Synthesis of organofunctional silane from rapeseed oil and its application as a coating material

The wood samples were ultrasonically washed one after another with acetone, ethanol and deionized water for 5 min with each of the solvents. Then the cleaned samples were dried in an oven at 60 °C for 12 h and directly used for further modifications. Then a series of three 5% solutions of silane RPTMS in ethanol were prepared as specified in Table 1.

¹H NMR (500 MHz), ¹³C NMR (126 MHz), and ²⁹Si NMR (99 MHz) spectra were recorded on a Bruker Avance III 500 MHz spectrometer at room temperature using chloroform-d (CDCl₃) as solvent.

The pH values of the solutions O1–O3 were measured using a pH meter FiveEasy F20 manufactured by Mettler-Toledo GmbH equipped with an electrode LE438. Before the measurements, the pH meter was calibrated with buffer solutions of pH 4.01, 7.00 and 9.21.

Scanning electron microscopy (SEM) images were taken on a FEI Quanta 250 FEG microscope equipped with an EDAX Energy Dispersive Spectroscopy detector (EDS). The images were taken in high vacuum mode with 10 kV accelerating voltage. EDS mapping was realized with electron beam energy of 20 keV using EDS Octane SDD detector (EDAX). The wood samples were prepared for SEM imaging by gluing onto the standard SEM carbon adhesive tape.

Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27 Fourier transform spectrometer equipped with a SPECAC Golden Gate diamond ATR (Attenuated total reflection) unit. In each case 16 scans were collected for a spectrum at the resolution of 2 cm⁻¹. Measurements were carried out at three sites for each sample and the spectra were averaged using the OPUS Data Collection Program. While interpreting the spectra, characteristic bands were ascribed according to the literature (Stuart 2004).

Static water contact angle (WCA) measurements on all samples were made using a Krüss GmbH DSA 100 Expert Drop Shape Analyzer equipped with a software-controlled (DAS4 2.0): x, y, z-axis table, quadruple dosing unit with zoom and focus adjustment, illumination, and a camera with780 × 580 px resolution. All presented data are arithmetic means from measurements made for 5 drops per sample. The measurements of contact angles were performed immediately after the deposition of a drop on a studied surface and the measurement time was about 1 s.

Water absorption was measured for wood samples dried at 110 °C to constant mass, weighed to the accuracy of 0.001 g, fully submerged in distilled water for 24 h. Then the samples were taken out, dried with filter paper and weighed again. Percentage of absorbed water was calculated using the following equation:where %M is the percentage of absorbed water (%); M_wet is the wet mass after submersion in water for 24 h (g); M_cond is the conditioned mass (initial dry mass) (g). All the obtained coatings were tested on three samples in parallel and the results were averaged.

The synthesis of RPTMS was performed using commercially available edible rapeseed oil pressed from seeds of the double-low varieties (with very low glucosinolate content in seed meal and with no erucic acid in the seed oil). The oil contained, on the average, 60–65% of oleic acid (monounsaturated fatty acid, C18:1) and 7% of saturated fatty acids—palmitic (C16:0) and stearic (C18:0) acids as well as polyunsaturated fatty acids—linoleic (C18:2, 20%), linolenic (C18:3, 10%) and eicosenoic acids (C20:1, 1%) (Bocianowski et al. 2012). Because of the problems with removal of glycerin, formed during vegetable oil saponification, this process was preceded by the reaction of oil transesterification leading to methyl esters of fatty acids (RME—rapeseed methyl esters) and the purified methyl esters were subjected to saponification in order to obtain a mixture of sodium salts of fatty acids. Then, as a result of nucleophilic substitution of 3-chloropropyltrimethoxysilane with sodium salts of fatty acids, in the presence of potassium iodide as a catalyst, the target organosilicon derivatives were obtained, according to the scheme 1.

The synthesis of similar silanes in the reaction of 3-chloropropyltrimethoxysilane with sodium salts of carboxylic acids, such as disodium fumarate and sodium undecylate, in the presence of iminium phase transfer catalysts and with toluene as a solvent, has been disclosed (Backer et al. 2016). Unfortunately, in the conditions described in the relevant patent it is impossible to obtain vegetable oil derivatives because of the problem with dissolving their sodium salts in toluene. After 48 h of the reaction in toluene, the sodium salts were not dissolved and no presence of the target product was detected. In order to dissolve the sodium salts of fatty acids it was necessary to use DMF as a solvent. After completion of the reaction, DMF was evaporated from the system and used in the synthesis with no further purification.

The obtained silane RPTMS was employed to create a coating that would protect wood against water. Hydrolysis and condensation reactions occurred in the prepared sol–gel solutions (Mai and Militz 2004a; Subasri et al. 2013). The former led to the formation of Si–OH groups from water. The subsequent two types of condensation reactions between Si–OH and Si–O–CH₃ moieties created a well-developed, cross-linked network consisting of Si–O-Si covalent bonds. Wood–O–Si covalent bonds were formed as a result of reactions occurring between –OH groups present in wood and Si–OH and Si–O–CH₃ groups which were present in the solution. In this way, the coating was attached to the surface of the modified element. Moreover, the aliphatic chain bonded to the silicon atom makes the surface of wood more hydrophobic compared to the unmodified sample.

Figure 1 shows SEM images of the surface of bare wood and samples coated with RPTMS (O1–O3). In the SEM image of bare wood, sharp edges of uncoated wood are visible, while in the case of SEM images of modified samples one can see a layer that coats the surface of wood. Additionally, in the SEM images of samples O1 and O2, spherical structures are visible on their surfaces. A likely reason for the presence of aforementioned structures is the retention of solution drops by the wood surface after the samples of wood were removed from solutions followed by the solvent evaporation during which the silane began to condense. The spherical structures are bigger in the case of the sample O2 due to hydrolysis of tetraethoxysilane in the solution. In the SEM image of sample O3 visible are characteristic cracked structures probably originated from silica. The extension of stirring time of the solution before the process of wood surface silanization could result in the formation of more condensed polysiloxane and silicate structures in the case when TEOS itself underwent condensation already in the solution. Generally, in the case of wood subjected to silanization with solutions O2 and O3, both silicate and organosilicon species are present on its surface.

Figure 2 shows the EDS spectra of bare wood and all the coated samples. The spectra of coated samples show an intensive peak assigned to Si atoms which provides evidence for the formation of organosilicon layer and a layer of silicate structures (in the case of samples O2 and O3) on the wood surface. Unfortunately, the EDS analysis of silanized wood indicates the presence on its surface of an additional coating that contains silicon atoms, however, their exact form remains unknown. In the FT-IR spectra (Fig. 3), the bands between 1100 and 1300 cm⁻¹ originating from Si–O-Si groups and wood-O-Si groups are present in the case of silane-coated samples. These bands are partially overlapped with a broad band between 950 and 1100 cm⁻¹ attributed to characteristic peaks of cellulose (see FT-IR spectra of bare wood). There are also strong bands in the range of 2800–3000 cm⁻¹ originating from C–H groups and in the range 1650–1700 cm⁻¹ ascribed to C=O groups.

Table 2 presents the water contact angle values of bare wood and O1–O3 samples. The WCA measurements show that unmodified surface of bare wood is hydrophilic because its WCA is less than 90°. A different behavior has been observed for the samples with modified surfaces, which exhibit hydrophobic properties. Their WCA values are greater than 90° and equal to 111°, 97° and 92° for O1, O2 and O3 samples, respectively. The higher hydrophobicity of sample O1 probably results from the presence of long hydrocarbon chains on its surface, while in the case of O2 and O3 samples the hydrocarbon chains can be partially covered by silicate structures due to the presence of TEOS. The hydrocarbon chains can be entangled in the structure of siloxane coating.

All the studied samples were subjected to water absorption measurements and results of the measurements have been collected in Table 3.

It results from the data presented in Table 3 that bare wood sample has shown high capability of absorbing water because its mass increased by over 50% after 24 h contact with water. The wood samples protected with silane RPTMS coatings showed a significant resistance to water absorption.

As was already mentioned, the reaction between hydroxyl groups present on the wood surface with Si–OH and Si–O–CH₃ groups present in solution results in the formation of Wood-O-Si covalent bonds (Mai and Militz 2004a; Subasri et al. 2013). Such bonds are stable (do not undergo hydrolysis), if the coating formed on the wood surface effectively protects them from the access of water. After completing the measurements of water soaking, dried wood samples were subjected to SEM–EDS and FT-IR analyses in order to determine changes that occurred in the formed organosilicon coatings.

Figure 4 shows EDS spectra of coatings before and after water soaking tests. Only in the case of coating O1, that was based on RPTMS alone, a reduction in the intensity of the peak originating from silicon atoms was observed. The intensity reduction is probably a result of leaching of unreacted silane or a partial hydrolysis of C–O–Si bond. Additionally, for a better identification of silicon-based structures formed on the wood surface, EDS mappings were performed of samples subjected to soaking in water. The images presenting carbon and silicon contents on the wood surface are available in Electronic Supplementary Material. In the images of sample O1 visible is homogeneous distribution of carbon and silicon on the whole mapped surface. On surfaces of O2 and O3 samples, there are areas with a higher saturation with silicon which is a result of the TEOS presence in silanizing solutions. Additionally, cracked structures present on the surface of sample O3, which were earlier ascribed to silicates, are not visible at all in the image showing carbon content in the coating, whereas in the image presenting silicon content in the coating these areas are very light which indicates a large silicon content in them.

The durability of formed coatings and their satisfactory resistance to the hydrolysis in mild conditions has also been confirmed by the results of FT-IR analysis performed for silanized samples after 24 h soaking in deionized water. It has been established that all characteristic bands that were present in the FT-IR spectra of O1–O3 samples (Fig. 3) were retained in the spectra of water-soaked samples, O1W–O3W, subjected to drying prior to spectra recording (Fig. 5). A minor decrease in the relative intensity of the discussed FT-IR spectra can be a result of leaching of unreacted silanes, partial hydrolysis of Si–O–C bonds, heterogeneity of wood surface and as a consequence also coatings formed on such a surface.