STUDY OF THE PROCESSES FOR OBTAINING N-HYDROLYZED FORMS BASED ON PAN WASTE AND THEIR CHARACTERISTICS

ABSTRACT

The article presents the results of a study on the hydrolysis of polyacrylonitrile (PAN) aimed at obtaining modified polymers with improved physicochemical properties. The influence of key process parameters, such as reaction duration and temperature, on the composition and degree of hydrolysis of PAN was examined. It was established that during hydrolysis, a gradual conversion of nitrile groups into amide and carboxyl groups occurs, accompanied by an increase in the degree of hydrolysis and changes in the functional characteristics of the polymer. The obtained hydrolyzed forms of PAN exhibit high reactivity and are promising for use in various industrial applications, including the creation of functional additives. The work demonstrates the feasibility of efficient processing of PAN waste and its transformation into valuable products, which is of significant importance for addressing environmental and technological challenges.

АННОТАЦИЯ

В статье представлены результаты исследования гидролиза полиакрилонитрила (ПАН) с целью получения модифицированных полимеров с улучшенными физико-химическими свойствами. Рассмотрено влияние ключевых параметров процесса, таких как продолжительность реакции и температура, на изменение состава и степени гидролиза ПАН. Установлено, что в процессе гидролиза происходит постепенное преобразование нитрильных групп в амидные и карбоксильные, что сопровождается увеличением степени гидролиза и изменением функциональных характеристик полимера. Полученные гидролизованные формы ПАН обладают высокой реакционной способностью и перспективны для использования в различных промышленных приложениях, включая создание функциональных добавок. Работа демонстрирует возможность эффективной переработки отходов ПАН и их преобразования в ценные продукты, что имеет важное значение для решения экологических и технологических задач.

Keywords: polyacrylonitrile, hydrolysis, degree of hydrolysis, functional groups, amide groups, acid number, nitrogen content.

Ключевые слова: полиакрилонитрил, гидролиз, степень гидролиза, функциональные группы, амидные группы, кислотное число, содержание азота.

Introduction. Recycling industrial waste into useful materials is becoming increasingly important in light of modern environmental and economic challenges. In particular, polyacrylonitrile (PAN) waste is of considerable interest due to its potential to create new functional materials through the hydrolysis process. These hydrolyzed products not only contribute to waste reduction but can also play a key role in the petroleum industry due to their potential depressant properties [1, 2].

Polyacrylonitrile (PAN) has a wide range of industrial applications due to its outstanding physical and chemical properties and affordability. However, its robust structure and high resistance to chemical agents and solvents limit its use. Hydrolysis processes are often used to modify the properties of PAN, converting it into products with functional groups that improve interfacial interactions and increase the reactivity of the material [3].

Studies show that H-hydrolyzed forms of polymers obtained from PAN can effectively reduce viscosity and prevent wax crystallization, which is extremely important for ensuring uninterrupted transportation of oil, especially in cold conditions. This makes them promising candidates for use in the compositions of depressant compositions, which are used at various stages of oil production and refining [4].

Thus, the research and development of new methods for obtaining and using H-hydrolyzed forms of PAN polymers is becoming a relevant area at the intersection of chemistry, materials science and the oil industry, opening up new opportunities for innovative waste disposal and improving the environmental safety of production processes.

Experimental part. PAN waste obtained from NavoiAZOT LLC is textile fibres or bundles of various colours. It is a linear polymer with a molecular weight of over 30,000 g/mol, a density of 1.14-1.15 g/cm³ and a melting point of up to 220 °C. It is characterised by high strength, low solubility, good weather resistance and comparative resistance to chemical influences. Compared to technical PAN, the waste is less hygroscopic (9-10% versus 15%) and has a moisture absorption of 0.9-1.0% at 20 °C and a relative humidity of 41%.

These wastes are resistant to aggressive environments, including concentrated acids (HNO₃, H₂SO₄) alkalis and organic solvents such as dimethylformamide and dimethylacetamide, which makes them promising raw materials for chemical processing. The use of 10% sulfuric acid in the acid hydrolysis process allows for the effective decomposition of PAN, preserving the structure of the hydrolysis products, and the production of modified polymers or composites for various industrial purposes.

The starting material was PAN in the form of fibers, and the reaction was carried out with a 10% sulfuric acid solution. The process took place in a round-bottomed flask with three necks, equipped with a reflux condenser to prevent liquid loss. The mixture was stirred with a mechanical stirrer, and a nitrogen flow provided an inert atmosphere, eliminating oxidation [5].

First, crushed PAN fibers were added to the flask in a ratio of 1:5 to 1:10 to the mass of sulfuric acid. Then, the acid solution was added and mixed thoroughly, heating to 50 °C. At this stage, the PAN fibers dissolved due to intensive mixing. Then the temperature was increased to 90 °C to carry out the main stage of hydrolysis. The reaction duration, varying from 1 to 6 hours, made it possible to determine the effect of temperature and time on the yield and properties of the hydrolyzed product.

After completion of hydrolysis, the mixture was cooled and neutralized with sodium hydrogen carbonate solution to neutral pH. Then an organic solvent (e.g., DMSO) was added to completely dissolve the hydrolyzed PAN and isolate insoluble impurities. The organic phase was separated, washed with water, and the solvent was evaporated on a rotary evaporator. The precipitate was dried at 50-60 °C until constant weight was achieved [6].

The yield of the polyacrylonitrile hydrolysis process was determined by the Kjeldahl method to establish the amount of nitrogen and by measuring the acid number. First, the nitrogen content in the initial mixture (N1) was determined, then in the system after hydrolysis was complete (N2). The difference between N1 and N2 allowed us to judge the degree of hydrolysis of the initial material. At the same time, the acid number (K) was measured, characterizing the number of acid groups formed in the hydrolyzed product. The combination of these data allowed us to estimate the efficiency of hydrolysis and the degree of conversion of polyacrylonitrile into its hydrolyzed forms.

Results and discussion. The results of the study allowed us to evaluate the influence of key parameters such as temperature, reaction time and reagent ratio on the PAN hydrolysis process. This section presents the experimental data and analysis of the obtained hydrolysis products.

Analysis of changes in acid number (AN) and nitrogen content (CN) depending on the duration of hydrolysis at a temperature of 70 °C (Fig. 1) shows patterns associated with the course of the main chemical reactions.

Figure 1. Change in acid number and nitrogen content depending on the duration of hydrolysis at a temperature of 70 °C

In the initial period of hydrolysis, during the first hour, the nitrogen content decreases significantly from 26.1 to 14.476 g/100 g. This indicates the destruction of nitrogen-containing groups in the structure of polyacrylonitrile under the action of hydrolysis. As the process continues, the rate of decrease in nitrogen content slows down, and by the fifth hour, the minimum CN value of 11.91 g/100 g is observed. This indicates the completion of the main stage of removal of nitrogen-containing components [7].

The acid number, on the contrary, demonstrates the opposite dynamics. Already at the initial stage of hydrolysis, its growth is observed, starting from 24.815 by the first hour and reaching 28.93 by the end of the second hour. This growth is due to the formation of new acid groups during hydrolysis. At later stages, starting from the third hour, the increase in the acid number continues, but with less intensity. The maximum value of the acid number, equal to 34.497, is reached by the fifth hour, which indicates the completion of the formation of acidic functional groups.

Thus, at the initial stages (up to two hours), the hydrolysis process is characterized by intensive destruction of nitrogen-containing structures and simultaneous formation of acid groups. At later stages (three to five hours), stabilization of the processes occurs, which indicates the completion of chemical transformations and the achievement of an equilibrium state. Increasing the duration of hydrolysis to five hours allows achieving a minimum nitrogen content with maximum accumulation of acid groups, which makes the hydrolysis product promising for further applications.

Table 1.

Changes in the content of amide groups and the degree of hydrolysis depending on the duration of the process

*-duration of hydrolysis .

The content of amide groups decreases with increasing duration of the hydrolysis process. At the initial stage, after 1 hour, the content of amide groups is 0.739 mmol/100 g, which indicates a high concentration of these functional groups in the starting material. As the duration of the process increases, their gradual decrease is observed, reaching a minimum value of 0.587 mmol/100 g by the fifth hour. This indicates the destruction of amide groups as a result of hydrolysis and their transformation into other functional groups, such as carboxyl groups [8].

The degree of hydrolysis, on the contrary, increases with the increase of the process time. In the first hour it is 30%, which confirms the initial stage of hydrolysis. As the reaction proceeds, the degree of hydrolysis increases, reaching 42% by the fifth hour. The growth of this indicator indicates the progressive decomposition of amide groups and the formation of acidic functional groups.

Table 2.

Change in the content of amide groups and the degree of hydrolysis from the process temperature (duration 3 h)

The content of amide groups gradually decreases with increasing process temperature. At the minimum temperature (1 hour), it is 0.660 mmol/100 g, which reflects a lower intensity of hydrolysis. With an increase in temperature to 3 hours, the content of amide groups decreases to 0.605 mmol/100 g, which indicates a gradual destruction of amide bonds. With a further increase in temperature to 5 hours, the content of amide groups decreases to 0.550 mmol/100 g, which indicates an acceleration of hydrolysis reactions at higher temperatures.

The degree of hydrolysis, on the contrary, increases with the temperature. At the minimum temperature, the degree of hydrolysis is 29%, which corresponds to the initial stage of hydrolysis. With the temperature increase to 3 hours, it reaches 39%, and with the temperature increase to 5 hours – 49%. This confirms that the increase in temperature promotes the intensification of hydrolysis, leading to a more complete destruction of amide groups and the formation of acid functional groups. The products obtained under such conditions are characterized by the following composition (Table 3) [2,5].

Analysis of the composition of hydrolyzed forms of polyacrylonitrile (PAN) depending on the duration of hydrolysis shows that the content of nitrile groups (-CH₂-CH(CN)) remains constant throughout the process, with a relative value equal to one. This indicates that the main framework of the polymer remains unchanged, and changes in the structure occur due to modification of functional groups.

Table 3.

The ratio of each compound in hydrolyzed PAN

*-duration of hydrolysis.

The content of amide groups (-CH₂-CH(CONH₂)) increases with increasing duration of hydrolysis. In the first hour, their ratio is 1.502 relative to nitrile groups, indicating the initial stage of transformation of nitrile groups into amide groups. As the duration of hydrolysis increases, the proportion of amide groups increases, reaching a maximum value of 2.335 in the fifth hour. This indicates a progressive process of hydrolysis of nitrile groups with the formation of amide compounds [4,7].

The content of carboxyl groups (-CH₂-CH(COOH)), on the contrary, demonstrates nonlinear dynamics. In the first hour of hydrolysis, their ratio is 2.505, which indicates the initial formation of carboxyl groups. However, with an increase in time up to the third hour, a decrease in their content to 2.051 is observed. This may be due to competition between the formation of amide and carboxyl groups or secondary reactions. After the third hour, the content of carboxyl groups increases again, reaching 2.272 in the fourth hour, and then slightly decreases to 2.225 in the fifth hour, which indicates stabilization of the transformation processes.

Conclusion. The conducted studies have shown that hydrolysis of polyacrylonitrile (PAN) is an effective method for modifying its structure and functional properties. It has been established that an increase in the duration of the process and an increase in temperature lead to a significant decrease in the content of amide and nitrile groups, accompanied by an increase in the degree of hydrolysis and the accumulation of carboxyl functional groups. Analysis of changes in the composition of hydrolyzed forms confirmed that the main transformations affect the functional groups, while the structure of the main PAN framework remains stable.

The obtained results indicate that the content of amide and carboxyl groups can be controlled by adjusting the hydrolysis parameters, such as temperature and reaction time. This opens up prospects for obtaining materials with specified physicochemical properties that can be used in various industrial applications. Hydrolyzed forms of PAN, which have high reactivity due to the presence of acidic functional groups, are of interest for the creation of new polymer compositions and functional additives.

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