21 IR-Fourier spectrometer with attachment of attenuated total internal reflection (ATIR) Miracle from Pike Technologies (Figures 1–3).
The tyre reclaim was obtained in a rubber mixer (tyre reclaim, technical conditions, ST LLP 070540009816-04-2011) [12].
The modified tyre reclaim was obtained in a twin-screw mixer with simultaneous cooling by heat transfer to the design elements of the twin-screw mixer [13].
The rubber compound was obtained on rollers (GOST 14333-79E, Rubber processing rollers).
The research was carried out using the following methods: (i)The stress-strain properties of the samples were determined on a breaking machine according to GOST 270-75 [14](ii)The resistance of the samples to thermal aging in air was estimated by changing the relative elongation at break and nominal tensile strength after holding them in a thermostat at 90°C for 72 hours; the test was carried out in accordance with GOST 9.024-74 [15](iii)Shore hardness (GOST 263-75)
3. Results and Their Discussion
Figure 4 shows the dependence of the decrease in the weight of the bleaching clay sample at different temperatures on the time of the content of oily substances on the firing time. With increasing time, the amount of burnt out oily substances increases.
The maximum extraction of substances occurs at the highest temperature. This is due to the fact that at temperatures above 350°C, the process of oxidation of both fats and impurities contained in the montmorillonite rock begins. The initial oil content in the bleaching clay was 20,0%; therefore, the clay’s thermal decontamination should be carried out at a temperature of 400°C for 1 hour, or for 1.5 hours at a temperature of 350°C.
The basis of the infrared spectroscopy method is infrared radiation. All heated bodies radiate energy in the infrared spectrum, and the wavelength depends on the heating temperature—a short wavelength corresponds to a higher temperature and radiation intensity [16].
Despite the fact that oils from sunflower seeds are produced by different manufacturers and using different technologies, they have very similar IR spectra when compared visually. For comparison, the spectrum of soap stock refining sunflower oil and a mixture composition of unsaturated and saturated fatty acids, bleaching clay, and diatomite are shown in Figures 1–3).
Figure 1 shows the IR spectra of the initial soap stock obtained during the refining of sunflower oil and the mixed composition of unsaturated and saturated fatty acids isolated from it.
An important feature of the infrared spectroscopy method is that the absorption bands of the same type of vibration of the atomic group of various substances are located in a certain range of the infrared spectrum (for example, 3720-3550 cm-1: the range of valence vibrations of groups –OH and 3050-2850 cm-1: groups -CH, -CH2, and -CH3 of organic substances), and the maximum of the absorption band of the atomic group indicates the nature of the substance [16, 17].
In the spectra of plant oils, intense bands are usually present in the region from 2800 to 3000 cm-1; the bands of valence vibrations of groups C-H, 1720-1750 cm-1, are due to the mixing of the vibrations of groups C-H and C-C, as well as the mixing of the vibrations of groups C-H and C=O in fatty acid molecules, 1400-1450 cm-1; there are bands of planar deformation vibrations of groups C-H. These bands are commonly used to confirm the composition and identification of oils by the infrared spectroscopy method (cm-1) [18, 19].
The analysis of the IR spectra (Figure 1) in the region of 3000-650 cm-1 confirms the presence of classes of organic compounds in accordance with the absorption bands of functional groups. Intense bands with maxima at 2970, 2924, and 2854 cm-1 can be attributed to the valence vibrations of methylene groups (νasCH2 and νsCH2). Unsaturated hydrocarbons are characterized by C=C bond bands in the region of 1643 cm-1. Bands in the region of 1465 cm-1 correspond to the deformation vibrations of group СН2. Bands in the region of 3363 cm-1 indicate the valence vibrations of the bond OH. In addition to the hydrocarbon chain, the carboxyl group contributes to the IR spectrum of fatty acids, both vibrations of the carbonyl group (C=O) and C-O and O-H bonds are taken into account [10]. The intense band in the region of 1755 cm-1 corresponds to the carbonyl group of ester or lactone, and the bands in the region 1303-1076 cm-1 correspond to the vibrations of the bond CO [20].
It is known [21] that similar mineral compositions of rocks predetermine the presence of the same absorption bands in the spectra. The shape of the profiles, the width of the bands, and their intensity indicate the presence of certain silicate and aluminosilicate molecular groupings in various states.
The analysis of the IR spectrum (Figure 2) shows that the main bands manifested in them are related to the valence bonds of silicon with oxygen and hydrogen with oxygen. The absorption band ~1450 cm-1, corresponding to the deformation vibrations of groups OH- at the vertices of silicon-oxygen tetrahedra, is a distinctive feature of silicates. On the groups OH of layered silicates, adsorption of water or other agents with oxygen atoms on the surface is possible by the mechanism of hydrogen bond formation. According to the literature data [21], montmorillonite has surface OH groups only on the “peripheral” parts of the particles.
In the literature [22], a study of the structural features of diatomite using the IR spectroscopy showed that the IR spectra of diatomite correspond to the spectra of cryptocrystalline and amorphized forms of silica, since unlike the spectra of crystalline forms of silica, they are more “poor,” and absorption bands compared to corresponding bands of crystalline forms of SiO2 are diffuse and less intense, but their position in the spectrum exactly coincides with the position of similar bands of β-quartz. In particular, in the region of 1200-4000 cm-1, an intense band is found at 1200-1000 cm-1, related to the valence and deformation O-Si-O and Si-O-Si vibrations of silicon-oxygen skeleton tetrahedra; a less intense double band in the region of 830-750 cm-1, due to Si-O-Si vibrations of rings from SiO4 tetrahedra; and strong absorption bands around 530 and 460 cm-1, attributed to the deformation vibrations of SiO4 tetrahedron.
A feature of the IR spectra of diatomite (Figure 3) was the manifestation of molecular water (in the form of absorption bands in the region of 3750-3500 and 1580-1600 cm-1), the presence of which in amorphous and cryptocrystalline forms of silica (in this case, in the composition of diatomite) is consistent with the large friability of their structure with rather large voids, sufficient to accommodate water molecules in them.
The data obtained on the properties of the waste masses, the composition of their organic part containing a complex mixture of fatty acids, supplement the information on the fat-and-oil industry wastes and form the basis for determining promising ways to use them as secondary material resources [16].
Thus, studies of the fat-and-oil industry wastes (soap stock, diatomite, bleached clay, and fatty acids isolated from soap stock) showed that the fat-and-oil industry wastes can provide the technical characteristics of the reference reclaim when used in a the modified tyre reclaim [23].
ECO-Tyre LLP developed a flexible method for obtaining a tyre reclaim based on affordable, cheap fat-and-oil industry wastes instead of several expensive components (activator, colophony, and bitumen) [12].
ECO-Tyre LLP is the largest company in the Republic of Kazakhstan for recovery/recycling of worn tyres and rubber-containing wastes. Working in the field of waste management, the company solves the issue of the region’s environmental safety, eliminating accumulations of out-of-order car tyres, and develops the direction of resource saving through the use of secondary raw materials in the production of consumer goods.
The prepared tyre crumb was initially mixed with soap stock, a waste of the fat-and-oil industry; then, the mixture was fed from the rubber mixer to the already heated rollers. During some time of processing on the rollers, devulcanization occurred in the mixture under the influence of pressure and temperature. As a result of processing, the mixture turned into a sheet, which can be further used for further processing, for example, for loading into an extruder in order to obtain rubber technical goods. Preliminary experiments included several stages of processing the mixture with soap stock, a waste of the fat-and-oil industry: processing on a mixer at a temperature of more than 180°C and then on the rollers [12].
The tyre reclaim composition is shown in Table 1.
Using soap stock, a waste of the fat-and-oil industry, according to Table 1, it can be said that when it was used, minimal chemical components were used. At the same time, quite little energy was spent and the hardware design did not take up large areas.
The choice of technological modes for obtaining a tyre reclaim is determined by the available equipment, as well as the requirements for the reclaim, due to its further use.
During some time of processing on the rollers, devulcanization occurred in the mixture under the influence of pressure and temperature. As a result of processing, the mixture turned into a sheet, which can be further used for further processing, for example, in order to obtain rubber technical goods. The devulcanization degree of the processed mixture was determined by the method of acetone-chloroform extraction. The results of its measurement are presented in Table 4.
According to Table 4, it can be said that, due to the high degree of devulcanization, it is possible to obtain rubber technical goods from the tyre reclaim.
The effect of soap stock on the technological and vulcanization characteristics of the reclaim and the physical and mechanical properties of its vulcanizates was studied.
The tyre reclaim properties are shown in Table 5.
From the data of Table 5, it can be seen that the plasticity, strength, and relative elongation of the tyre reclaim increase.
ECO-Tyre LLP developed a flexible method for obtaining a modified tyre reclaim based on affordable, cheap fat-and-oil industry wastes instead of expensive carbon black [13].
A modifier was added to the obtained tyre reclaim [12] (Table 2). The fat-and-oil industry wastes (diatomite, bleaching clay, and fatty acids isolated from soap stock) were used as a modifier. The mixing of the components (reclaim and modifier) was carried out in a twin-screw mixer [13].
In the twin-screw mixer, the reclaim was mixed with the modifier, with simultaneous physical and chemical reactions [24] between the active centers of raw rubber macromolecules, including various radicals that arose during the destruction of polymers and modifiers. As a result of such reactions, structuring occurred in polymer chains, which ultimately led to the improvement in the reclaim properties—increase in its viscosity and physical and mechanical properties. Such active mixing allowed to obtain a homogeneous mixture of the reclaim and the modifier, improve the modification process, and obtain a product of uniform properties and high quality. Further, as the mixture passed through the mixer’s internal volume, the temperature decreased. At the mixer’s outlet, the modified tyre reclaim had a temperature of 60-80°C. Intensive cooling was carried out due to heat transfer by the mixer’s water-cooled elements. A rapid decrease in the reclaim’s temperature in the twin-screw mixer contributed to the termination of the raw rubber macromolecule destruction reactions, which also led to the improvement in the reclaim’s physical and mechanical properties. The unloading was carried out using the end screw sections, the ridges of which were oriented in the opposite direction than the ridges of the mixer’s conveying sections.
The modified tyre reclaim composition is shown in Table 2. In the modified tyre reclaim formulation, carbon black was replaced with the fat-and-oil industry wastes (diatomite, bleaching clay, and fatty acids isolated from soap stock).
Table 6 shows the technical characteristics of the obtained modified tyre reclaims, as well as the reclaim [13].
According to Table 6, it is possible to state the increase in the technological and technical indicators of the reclaim and samples obtained from the reclaim.
Further, the research consisted in the development of a rubber compound formulation for manufacture of sleeper pads for railway tracks.
From the obtained tyre reclaim [12] and modified tyre reclaim [13], a rubber compound was made (Table 3).
ECO-Tyre LLP uses the tyre reclaim and modified tyre reclaim in the formulation of the rubber mixture for manufacture of sleeper pads for railway tracks.
The test results of the obtained rubber mixture are shown in Table 7.
Extended physical and mechanical tests of experimental rubbers led to the conclusion that it is most expedient to use the fat-and-oil industry wastes in the formulation of rubber compounds for production of sleeper pads for railroad tracks, since when using a tyre reclaim and a modified tyre reclaim, the indicators of rubber properties practically do not change and comply with control standards.
4. Conclusion
In this work, the authors studied the possibility of using the fat-and-oil industry wastes (soap stock, diatomite, bleaching clay, and fatty acids isolated from soap stock) in the formulations of a tyre reclaim and a modified tyre reclaim.
The authors obtained the tyre reclaim and the modified tyre reclaim using the fat-and-oil industry wastes. The test results showed that the fat-and-oil industry wastes provide the technical characteristics of the reference reclaim when used in the tyre reclaim and the modified tyre reclaim.
Extended physical and mechanical tests of experimental rubbers led to the conclusion that it is most expedient to use the fat-and-oil industry wastes in the formulation of rubber compounds for production of sleeper pads for railroad tracks.
Data Availability
Data is available on request to Sagitova Guzaliya (email: [email protected]).
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.