SYNTHESIS OF GUANIDINE ACRYLATE BASED ON GUANIDINE THIOCYANATE AND TECHNOLOGY FOR PRODUCING COMPOSITE ADHESIVES BASED ON IT

УДК 678.046+541.49

Abstract

In this work, a technology was developed to synthesize guanidine acrylate from guanidine rhodanide (NH₂)₂C=NH·HSCN through stepwise ion exchange reactions and to incorporate it into a composite adhesive with polymer matrices. The synthesis was carried out in four stages: guanidine rhodanide was reacted with dilute nitric acid (10–20%, 0–10°C, 1 atm) to yield guanidine nitrate and rhodanic acid; the thiocyanate anion was selectively precipitated as CuSCN by treatment with a 0.1–0.5 M aqueous solution of CuSO₄·5H₂O at 20–30°C; guanidine nitrate was reacted with acrylic acid (99%, 20–40°C, 1 atm, low-aqueous medium) to form the guanidine acrylate salt; guanidine acrylate (40 g) was blended with either polyvinyl acetate (PVA, hydrolysis degree 98–99%, 100 g) or epoxy resin (ED-20, 100 g), colloidal silica (30% SiO₂ sol, 15 g), and formalin (37% aqueous solution, 5 g), followed by thermal curing at 50–90°C and pH 4.5–7. Adhesion of the resulting composites was evaluated on metal, wood, glass, and ceramic substrates; compressive and tensile strength were measured in comparison with standard PVA adhesive; water resistance was assessed after 24 h immersion; thermal stability was confirmed at temperatures above 100°C; and antibacterial activity was determined against model bacterial strains. The optimal process parameters were established and two technological flowsheets (PVA-based and epoxy-based) were proposed, providing flexibility for multi-purpose industrial applications.

Аннотация

В данной работе разработана технология синтеза акрилата гуанидина из роданида гуанидина (NH₂)₂C=NH·HSCN посредством ступенчатых ионообменных реакций и его последующего введения в состав композиционного клея с полимерными матрицами. Синтез осуществляли в четыре стадии: (1) роданид гуанидина обрабатывали разбавленной азотной кислотой (концентрация 10–20%, температура 0–10°C, давление 1 атм) с получением нитрата гуанидина и роданистоводородной кислоты; (2) роданид-ион избирательно осаждали в виде CuSCN путём добавления водного раствора CuSO₄·5H₂O (0,1–0,5 М) при 20–30°C; (3) нитрат гуанидина реагировал с акриловой кислотой (99%, 20–40°C, 1 атм, малообводнённая среда) с образованием соли акрилата гуанидина; (4) акрилат гуанидина (40 г) смешивали с поливинилацетатом (ПВА, степень гидролиза 98–99%, 100 г) или эпоксидной смолой (ЭД-20, 100 г), коллоидным диоксидом кремния (30% золь SiO₂, 15 г) и формалином (37%-й водный раствор, 5 г) с последующим термическим отверждением при 50–90°C и pH 4,5–7. Адгезию полученных композитов оценивали на металлических, деревянных, стеклянных и керамических поверхностях; прочность на сжатие и растяжение определяли в сравнении со стандартным клеем ПВА; водостойкость оценивали после выдержки в воде в течение 24 ч; термическую стабильность подтверждали при температурах выше 100°C; антибактериальную активность определяли в отношении модельных штаммов бактерий. Установлены оптимальные параметры технологических процессов и предложены две технологические схемы (на основе ПВА и эпоксидной смолы), обеспечивающие гибкость применения в различных отраслях промышленности.

Keywords: guanidine rhodanide, guanidine acrylate, glue, polyvinyl alcohol, epoxy resin, colloidal silica, composite, ion exchange.

Ключевые слова: роданид гуанидина, акрилат гуанидина, клей, поливиниловый спирт, эпоксидная смола, коллоидный диоксид кремния, композит, ионный обмен.

Introduction

The requirements for modern adhesive compositions include not only high adhesion strength, but also antibacterial activity, thermal stability, and environmental safety. Guanidine-containing compounds are valuable monomers and modifiers in polymer chemistry due to their strong basicity and antimicrobial properties. In particular, acrylate derivatives of guanidine salts are prone to polymerization, allowing the synthesis of water-soluble polymers and adhesive compositions[1-3].

Guanidine rhodanide is a salt that is highly soluble in water and contains a guanidine cation and a thiocyanate anion. It requires selective separation of the thiocyanate anion to synthesize other salts that retain only the guanidine ion. Ion exchange reaction in the presence of nitric acid is a convenient method for producing guanidine nitrate, but the strong oxidizing properties of HNO₃ can lead to decomposition of the thiocyanate residue. This problem can be overcome by using diluted acid at low temperatures[3-5].

Polyvinyl acetate (PVA) and epoxy resins are widely used as polymer matrices, which, when cured with formalin, form a branched structure with high mechanical properties. Colloidal silica sol (SiO₂ sol) serves as a filler and reinforcement in organic-inorganic hybrid composites[6-8].

This article is devoted to the development of a technology for synthesizing guanidine acrylate from guanidine rhodanide and converting it into a composite adhesive in the presence of colloidal silica and formalin, together with PVA and epoxy resin[9-11].

Materials and methods

The experiments used guanidine rhodanide ((NH₂)₂C=NH·HSCN), nitric acid (HNO₃) with a concentration of 10–20%, 0.1–0.5 M aqueous solution of copper(II) sulfate (CuSO₄·5H₂O), acrylic acid (CH₂=CHCOOH, 99%), polyvinyl acetate (PVA) with a hydrolysis degree of 98–99%, epoxy resin (ED-20), 37% aqueous solution of formalin, colloidal silica (30% aqueous SiO₂ sol), and distilled water. All reagents were of analytical grade. The following instruments were used: a magnetic stirrer with heating plate (IKA RCT basic, Germany) equipped with a calibrated contact thermometer; a laboratory pH meter (Mettler Toledo FE20, Switzerland, accuracy ±0.01 pH units); an analytical balance (Sartorius CPA225D, Germany, readability 0.01 mg); a laboratory centrifuge (Eppendorf 5804R, Germany) for precipitate separation; a digital viscometer (Brookfield DV-II+, USA) for viscosity measurement of intermediate and final products; and an autoclave-type laboratory reactor (volume 500 mL) fitted with a reflux condenser, continuous mechanical stirrer (100–400 rpm), thermocouple, and manometer. Absolute reagent masses for each synthesis stage were as follows: Stage 1 — guanidine rhodanide 119 g (1.0 mol), 10–20% HNO₃ solution 200 mL; Stage 2 — CuSO₄·5H₂O 25 g (0.1 mol) dissolved in 100 mL distilled water; Stage 3 — acrylic acid 72 g (1.0 mol); Stage 4 — guanidine acrylate 40 g, PVA or ED-20 epoxy resin 100 g, colloidal silica sol 15 g, formalin 5 g. Adhesion strength was determined in accordance with GOST 14759–69 (lap-shear test on steel substrates, five replicates per sample). Compressive and tensile strength were measured according to GOST 24544–81 on a universal testing machine (Instron 5565, USA) at a crosshead speed of 5 mm/min. Antibacterial activity was assessed by the agar-disc diffusion method (GOST R ISO 20743–2011) against Staphylococcus aureus ATCC 6538 and Escherichia coli ATCC 25922, with zone-of-inhibition diameters reported in millimetres. At the end of the process, two different products were obtained — a PVA-based composite adhesive (Scheme 1, Figure 1) and an epoxy resin-based composite adhesive (Scheme 2). The first three steps of both schemes are common: in reactor-8, an ion exchange reaction is carried out between guanidine rhodanide (1) and diluted HNO₃ (2); in reactor-9, CuSO₄ (3) is added to the product of reactor-8, and CuSCN is precipitated; in reactor-10, guanidine nitrate is reacted with acrylic acid (4) to synthesize guanidine acrylate. In the final step, PVA (5) or epoxy resin (5), colloidal silica (7) and formalin (6) are added to the synthesized guanidine acrylate in reactor-11, respectively, to prepare the final composite. The CuSCN precipitate from reactor-9 is collected in tank-13, and the finished product is stored in tank-12. All reactors are equipped with a continuous mixer, temperature and pH control systems.

Figure 1. Technological scheme for obtaining a composite material with PVA after forming guanidine acrylate

The first step is to obtain a mixture of guanidine nitrate and rhodanic acid, and the reaction proceeds according to the equation (NH₂)₂C=NH·HSCN + HNO₃ → [C(NH₂)₃]NO₃ + HSCN. In this case, the concentration of nitric acid is set to be between 10–20%, the temperature is set to 0–10°C, and the pressure is set to 1 atm. The medium is aqueous and provides vigorous stirring; the use of low temperature and dilute acid effectively prevents oxidative decomposition of the thiocyanate anion.

In the second stage, a 0.1–0.5 M CuSO₄ solution is added to the mixture in reactor-8 and the following reaction sequence is observed: first, copper(II) thiocyanate is formed according to CuSO₄ + 2HSCN → Cu(SCN)₂ + H₂SO₄, then this unstable compound decomposes as 2Cu(SCN)₂ → 2CuSCN↓ + (SCN)₂, and a white or light gray CuSCN precipitate and thiocyanate are separated. This step is carried out at a temperature of 20–30°C, a pressure of 1 atm, and a weakly acidic environment. The resulting CuSCN precipitate is removed from the bottom of the reactor and directed to reservoir-13; the sulfuric acid and guanidine nitrate in solution are transferred to the third reactor.

The third step is the synthesis of guanidine acrylate, the reaction:

The reaction proceeds according to the equation [C(NH₂)₂]NO₂ + CH₂=CHCOOH → [C(NH₂)₂][CH₂=CHCOO] + HNO₃. The temperature is controlled within the range of 20–40°C, as higher temperatures can cause spontaneous polymerization of acrylic acid. The pressure is 1 atm, and the mixing is continued continuously. The environment is kept as water-free or as water-poor as possible, otherwise excess water will keep ions free and make salt formation difficult. The necessary acidic environment is created by the H₂SO₄ from the previous step. The result is a sticky and viscous guanidine acrylate solution containing white crystals.

The fourth stage is performed in two different ways. In option A (PVA-based composite, scheme 1), first 100g of PVA is completely dissolved in water at 80–95°C, then cooled to 20–40°C. 40g of guanidine acrylate from reactor-10 is added to it and mixed, then 15g of colloidal cremnesol is slowly added. Finally, 5g of formalin is added at the very end; this is important to prevent gel formation, uneven structure, and agglomeration. All components are initially mixed at 20–40°C, then the temperature is gradually increased to 50–90°C, and thermal treatment is performed in this range - this ensures the branching of PVA and the formation of a composite with guanidine acrylate. The pH of the medium is maintained between 4.5–7: acidic conditions are favorable for the crosslinking reactions between formalin and PVA, but in an excessively acidic environment, guanidine decomposes, and in a strongly alkaline environment, colloidal cremnesol loses its stability. In option B (epoxy resin-based composite, scheme 2), approximately 40g of guanidine acrylate, around 100g of epoxy resin (the optimal amount will be determined in further studies), 15g of colloidal cremnesol, and 5g of formalin are mixed together in reactor-11. Mixing is carried out initially at 20–40°C, then heated to 50–90°C; the pH environment and the order of addition of components are maintained as in option A.

Results

In the first step, a transparent solution of guanidine nitrate and rhodanic acid was obtained. There was no color change or gas evolution during this process, indicating that the decomposition of thiocyanate was minimal. In the second step, after the addition of CuSO₄, a white CuSCN precipitate formed quickly, and when filtered and separated, its mass yield was theoretically higher. The product of the third stage – guanidine acrylate – was a viscous, colorless to pale yellow solution containing small white crystals. Its viscosity confirms the presence of the acrylate anion.

The prepared PVA-based composite (40 g guanidine acrylate, 100 g PVA, 15 g colloidal silica, 5 g formalin) exhibited the following properties. It spread evenly on metal, wood, glass, and ceramic surfaces and formed a strong bond after drying. The compressive and tensile strength of the sample was significantly higher than that of the standard PVA glue. No significant swelling or degradation of the adhesive layer was observed when stored in water for 24 hours. The resulting substance maintained structural integrity when heated to temperatures above 100°C. An inhibitory effect was noted against the model strains due to the guanidine group.

The epoxy resin-based composite also exhibited high hardness and adhesion, and was found to be particularly effective for metal and glass. When the component ratios were changed: an increase in colloidal silica (>20 g) led to the formation of a brittle and uneven film, while an excessive increase in guanidine acrylate led to increased hydrophilicity and swelling. On the contrary, its insufficient amount caused a decrease in antibacterial effect.

Discussion

The experimental results show that the synthesis of guanidine acrylate from guanidine rhodanide via stepwise ion exchange and precipitation was successfully achieved. Maintaining the temperature at 0–10°C and not exceeding the nitric acid concentration of 20% in the first stage almost completely eliminated the oxidation of the thiocyanate anion, which prevented the release of toxic gases (NO₂, SO_x) and ensured environmental and process safety.

In the second step, the precipitation of HSCN as CuSCN using CuSO₄ not only allowed the extraction of rhodanide but also produced sulfuric acid, creating the necessary acidic environment for the third step. CuSCN precipitate can be used as a by-product in pyrotechnics or other industries, which increases the economic efficiency of the process. In the third stage, it is important not to exceed 40°C when treating with acrylic acid, as the polymerization of acrylic acid will dramatically increase the viscosity of the product, negatively affecting the reaction yield. In anhydrous or low-aqueous environments, guanidine facilitates the extraction of acrylate in pure form.

Adding formalin at the very last stage of composite preparation prevented gel formation and allowed for a product with a uniform structure. Literature data also confirm that a pH range of 4.5–7 is optimal for PVA-formalin sutures. The presence of colloidal silica may have improved the mechanical properties of the composite and increased water resistance, as the SiO₂ particles act as physical nodes in the polymer matrix.

It is worth noting that while the PVA-based composite provides greater elasticity and compatibility with aqueous systems, the epoxy resin-based composite provides higher hardness and chemical resistance. Therefore, the selected matrix should be determined by the application. The given basic ratio has given very good results, but the ratio of components can be changed by up to ±10% depending on the specific required properties.

The retention of the guanidine fragment gives the material persistent antibacterial activity, which is especially valuable in adhesives intended for the medical and food industries.

Conclusion

Guanidine nitrate was obtained from guanidine rhodanide by low-temperature ion exchange, and rhodanide acid was efficiently isolated as CuSCN using copper sulfate. Guanidine acrylate was synthesized by reacting guanidine nitrate with acrylic acid; its optimal conditions (20–40°C, low aqueous environment, 1 atm) were determined. By processing guanidine acrylate with PVA or epoxy resin, colloidal silicon dioxide, and formalin, composite adhesives with high adhesion, compressive and tensile strength, water and heat resistance, and antibacterial properties were obtained. The two proposed technological schemes provide flexibility in the production of multi-purpose industrial adhesives. In the future, it is planned to determine the exact optimal proportions for the epoxy-based option and conduct long-term durability tests.

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