PREPARATION AND CHARACTERISATION OF PHOSPHORUS-CONTAINING STARCH-BASED BIODEGRADABLE POLYMERS

ABSTRACT

This study is dedicated to the synthesis of environmentally safe starch-based biomedical modifiers, and the processes of chemically modifying starch using ammonium polyphosphate were investigated. The starch–ammonium polyphosphate etherification reaction was carried out at various mole ratios under the influence of temperature and time. The yields, swelling degree, and chemical composition of the reaction products were analyzed. The synthesized starch–ammonium polyphosphate gel was used as a biomedical modifier to modify polyethylene-based composites. Infrared spectroscopy, thermogravimetric (TGA), and differential thermal analysis (DTA) of the resulting composites showed that they have high thermal stability. The research results justify the prospects for using the modified polymer composites as environmentally friendly, biodegradable, and thermally stable materials in the packaging, agricultural, and industrial sectors.

АННОТАЦИЯ

Данное исследование посвящено синтезу экологически безопасных биомодификаторов на основе крахмала, а также изучению процессов химической модификации крахмала с использованием полифосфата аммония. Реакция этерификации крахмала с полифосфатом аммония проводилась при различных мольных соотношениях под воздействием температуры и времени. Были проанализированы выходы, степень набухания и химический состав продуктов реакции. Синтезированный гель крахмал-полифосфат аммония был использован в качестве биомедицинского модификатора для модификации композитов на основе полиэтилена. Инфракрасная спектроскопия, термогравиметрический анализ (ТГА) и дифференциальный термический анализ (ДТА) полученных композитов показали, что они обладают высокой термической стабильностью. Результаты исследования подтверждают перспективность использования модифицированных полимерных композитов в качестве экологически чистых, биоразлагаемых и термостабильных материалов в упаковочной, сельскохозяйственной и промышленной отраслях.

Keywords: starch, ammonium polyphosphate, biomodifier, etherification, polymer composites, thermal stability, biodegradability.

Ключевые слова: крахмал, полифосфат аммония, биомодификатор, этерификация, полимерные композиты, термическая стабильность, биоразлагаемость.

Introduction. The growing global concern for environmental sustainability, coupled with the problems associated with the long-term persistence of conventional synthetic polymers in the environment, has sharply increased scientific and practical interest in the development of biodegradable materials. In this respect, the development of new functional materials based on renewable natural polymers, particularly polysaccharides, is considered a priority area in modern polymer chemistry [1]. Starch is a polysaccharide derived from plant sources, which is inexpensive and environmentally safe, and has wide application potential in many industrial sectors. However, its high hydrophilicity, low mechanical strength, and limited thermal stability restrict the practical applications of starch-based materials. Therefore, improving the structural and functional properties of starch through chemical modification of its macromolecules is a pressing scientific challenge [2].

Modifying starch with phosphorus-containing compounds, particularly using ammonium polyphosphate, allows for the introduction of phosphate and ammonium groups into the macromolecular chains. As a result, gel-forming, highly swelling and heat-resistant biomedical modifiers are obtained. When such compounds are combined with synthetic polymers, they significantly improve the thermal and operational properties of composite materials.

In this work, the kinetic and thermodynamic conditions of modifying starch with ammonium polyphosphate were investigated, and the effect of the resulting biomedical modifier on the properties of polymer composites based on polyethylene and polyacrylonitrile was comprehensively studied [5–6].

Experimental Section. Starch and ammonium polyphosphate were chosen as the main reagents in the studies. Ammonium polyphosphate was synthesized in the laboratory from a specific ratio of orthophosphoric acid and ammonium hydroxide. Subsequently, the obtained substance was subjected to an etherification reaction with starch in various molar ratios.

The reaction was carried out in a closed system—an autoclave—at 130 °C and a pressure of 1.5–1.7 atm for 2.5 hours. Upon completion of the process, the reaction mixture was gradually cooled to room temperature and a gel-like mass was formed by adding distilled water. For purification, the product was washed with a 70% ethanol solution and then dried at room temperature.

The obtained results showed that increasing the molar ratio of starch to ammonium polyphosphate leads to an increase in the product yield. It was noted that the highest yield was achieved at a starch: APP ratio of 1:1.5, reaching 80%. Analysis of the effect of temperature showed that the yield increases when the reaction temperature is raised to 130–135 °C, with no significant change at higher values.

The phosphorus and nitrogen contents in the chemical composition of starch–ammonium polyphosphate samples were determined using standard analytical methods. An intensification of the reaction conditions was observed to increase the proportion of phosphate and ammonium groups. This phenomenon is explained by the multifunctional polyelectrolytic properties of ammonium polyphosphate.(See Table 1.)

Table 1.

Yield of starch–ammonium polyphosphate etherification reaction products (K: APP)

Note*. Theoretically, the yield of the reaction products is relatively high, amounting to 80–82% depending on the esterification conditions.

In our subsequent experiment, the effect of temperature on the yield of starch ammonium polyphosphate was investigated. The results obtained are presented in Table 2 below: From the table, it can be seen that in the starch ammonium polyphosphate esterification reaction the temperature range of 80–140 °C. When the reaction is carried out for up to 2–3 hours at a temperature in the range of 80–140 °C, it can be seen from the table that the product yield reaches approximately 80% at a molar ratio of 1:1.5.

Table 2.

Effect of temperature on product yield

Figure 1. IR spectra of starch and ammonium polyphosphate

The IR spectroscopy of the substances obtained during the experiment was analysed. A valence vibration belonging to the bonded OH group is present in the region of 3089.96 and 3055.24 cm^-1, the –C–H– group valence vibration in the 2922.16–2852.72 cm⁻¹ region, the starch ring valence vibration at 1653 cm⁻¹, and the NH₄ group valence vibration at 3234.62 cm⁻¹, The deformation vibration at 1435.04 cm⁻¹ shows characteristic absorption bands of the bending and stretching vibrations. The P–O–C bonds also gave characteristic valence and deformation absorption bands at 1072.42 cm⁻¹. In IR spectroscopy, the characteristic valence vibration of the C–N bond at 800.46 cm⁻¹ and the region corresponding to the P=O bond at 1259.52 cm⁻¹ allow us to observe the bonds associated with the starch-bound ammonium polyphosphate.

TGA and DTA analysis: To evaluate the thermal stability of modified polyacrylonitrile, analyses were performed using thermogravimetric analysis and differential thermal analysis (using a Shimadzu DTG-60/60H instrument with an accuracy of 99%).

It was found that the weight-loss kinetics of the polyethylene and polyethylene starch–ammonium polyphosphate composition depend on temperature (Figure 2). As can be seen from the results, the weight loss of polyethylene is observed in the temperature range of 226.17–601.73 °C.

In the first stage of the degradation process of the modified polyethylene, a weight loss of 6.775% was observed in the temperature range of 28.01–219.12 °C. In the subsequent stage, this reached 15.808% in the range of 219.12–406.87 °C. Furthermore, it was noted that when the thermal decomposition temperature of the polyethylene-APF-starch composites reached 601.6°C, the polyethylene had completely decomposed, while the composite's decomposition had reached 62.9%. The results of the analysis show that the composite is 36.07% thermally stable compared to polyethylene [4].

Figure 2. Thermal analysis of PE modified with starch–ammonium polyphosphate (PE: St–APP)

TGA and DTA analysis results for the polymer samples modified with starch, ammonium polyphosphate, and F-0220 polyethylene are presented in Tables 3 and

Table 3.

TGA analysis of PE: St-APP polymer

Table 4.

DTA analysis of PE: St-APP polymer

Results and Discussion. The synthesized starch–ammonium polyphosphate biomedical modifier was used to modify polyethylene-based polymer matrices. The structural changes in the resulting composites were evaluated by infrared spectroscopy. The appearance of absorption bands characteristic of phosphate and ammonium groups in the IR spectrum confirms the formation of chemical bonds between the starch macromolecules and the ammonium polyphosphate.

The thermal stability of the materials was investigated using thermogravimetric and differential thermal analysis methods. The results of the study showed that the thermal decomposition temperature of the polymer composites modified with starch–ammonium polyphosphate occurs at higher values compared to pure polyethylene. It was found that the overall thermal stability of the composites increased by approximately 36% as a result of the modification. This indicates that the starch–ammonium polyphosphate acts as an effective component against thermal effects within the polymer system, thereby expanding the potential for the industrial application of this biomodified.

Conclusion. As a result of the scientific research conducted, a new biomedical modifier material with a gel structure based on starch and ammonium polyphosphate was synthesized. The optimal conditions for the starch–ammonium polyphosphate etherification reaction were determined, and it was shown that the highest product yield is achieved at a starch: APF molar ratio of 1:1.5 and a temperature of 130–135 °C. The results of chemical and thermal analyses confirmed that the obtained biomodification significantly enhances the thermal stability of polyethylene-based composites. The research findings scientifically substantiated the potential of starch–ammonium polyphosphate-based materials for creating environmentally safe, biodegradable, and heat-resistant composites.

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