STUDY OF THE PROPERTIES OF SULFUR OBTAINED BY DEVULCANIZATION OF AGED RUBBER WASTE

SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 174 STUDY OF THE PROPERTIES OF SULFUR OBTAINED BY DEVULCANIZATION OF AGED RUBBER WASTE G. Toshtemirova1, M. Tokhirov2, R. Adilov3 Laboratory assistant, "Clear" LLC1 Doctor of philosophy (PhD) in technical sciences, Chief Specialist of the Academic Activity Management Department, Institute “International School of Finance Technology and Science” 2 Doctor of technical sciences, Professor, Department “Technology of high-molecular compounds and plastics”, Tashkent Institute of Chemical Technology3 https://doi.org/10.5281/zenodo.17290718 Abstract. In this study, the physicochemical properties of free sulfur, isolated by chemical devulcanization of vulcanized rubber waste, were comprehensively studied. The devulcanization process was carried out using a mixture of FeCl3 and CH3COOH, and the effectiveness of the process was assessed using iodometric titration, IR spectroscopy, TGA/DTA, SEM-EDS, and XRD methods. The results showed that the optimization of devulcanization parameters (reaction time, temperature, acid concentration) directly affects the degree of C-C and C-C bond breaking. Based on iodometric analysis, up to 80% of the bound sulfur was converted to free state. The results of IR spectroscopy and thermogravimetric analysis showed changes in the structural and thermal stability of sulfur, while SEM-EDS clearly described morphological changes. The obtained results revealed the possibility of using devulcanized sulfur as an environmentally safe and promising raw material for industry. Keywords: devulcanization, rubber waste, sulfur, iodometric titration, IR spectroscopy, TGA/DTA, SEM-EDS, XRD, physicochemical properties, environmental safety. Introduction. In recent years, the issue of processing rubber waste and producing products with high added value has become one of the most pressing environmental and economic problems on a global scale. In particular, vulcanized rubber waste causes significant damage to the environment due to its inertness and very slow decomposition in natural conditions[1][2]. Therefore, the development of efficient, environmentally friendly, and cost-effective technologies for processing such waste is considered one of the priority areas of modern scientific research [3]. The high mechanical strength of vulcanized rubbers is determined by the presence of many covalent C-C and C-C bonds in their structure. The process aimed at purposefully breaking these bonds and restoring the processing properties of rubber is called devulcanization[4][5]. Devulcanized rubber materials are used as a promising raw material in the production of polymer composite materials (PCM) with high mechanical, thermal, and chemical properties by composition with polymer matrices [6], [7]. Among the various devulcanization methods, the method of chemical bond breaking is of particular importance for increasing the productivity of C-C and C-C bonds using a mixture of hydrochloric and acetic acids. This method is carried out at relatively low temperatures, preserves the mechanical properties of the restored elastomer, and allows for the control of the physicochemical properties of composite materials by optimizing the process parameters [8]-[10]. The purpose of the research. The main goal of this research is an in-depth analysis of the physicochemical properties of sulfur, isolated as a result of the chemical devulcanization of aged SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 175 rubber waste. The main task of this work is to assess the effectiveness and selectivity of the process, expand the possibilities of its industrial and environmental application by determining the morphological, thermal, and chemical properties of the isolated sulfur, as well as to create a scientific basis for the development of environmentally friendly and economically viable technologies for processing rubber waste. Research object and methods. Vulcanized car tires and technical rubber waste were selected as the object of this study. These wastes have high strength and decompose very slowly in the natural environment due to the presence of numerous covalent C-C and C-C bonds in their structure [11], [12]. Therefore, during the processing of waste, the method of chemical devulcanization was used for the purpose of selectively breaking their sulfur bonds and restoring their processing properties [13]. This method was carried out as follows: Chemical devulcanization is another devulcanization method of obtaining devulcanized rubber, where the treatment is carried out using a chemical or devulcanizing agent to break down the sulfide modification network of the rubber tire, and often this method is combined with the thermal and mechanical devulcanization process to increase efficiency. In trade, these chemicals come from the classes of aliphatic, alkylphenol sulfides, amines, aromatic mercaptans, zinc salts, di/sulfides, and unsaturated compounds. Since these compounds have the property and tendency to react with radicals formed as a result of the breaking of bonds, they are often used to prevent the phenomenon of recombination of molecules. Table 1 presents a list of some chemicals studied as devulcanizing agents according to the classification group, Figure 1 shows the specific approach to chemically based devulcanization. Classification of some devulcanizing agents Table 1 Amine and ammonium compounds Organic sulfides and mercaptans Sulfur-free organic and inorganic compounds Thiolamine Xylene thiols Sodium dissolved Tetra-butyl-ammoniacinate (II) Diphenyl disulfide Lithium aluminum hydride N, N-dialkyl aryl amines Phenol sulfides Supercritical CO2 Figure 1. Chemical devulcanization scheme The devulcanization process was carried out using a mixture of hydrochloric and acetic acids (FeCl3/Cl2 + CH3COOH). This mixture selectively breaks the S-S and C-S bonds in vulcanized rubber through the protonation and oxidation stages, releasing free sulfur in the form SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 176 of hydrogen sulfide [14]. The isolated H2S gas was captured in a solution of cadmium acetate and sodium acetate and quantitatively analyzed by iodometric titration [15]. In order to optimize the process parameters, the influence of temperature, time, and reagent concentration was studied separately. The resulting sulfur was investigated using the following modern analytical methods: Iodometric titration - for determining the amount of free sulfur and assessing the effectiveness of devulcanization [14], IR spectroscopy - for analyzing the bonds in the molecular structure of sulfur [15], TGA/DTA - for determining the thermal stability and decomposition temperature of sulfur [16], SEM-EDS - for studying the morphology and elemental composition of particles [17], XRD - for analyzing the degree of crystallinity and phase composition [18]. The integration of these methods allows for a comprehensive assessment of the physicochemical properties of the isolated sulfur and determines the possibilities of its industrial and ecological application [19], [20], [21]. Results and analysis. As a result of the devulcanization process, the sulfur bonds inside the rubber structure - especially the C-C and C-C bonds - are broken, and a series of free or weakly bound sulfur compounds are formed. The quantitative and qualitative characteristics of this sulfur can be determined, and the degree of their separation or preservation in the polymer matrix can be assessed using the following analytical methods. For this, titration was carried out by the most effective classical method of determining the amount of free sulfur in the devulcanized sample using the iodometric titration method. The results of determining the sulfur content in the sample before and after devulcanization using the iodometric titration method are presented in Table 2. Initially, the change in the concentration of sodium thiosulfate solution from 0.01 M to 0.005 M was studied. Table of the dependence of sulfur content on the amount of sodium thiosulfate solution based on iodometric titration in devulcanized rubber samples. Table 2 № Titration solution Sulfur before devulcanizatio n (%) Sulfur after devulcanization (%) Change (%) 1 Na₂S₂O₃, 0.005 M 2.80 0.85 69.6 2 Na₂S₂O₃, 0.01 M 2.75 0.72 73.8 3 Na₂S₂O₃, 0.01 M 2.60 0.65 75.0 4 Na₂S₂O₃, 0.01 M 2.78 0.59 78.8 Analyzing the data in Table 1, sample No. 1 used a 0.005 M thiosulfate solution, and the sulfur yield was 69.6%. The remaining three samples were titrated at a concentration of 0.01 M, in which the amount of sulfur released was higher (73.8-78.8%). This indicates that when titrating with a low concentration of thiosulfate solution, the accuracy is low, and due to incomplete neutralization of iodine or uncertainty in the titration, a smaller amount of free sulfur is recorded. These tests showed that 73.8%, 75.0% and 78.8% of sulfur was released, respectively. Based on the difference in the previous amount of S (2.60 - 2.78%) and the subsequent decrease in the amount, a significant increase in the level of released sulfur was observed. These frequency tests showed a stable and reliable level of accuracy for a 0.01 M Na2S2O3 solution. In particular, sample No. 4 with an excretion rate of 78.8% indicates the highest effectiveness of SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 177 devulcanization. When titrating with 0.005 M Na2S2O3, the accuracy of sulfur is lower, since in this case, incomplete neutralization of iodine, prolonged titration, and indistinct color of the indicator are observed. A 0.01 M Na2S2O3 solution is the most optimal, allowing for a more complete determination of the amount of sulfur released, with no significant difference in repeated tests. The changes in the table directly confirm the effectiveness of the devulcanization process in breaking sulfide bonds. At the next stage of our research, the influence of changes in the concentration of hydrochloric acid on the amount of sulfur released from devulcanized rubbers was also studied. The obtained results are presented in Table 3. Table of the dependence of sulfur content on hydrochloric acid concentration based on iodometric titration in devulcanized rubber samples. Table 3 № HCl conc. (%) S content before devulcanizati on (%) S content after devulcanization (%) Sulfur content (%) 1 5% 2.75 0.82 70.2 2 10% 2.72 0.63 76.8 3 15% 2.69 0.51 81.0 4 20% 2.66 0.47 82.3 As can be seen from Table 3 below, the total sulfur content in the samples before devulcanization was ~2.66-2.75%, which was in bound (C-S, S-S) forms. The amount of sulfur after devulcanization decreased with increasing HCl concentration: 0.82% at 5% HCl and 0.47% at 20% HCl. This means that an increase in the amount of HCl increases the rate and depth of the devulcanization reaction, while the amount of bound sulfur decreases significantly. When calculating the sulfur yield as a percentage, the maximum result was recorded at 20% HCl (~82.3%). In general, the amount of sulfur determined by the iodometric method in the sample before devulcanization by iodometric titration averaged 2.6-2.8%. After devulcanization, a decrease in sulfur content to 0.6-0.8% was observed. This indicates that as a result of the process, up to 80% of the bound sulfur was broken off and transferred to a free form. Iodometric titration made it possible to quantitatively assess the effectiveness of the process, and a significant decrease in the sulfur content confirmed the successful course of the process. At the next stage of our research, the structure of devulcanized sulfur and the structure of pure sulfur were determined using the IR spectrum to determine the presence or absence of sulfur bonds (C-S, S-S). They manifested themselves in the spectrum with characteristic vibrational lines. The IR spectra of the rubber crumb, not initially devukanized, are shown in Fig. 2. Fig. 3 shows the IR spectra of pure sulfur and sulfur after devulanization. In the initial (vulcanized) sample, intense peaks were recorded in the range of 500-600 cm−1 for the C-C bond and 650-700 cm−1 for the C-C bond. The IR spectra of sulfur in Fig. 3 showed a significant decrease in the intensity of these peaks in the post-devulcanization spectra. Also, the appearance of new C=C bonds or aromatic groups may have been detected in the range of 1600-1700 cm−1. Changes in the IR spectrum confirm the decomposition of sulfur bonds and directly indicate structural changes. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 178 Figure 2. IR spectra of rubber crumb before reaction Figure 3. IR spectra of pure sulfur Figure 4. IR spectrum of sulfur isolated from devulcanized rubber crumbs SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 179 Continuing our research, the nature of the loss of sulfur components under the influence of heat was studied using thermogravimetric analysis. Thermal stability was analyzed by differentialthermal and thermogravimetric methods on the DTG-60 Shimadzu device of Japan in an argon medium. According to the analysis results, sulfur evaporated in the range of 200-400°C. The obtained result can be seen in Figure 5. Figure 5. TGA analysis by sulfur content Before devulcanization, up to ~5% of the mass loss in the sample was due to sulfur. In the sample after devulcanization, this indicator decreased to ~1.2-1.5%. This result also shows that a large part of the sulfur was released from the structure as a result of devulcanization. With the help of TGA, the change in sulfur relative to temperature was determined, and this analysis confirmed the iodometric results on a thermal basis. Based on the results of SEM (Scanner Electron Microscopy) and EDS (Energy-Dispersive Spectroscopy), the changes in the microstructure and sulfur distribution of vulcanized and devulcanized rubber were compared as follows: 1. Non-devulcanized rubber (starting position): SEM image: Sulfur-containing aggregates (white spots) are clearly visible on the rubber surface. These aggregates were unevenly distributed, indicating the accumulation of sulfur during the vulcanization process. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 9 SEPTEMBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 180 EDS analysis: The sulfur concentration is 2.4%, which corresponds to the additional amount of sulfur involved in vulcanization. 2. Devulcanized rubber: SEM image: increased porosity (voids) on the surface, which is a result of the release of sulfur during devulcanization and the destruction of polymer networks. The sulfur stains have significantly decreased, confirming the breakdown of chemical bonds. EDS analysis: Sulfur content decreased to 0.7%, which showed a decrease of approximately 71%. SEM images clearly showed the disappearance of sulfur aggregates on the surface of devulcanized rubber and the porosity of the structure. EDS data confirmed a quantitative reduction in sulfur content, which indicates the effectiveness of the devulcanization process. The results proved that the devulcanization process plays an important role in removing sulfur from rubber and changing its physicochemical properties. Conclusion. In this study, the process of chemical devulcanization of vulcanized rubber waste was studied, and as a result, the physicochemical properties of the isolated free sulfur were comprehensively analyzed. It was established that the devulcanization method based on a mixture of FeCl3 and CH3COOH selectively breaks C-C and C-C bonds, ensuring effective separation of sulfur. The quantitative composition, thermal stability, degree of crystallinity, and morphology of the obtained sulfur were assessed using modern analytical methods, such as iodometric titration, IR spectroscopy, TGA/DTA, SEM-EDS, and XRD. The results showed that the optimization of devulcanization parameters (temperature, reagent concentration, time) directly affects the degree of sulfur release and allows for a significant improvement in its quality indicators. 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