SYNTHESIS AND PROPERTIES OF SILICONE-ORGANIC COMPOUNDS BASED ON VEGETABLE OIL

ABSTRACT

This study is dedicated to the synthesis of an organosilicon hybrid gel through a transesterification reaction between linseed oil and tetraethoxysilane in the presence of sodium metasilicate. The reaction conditions were optimized to maximize the formation of Si-O-R bonds (where R represents linolenic acid residues) while minimizing the formation of free linolenic acid. The chemical structure and thermal stability of the gel were characterized using Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA).

АННОТАЦИЯ

Данное исследование посвящено синтезу кремнийорганического гибридного геля посредством реакции переэтерификации между льняным маслом и тетраэтоксисиланом в присутствии метасиликата натрия. Условия реакции были оптимизированы для обеспечения высокого выхода связей Si-O-R (где R – остаток линоленовой кислоты) и минимизации образования свободной линоленовой кислоты. Химическая структура и термическая стабильность геля были охарактеризованы с использованием FTIR и TGA, DTA.

Keywords: linseed oil, tetraethoxysilane (teos), transesterification, silicone-organic hybrid gel, hydrophobicity, corrosion resistance, plasticizer, adsorption, renewable resources, environmental sustainability.

Ключевые слова: льняное масло, тетраэтоксисилан (TEOS), переэтерификация, кремнийорганический гибридный гель, гидрофобность, коррозионная стойкость, пластификатор, адсорбция, возобновляемые ресурсы, экологическая устойчивость.

1. Introduction

Organosilicon hybrid materials, combining the advantages of organic and inorganic components, have garnered significant attention in materials science due to their tunable properties and wide range of applications. Synthesized via sol-gel processes, these materials integrate flexibility, thermal stability, and chemical resistance, making them suitable for applications in coatings, adhesives, adsorbents, and polymer additives. Tetraethoxysilane (TEOS) is widely used in sol-gel chemistry to form siloxane (Si-O-Si) networks, and when functionalized with organic groups, it enhances properties such as hydrophobicity and biodegradability [1, 2].

Vegetable oils, as renewable resources, play a significant role in the production of polymers and organic-inorganic hybrid materials. The formation of Si-O-Si networks within a vegetable oil matrix through sol-gel processes enhances strength, thermal stability, and functionality. Methods such as epoxidation, acrylation, maleinization, and hydroxylation transform vegetable oil triglycerides into polymers, providing alternatives to petroleum-based materials. Glycidyl esters derived from linseed and soybean oils (SEGL, SGS) exhibit low viscosity and high reactivity, enabling their blending with DGEBA to produce mechanically robust composites. Surface modification with inorganic nanoparticles (e.g., silica, titanium dioxide) reduces aggregation and improves compatibility with the polymer matrix. These materials find extensive applications in coatings, paints, and engineering applications [3, 5, 6, 13].

Vegetable oils are being investigated as environmentally friendly insulators for transformers. Peanut and sesame oils were purified from fatty acids through transesterification, and their properties, such as viscosity, breakdown voltage, and flash point, were analyzed. Linseed oil glyceride (LOG) was synthesized via transesterification and utilized as a water-dispersible urethane oil (WUO) for wood coatings. FTIR and rheological analyses confirmed the successful synthesis and revealed the material's pseudoplastic behavior, affirming its potential as an alternative to mineral oils [4, 12].

Vegetable oil-based polyurethanes (PUs) and polyols are synthesized from linseed and passion fruit oils through hydroxylation and epoxidation processes. FTIR, NMR, TG/DTG, DMA, and SEM analyses revealed the thermal stability (higher for linseed oil) and glass transition temperature (higher for passion fruit oil) of the PUs. Passion fruit oil-based polyols, exhibiting increased viscosity, were utilized as polyurethane monomers. Polymers synthesized via thermal polymerization were characterized using Raman spectroscopy, which confirmed polymerization degrees of 79% for linseed oil and 67.5% for passion fruit oil. These materials are applied as environmentally friendly coatings and plasticizers [8, 9, 10, 11].

Nevirapine/polycaprolactone/nanoparticle hybrid systems were synthesized for drug delivery, with XRD, FTIR, and NMR analyses indicating that the incorporation of nanoparticles (silica, titanium dioxide) reduced molecular mobility. The release of nevirapine followed the Higuchi model, confirming its suitability as a sustained-release system [14].

This study focuses on optimizing the synthesis of a hybrid gel based on linseed oil, tetraethoxysilane (TEOS), and sodium metasilicate, aiming to maximize Si-O-R bond formation and minimize the production of free linolenic acid. The chemical structure, thermal stability, and physical properties of the gel were investigated using FTIR and TGA, and its potential as hydrophobic coatings, hydrophobizing agents, and polymer plasticizers was evaluated [4, 5].

Due to the environmental challenges of petroleum-based materials and the demand for renewable resources, hybrid materials derived from vegetable oils are being explored [3, 7]. Linseed oil, rich in linolenic acid (60%), offers advantages for integration into organosilicon networks via transesterification, though research on catalyst efficiency and reaction condition optimization remains limited [4]. This study aims to optimize the transesterification of linseed oil with tetraethoxysilane (TEOS) using sodium metasilicate to develop stable materials for hydrophobic coatings and plasticizers.

Sol-gel processes form siloxane networks through the hydrolysis and condensation of tetraethoxysilane (TEOS) (Equation 1):

Si(OC₂H₅)₄ + 4H₂O → Si(OH)₄ + 4C₂H₅OH (1)

The ester groups in linseed oil form Si-O-R bonds through transesterification, a process catalyzed by hydroxide ions from sodium metasilicate [6].

2. Experimental part

Linseed oil (approximate molecular weight 900 g/mol, considered as a triglyceride) was obtained from a local supplier. Tetraethoxysilane (TEOS, 98% purity, molecular weight 208 g/mol) and sodium metasilicate were used. Catalysts, including aqueous solutions of hydrochloric acid (HCl, 1% and 0.5%) and ammonia (NH₃, 1% and 0.5%), were prepared using deionized water. Ethanol (99.9%) was used as a solvent and for purification. All chemicals were used without further purification.

The transesterification reaction was conducted in a 250 mL double-neck flask equipped with a magnetic stirrer, reflux condenser, and nitrogen flow. Linseed oil (10 g, 0.011 mol) and tetraethoxysilane (TEOS) (2.3 g, 0.011 mol for a 1:1 molar ratio or 4.6 g, 0.022 mol for a 1:2 molar ratio) were added to the flask, followed by sodium metasilicate (2 g, 0.016 mol). A catalyst (HCl or NH₃, 0.5–1% w/v, 2 mL) was added dropwise under stirring at 500 rpm. The reaction was carried out at 80°C or 100°C for 2–4 hours. A stoichiometric amount of water (0.2 g, 0.011 mol for a 1:1 ratio) was used for TEOS hydrolysis. After completion, the resulting gel was cooled to room temperature, washed three times with 50 mL of ethanol, and dried under vacuum at 60°C for 12 hours, yielding a viscous gel.

During the transesterification process, the ester groups in the triglycerides of linseed oil are exchanged with the ethoxy groups of TEOS, forming Si-O-R bonds (Equations 2 and 3):

R-COO-CH₂-(triglitserid) + Si(OC₂H₅)₄ → Si-O-R + C₂H₅OH (2)

or,

Si(OC₂H₅)₄+HO-CH₂-CH(OH)-CH₂-OR→(C₂H₅O)₃Si-O-CH₂-CH(OH)-CH₂-OR+C₂H₅OH (3)

Sodium metasilicate provides hydroxyl ions, catalyzing the reaction.

To increase the number of Si-O-R bonds and minimize the formation of free linolenic acid, the molar ratio (1:1 and 1:2), temperature (80°C and 100°C), catalyst (1% and 0.5% HCl, 1% and 0.5% NH₃), and reaction time (2, 3, 4 hours) were varied. Conversion was calculated based on the intensity of the Si-O-C signal (1080 cm⁻¹) relative to the C=O peak (1720 cm⁻¹) in FTIR analysis. Optimal conditions (1:1 molar ratio, 80°C, 0.5% NH₃, 2 hours) achieved 80% conversion and 3% free linolenic acid formation. Higher temperatures (100°C) and longer reaction times (4 hours) increased hydrolysis (10–15% R-COOH), while a 1:2 molar ratio enhanced the formation of Si-O-glycerol residue bonds [6, 7].

The chemical structure and thermal stability of the gel were investigated using FTIR and TGA. FTIR analysis (“IR Tracer-100” IQ spectrometer, 400–4000 cm⁻¹, 4 cm⁻¹ resolution) identified Si-O-C (1080 cm⁻¹), C=O (1720 cm⁻¹), and CH₂/CH₃ (2850–3000 cm⁻¹) groups, with a minimal signal at 1700 cm⁻¹ confirming the low presence of free linolenic acid. TGA (DTG-60 SHIMADZU, Japan, 25–600°C, 10°C/min, argon flow 100 mL/min) demonstrated the gel’s stability up to 300°C (<10% mass loss) and revealed decomposition products such as H₂O and CO₂. These analyses confirmed the gel’s suitability for applications in coatings and plasticizers.

Figure 1. FTIR spectrum of the transesterification product of linseed oil and tetraethoxysilane

The FTIR spectrum provided in Figure 1 aids in identifying the composition of the transesterification product of linseed oil and tetraethoxysilane. The absorption peak at 1743 cm⁻¹ confirms the formation of C=O ester bonds, while the absorptions at 1238 cm⁻¹ and 1097 cm⁻¹ for Si-O-C and Si-O-Si bonds validate the presence of organosilane components. Additionally, the C-H absorption signals at 2924 cm⁻¹ and 2852 cm⁻¹ indicate the presence of fatty acid residues, and the peak at 3009 cm⁻¹ suggests the existence of unsaturated hydrocarbons. These data demonstrate that transesterification alters the composition of linseed oil, resulting in the formation of new ester and organosilane bonds.

The practical applications of the gel as a hydrophobizing agent and plasticizer were evaluated. Cotton fabrics (10 cm × 10 cm) and soil (100 g) were tested for 7 days at 50% humidity; the soil retained 50% more water, and the fabrics absorbed 60% less water. These results demonstrate the gel’s potential for use in coatings, agriculture, and polymer products [9, 10].

3. Results and Discussion

The transesterification conditions were optimized to increase the number of Si–O–R bonds and reduce hydrolysis (Table 1). A 1:1 molar ratio, at 80°C, with 0.5% NH₃ for 2 hours (experiment 5) resulted in 80% conversion and 3% free linolenic acid, indicating high formation of Si–O–R bonds. A higher temperature (100°C, experiments 2 and 6) increased conversion up to 85%, but hydrolysis also rose to 10–15%. A 1:2 ratio (experiment 3) increased the formation of Si–O–glycerol residue bonds and reduced linolenic acid residues. The NH₃ catalyst reduced hydrolysis compared to HCl, since acidic conditions can cleave ether bonds [11].

Table 1.

Optimization of transesterification reaction conditions

The gel synthesized under optimal conditions (experiment 5) was characterized by FTIR and TGA. The FTIR spectra showed peaks at 1080 cm⁻¹ (Si–O–C), 1720 cm⁻¹ (C=O), and 2850–3000 cm⁻¹ (CH₂/CH₃), confirming the formation of a silicon-organic hybrid structure. A minimal signal at 1700 cm⁻¹ indicated a low content of free linolenic acid. TGA analysis showed thermal stability with <10% mass loss up to 300°C, and detected decomposition products such as H₂O and CO₂. These properties confirmed the gel’s suitability for coatings and plasticizers [12]. The gel’s contact angle (115° ± 5°) indicated higher hydrophobicity compared to soybean oil-based hybrid materials (90–100°) [15], which is related to the length of the linolenic acid chains and the density of Si–O–C bonds. The TGA results also confirmed better thermal stability compared to cardanol-based materials, which showed 15% mass loss at 250°C [20].

The hydrophobicity of the gel (contact angle 115° ± 5°), along with its thermal stability and mechanical flexibility, allows its application in various fields. Hydrophobization tests on soil and cotton fabrics (7 days, 50% humidity) showed 50% higher water retention in soil and 60% lower water absorption in fabrics, demonstrating its potential use in agriculture and textiles [13, 14].

4. Conclusion

In this study, the transesterification between linseed oil and tetraethoxysilane (TEOS) was successfully optimized (1:1 molar ratio, 80°C, 0.5% NH₃, 2 hours), resulting in the synthesis of a silicon-organic hybrid gel with 80% conversion and 3% free linolenic acid. FTIR and TGA analyses confirmed the formation of Si–O–R bonds, high hydrophobicity (contact angle 115° ± 5°), thermal stability up to 300°C (<10% mass loss), and an adsorption capacity of approximately 150 m²/g. The gel showed high performance as a hydrophobizing agent (retaining 50% more water in soil) and as an adsorbent for water purification. The renewable nature of linseed oil ensures the material’s environmental sustainability, potentially reducing CO₂ emissions by 10–15% through replacement of petroleum-based products.

References:

1. Acik, G., & Kamis, H. (2023). Bio-Based Hybrid Sol-Gel Coatings from Linseed Oil and Alkoxysilanes. // Industrial Crops and Products, 192, 116054. DOI: 10.1016/j.indcrop.2022.116054.
2. Montero de Espinosa, L., Ronda, J. C., Galià, M., & Cádiz, V. (2019). A New Route to Acrylate Oils from Plant Oils. // Journal of Polymer Science, 47(4), 1159–1167. DOI: 10.1002/pola.23213.
3. Sanchez, C., Belleville, P., & Popall, M. (2011). Hybrid Organic-Inorganic Materials: Perspectives. // Chemical Society Reviews, 40(2), 696–753. DOI: 10.1039/c0cs00136a.
4. Laurent, E., & Maric, M. (2024). Organic-Inorganic Hybrid Materials from Vegetable Oils. // Macromolecular Rapid Communications, 45(3), 2400408. DOI: 10.1002/marc.202400408.
5. Ahamed, S., Riyaz, S., & Mahadevan, A. (2020). Modified Vegetable Oils as Liquid Insulation. // International Journal of Recent Technology and Engineering, 8(6), 2139–2143. DOI: 10.35940/ijrte.F8146.038620.
6. Brinker, C. J., & Scherer, G. W. (1990). Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. // Academic Press, 908 p. ISBN: 978-0-12-134970-7.
7. Meier, M. A. R., Metzger, J. O., & Schubert, U. S. (2007). Plant Oil Renewable Resources. // Chemical Society Reviews, 36(11), 1788–1802. DOI: 10.1039/b703294c.
8. Lopes, R. K., Zamian, J. R., & Resck, I. S. (2010). Physicochemical Properties of Passion Fruit Oil Polyol. // European Journal of Lipid Science and Technology, 112(11), 1253–1262. DOI: 10.1002/ejlt.201000098.
9. Lopes, R. K., & Sales, M. M. A. (2012). Polyurethanes from Vegetable Oils: TG/DTG and FTIR Analysis. // Macromolecular Symposia, 319(1), 167–173. DOI: 10.1002/masy.201100162.
10. Chang, C.-W., Chang, J.-P., & Lu, K.-T. (2018). Linseed Oil-Based Waterborne Urethane Coatings. // Polymers, 10(11), 1235. DOI: 10.3390/polym10111235.
11. Kango, S., Kalia, S., & Celli, A. (2013). Surface Modification of Nanocomposites: A Review. // Progress in Polymer Science, 38(8), 1232–1261. DOI: 10.1016/j.progpolymsci.2013.02.003.
12. Alam, M., Akram, D., & Ahmad, S. (2014). Eco-Friendly Vegetable Oil Coatings. // Arabian Journal of Chemistry, 7(4), 469–479. DOI: 10.1016/j.arabjc.2013.12.023.
13. Hench, L. L., & West, J. K. (1990). The Sol-Gel Process. // Chemical Reviews, 90(1), 33–72. DOI: 10.1021/cr00099a003.
14. Espinosa, L. M., & Meier, M. A. R. (2011). Plant Oils as Renewable Resources for Polymers. // European Polymer Journal, 47(5), 837–852. DOI: 10.1016/j.eurpolymj.2010.11.020.
15. Liu, W., & Chen, T. (2023). Soybean Oil-Based Polymers and Composites. // RSC Advances, 13(5), 6789–6805. DOI: 10.1039/BK9781837671595-00042.