Synthesis of three novel silane-based carboxylic acids for application in methacrylated epoxy-based oligomers and their ability in the UV curable hybrid coatings


 In this study, bromoacetic acid was reacted with 3-(mercaptopropyl) trimethoxy silane (MPTS) and trimethoxy silyl propyl thioacetic acid (TSTA) was produced. Also, bromoacetic acid was reacted with 3-(triethoxysilyl) propylamine (APTS), and triethoxysilyl propylamino acetic acid (TSPA) was synthesized. Finally, from a reaction between trimellitic anhydride (TMA) and APTS, trimellitylimidopropyl triethoxysilane (TMIS) resulted. In all reactions mentioned above, a carboxylic acid head and a trialkoxy silane tail including reactants were obtained. Furthermore, hybrid coatings based on methacrylated bisphenol A epoxy (MBAE) and synthesized carboxylic acids were obtained by photopolymerization. Polycarbonate substrates were utilized for preparation of transparent hybrid films. Then, the solvent resistance, hardness, gel content as well as the adhesion of coatings were measured as physical and mechanical properties. According to the obtained results, these properties of hybrid coatings improved with the increase in alkoxysilanes and sol–gel precursor contents. The surface morphology was characterized by scanning electron microscopy (SEM). The results showed that silica particles were not dispersed homogenously at the molecular level in the hybrid system. Also, the thermogravimetric analysis results indicated that alkoxysilanes enhanced the thermal oxidative stability of the hybrid coatings.


INTRODUCTION
During the synthesis and use of paints, a considerable amount of detrimental vapors are released from organic-based raw materials [1]. Such vapors can be reduced in the coating industry by Ultraviolet (UV) curing technology [2]. UV-curable coatings compared with the traditional ways have remarkable benefits such as low energy consumption, a higher pace in the curing process and no need to a solvent [3][4][5][6][7]. On the other hand, renewable resources have been used in additives and adhesives industries as alternatives to crude oil [8] vegetable oil is a safe resource among them. Vegetable oil-based coatings can reduce vapor emissions and improve the degeneration capacity of paints [9]. Furthermore, vegetable oils are film formers and their combination with UV-curable technology could be an environment-friendly technique for the current paint industry problems [10][11][12][13].
Vegetable oils are composed mainly of triglycerides with multiple fatty acid chains including 14-20 carbon atoms with some functional groups such as double bonds and epoxy groups [14]. Double bonds are cross-linkage by mechanism of self-oxidizing due to the oxygen-sensitivity of this functional group [15][16][17]. Although, Self-oxidation is a time taking process therefore driers are used to fasten the curing rate. Grafting acrylate fractions with more active double bonds or any other effective groups such as siloxanes etc. to improve material properties is an advantage of double bonds in vegetable oils [18][19][20][21][22][23][24]. A typical methacrylated vegetable oil oligomer is methacrylated epoxidized soybean oil (MESO) generally attained by the ring opening reaction of methacrylic acid with epoxidized soybean oil (ESO) for preparing UV-curable coatings. A polymer with similar properties to report petroleum-based composition from acrylated epoxidized soybean oil (AESO), curing with a rosin-based acrylamide was prepared by Shang and coworkers [25]. 4 Also, Qiu et al [26]. developed an AESO-based resin without styrene with an improved fiber/matrix interface adhesion. Their study demonstrated that AESO was preferable as a modifier to the epoxy resin because of its good mechanical properties [27].
Additionally, paint compositions incorporating silicone monomers have attracted special attention due to some of their properties such as excellent flexibility, low surface energy and very good thermal stability [28][29][30][31]. According to the Isin and co-workers the Si atoms of silicone containing macromolecules which arranged in the backbone or pendant groups present these properties to matrix [32].
In the present study, our basic goal was better understanding the effect of excess amount of silanes on the hybrid coatings in the presence of TEOS and sol gel method. For this purpose, firstly, three novel silane based carboxylic acids were prepared to introduce alkoxysilanes moieties in the epoxy chains by ring opening reactions. Accordingly, ESO was reacted with equal amounts of silane based carboxylic acids and methacrylic acid.
Epoxy resin was also reacted with methacrylic acid. TEOS was hydrolyzed in the presence of water and ethanol and coreacted with 3-methacryloxypropyltrimetoxysilane (TSPM).
The UV induced polymerization was used to cure organic-inorganic hybrid coatings. The structural characteristics of silane based carboxylic acids were analyzed by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) as well as FT-IR for hybrid materials. Thermal properties of hybrid coatings were performed by DSC and thermal gravimetric analysis (TGA) techniques. Epoxidized soybean oil (ESO, epoxide content 4.20 mol/kg) was purchased from Aladdin Industrial Co., China. The photoinitiator Phenylbis (2, 4, 6 trimethylbenzoyl) phophine oxide (PTBP) was provided from Sigma Aldrich. The substrates used in this work were polycarbonate panels (3 mm×50 mm×100 mm) and were obtained from Isfahan province, Iran.

Characterization
The FT-IR spectra were recorded with the help of a Nicolet Impact 400D Model spectrophotometer (Nicolet Impact, Madison, USA) in the spectral range between 4000 and 400 cm -1 characterized by FT-IR. NMR Spectra were recorded on a Bruker 400 MHz To better understand the physical and chemical properties of cross-linked films, some polycarbonate panels were used for applying the coating formulations using an applicator and cured in a bench type UV processor (Dadkhah electronic Isfahan-Iran, 120W/cm, λmax = 365 nm, medium pressure mercury UV lamps). In addition, Soxhlet extraction method was used for determination of gel contents of the UV cured hybrid films. 1.40 g (10 mmol) bromoacetic acid and 25 mL dehydrated THF were charged into a 100 mL three necked round bottom flask equipped with a dropping funnel, a reflux condenser and a nitrogen inlet. The acid and THF were stirred strongly until a homogenous mixture were obtained. Then, 2g (10 mmol) MPTS was diluted by a 5 mL THF and added drop-wise to the mixture. This mixture was stirred magnetically at reflux temperature for 24 hrs. under nitrogen atmosphere. 3-5 drops of TEA was used as acid scavenger and a viscous liquid was collected as a condensation reaction result. The THF was removed using a rotary vacuum evaporator and the product was concentrated. For further purification of this yellow powder, it was dissolved in dry ethyl acetate and then was precipitated in nhexane. These techniques of dissolution and precipitation were repeated two times. The result was a pure white powder which was then dried in a vacuum oven. The reaction path and chemical structure of TSTA is given in Scheme 1 and Fig. 1, respectively. A temperature of 60 to 65 °C was automatically adjusted and the reaction time was 24 hrs.
Subsequently, the product was concentrated to remove the solvent THF using a rotary vacuum evaporator; the result was a yellow viscous liquid sample. Then, the sample was dissolved in absolute ethanol, and 10 mL of hexane was added for precipitation.
Furthermore, the solution was filtrated and white powder was obtained. The above procedures of dissolution and filtration were repeated two times. At last, the pure white powder was obtained and dried in a vacuum. The reaction path and structure are shown in

Synthesis of triethoxysilyl propyl trimellytilimide (TMIS)
Into a 100 mL three necked round bottom reaction vessel fitted with a stirrer, a reflux condenser, a nitrogen gas inlet tube and a dropping funnel were placed 0.5g of TMA, 20g of THF and 0.56g of APTS. This mixture was stirred magnetically at reflux temperature for 24 hrs. under nitrogen atmosphere. 3-5 drops of TEA was used as acid scavenger and a viscous liquid was collected as a condensation reaction result. The THF was removed using a rotary vacuum evaporator and the product was concentrated. For further purification of this white powder, it was dissolved in dry ethyl acetate and then was precipitated in n-hexane. These techniques of dissolution and precipitation were repeated two times. Finally, the pure white powder resulted which was then dried in a vacuum oven.
The reaction path and chemical structure of TMIS are given in Scheme 3 and Fig. 3.

Preparation of bisphenol A epoxy methacrylate oligomer (BAEM)
The BAEM oligomer was synthesized according to Kahraman and coworkers as follows [33]: 50g bisphenol A epoxy resin, 0.05g hydroquinone as a radical scavenger and 9 0.5g triphenyl phosphine as a catalyst were added into a 250 mL three-necked round bottom reaction vessel equipped with a dropping funnel, a mechanical stirrer and a nitrogen inlet.
Before the addition of epoxy resin to the vessel, it was heated to attain lower viscosity for easier handling. To form a homogenous compound, the contents of the reactor were strongly stirred at 450 rpm. Firstly, a green and next a yellowish product resulted.

Synthesis of siliconated/methacrylated soybean oil (SMSO)
A 250 mL three-necked round bottom flask equipped with a mechanical stirrer, a dropping funnel and a nitrogen inlet was charged with 5g epoxidized soybean oil and 0.05 g triphenyl phosphine. All reactants were stirred vigorously until homogenous mixture was obtained. Then temperature was raised to 40 °C and 0.5 mol of each three siliconecontaining carboxylic acids TSTA, TMIS, and/or TSPA were added into flask drop-wise.

TEOS Prehydrolysis
For the prehydrolysis of TEOS, 3.9g (216.6 mmol) water, 6 mL ethanol and 15g (72 mmol) TEOS homogeneously were mixed. In addition, 0.15g p-toluene sulfonic acid as catalyst was added to the mixture at 20 °C. The ratio of water/silicone is calculated as r = 3 when the mixture was stirred overnight and allowed to warm to room temperature. The reaction path is given in Scheme 7.

Synthesis of hybrid coatings
The similar formulations of UV curable hybrid coatings in the absence and in the presence of TSTA, TSPA and TMIS were applied. Also, in these experiments, the equal amounts of each silicone containing carboxylic acids were used while the difference between formulations is only the kind of silicone. As can be seen in Table 1, the hybrid coatings were prepared by mixing BAEM 5g, SMSO 1g, TMPA 1g, pre-hydrolyzed TEOS (0.0-1g), TSPM (0.0-0.3g) and photoinitiator PTBP 0.20g. The samples were prepared into a 100 mL beaker with adequate stirring. Along the synthesis, the beaker content heated to 40 °C was kept under gentle vacuum for 10 min. to remove air bubbles formed during mixing. The products were applied onto polycarbonate panels after homogenization, using a wire gauged bar applicator obtaining a layer thickness of 30 μm. A UV processor cabinet equipped with a medium pressure mercury lamp (120 W/cm) was used for examining the hardness of the wet coatings applied by five passes. The belt had a speed about 2m/min and the light dose was calculated as 720 mJ/cm 2 . In addition, to determine the mechanical properties, hybrid free films were synthesized by pouring the UV curable liquid formulations onto a glass mold (20 mm×50 mm×2 mm). The mixture in the mold was covered by transparent, 100 μm thick Teflon TM film before irradiation with a high-pressure UV lamp (OSRAM, 300 W), to prevent the inhibiting effect of oxygen. To obtain moderately controlled film thickness and smooth surface, a quartz glass plate was placed over. Consequently, 100 μm thick free hybrid films were obtained after 240 s irradiation under UV lamp. Finally, the resulted coated panels and the free film were annealed at 60 °C 14 hrs, 70 °C 1 hr and 80 °C for 1 hr and then stored at room temperature. Via annealing, post-curing of silanol groups were performed.  Figure 3(C), a peak appeared at 3259 cm -1 which could be attributed to -OH absorption. Also, the absorption peak at 2937 cm -1 could be attributed to C-H bond vibration, the peaks at 1713, 1627 and 1033 cm -1 could be attributed to C=O, C-N and Si-O-C, respectively. In addition, the absence of NH2 peak at 3375 cm -1 and presence of a new peak at 1627 cm -1 demonstrates the formation of TMIS absolutely.

NMR analysis of TSTA, TSPA and TMIS
The 1 H-NMR of TSTA is shown in Figure 4. It can be seen that all the relevant peaks of this carboxylic acid could be found in this Figure. The peak at 0.0 ppm resulted from the group of -CH3 in the chain of Si-CH3 and C-CH3 in the molecule of TSTA. The signals of the Si-CH2 resonance were obtained at 0.5, 1.6 and 3.2 ppm, respectively. A sharp signal is shown at 3.5 ppm belonging to Si-OCH3 bond. Also, a peak is observed at 12.4 ppm which belongs to -COOH bond in the molecule of TSTA. Figure 5 shows the 1 H-NMR spectra of TSPA. The peak at 0.0 ppm results from the group of -CH3 in the chain of Si-CH3 in the molecule of TSPA. Also, three peaks have been observed at 0.5, 1.3 and 2.6 ppm belonging to Si-CH2 and a signal has been obtained at 3.2 ppm, which belongs to the -NH bond. A sharp peak could be observed at 3.6 ppm which belongs to the NH-CH2 bond. In addition, two peaks are shown at 5 and 5.4 ppm which belong to Si-OC2H5 bond. Moreover, the peak at 13.1 ppm resulted from the group of COOH bond. Figure 6 shows the 1 H-NMR spectra of TMIS. The peak at 0.0 ppm results from the group of -CH3 in the trimetheyl silane (TMS). Three peaks could be observed at 0.5, 1.6 and 4.2 ppm which belong to Si-CH2, in addition, a sharp signal was obtained at 3.8 ppm, which belongs to the Si-O-CH2 bond. Also, three peaks could be observed at 7.9, 8.2 and 8.3 ppm which belong to the aromatic ring, respectively.

Thermal properties
To evaluate the thermal stability of copolymers, TGA and DSC are chosen as popular techniques [34]. The thermal properties of TSTA, TSPA and TMIS modified UV curable hybrid coatings and an unmodified sample was evaluated by means of TGA/DTA and DSC under nitrogen atmosphere (Figs. 7 and 8). The unmodified hybrid coating (Fig.   7A) shows a stable situation up to 370 °C. The chemical decomposition starts after this temperature and the maximum decomposition is at around 600 °C. The TSTA modified hybrid coatings (Fig. 7B) shows a stable situation up to 385 °C. The chemical decomposition starts after this temperature and the maximum decomposition is also at around 600 °C. The same thermal behaviors were observed for TSPA and TMIS modified hybrid coatings but they show stable situations at 400 and 420 °C, respectively. Based on the results, it is concluded that the existence of silicone containing carboxylic acids moieties in the hybrid coatings causes some thermal stability. In addition, by increasing the amount of alkoxysilanes, the thermal stability also increases.
The DSC curves of the TSTA, TSPA and TMIS modified UV curable hybrid coatings and unmodified sample are shown in Fig. 8. According to Fig. 8D, the DSC curve of the unmodified hybrid coating reveals an endothermic shift around 430 °C, which corresponds to Tg. The Tg is around 428 °C, 425 °C and 410 °C for TMIS, TSPA and TSTA modified UV curable hybrid coatings, respectively. The higher Tg for unmodified hybrid coating was interpreted as the result of the more difficult micro-Brownian motion of the stiffer chains rather than silane containing polymer chains. According to these results, it 18 could be concluded that the presence of TSTA, TSPA and/or TMIS moieties causes a change in the thermal behavior and it particularly affects Tg.

Morphological studies by SEM
The morphology of the hybrid coatings in the presence and absence of silicone was investigated by SEM from fractured surface. Fig.9 (A-D) shows the morphology of (A) hybrid coating without silicone based carboxylic acid, (B) TSTA, (C) TSPA and (D) TMIS, respectively. According to this micrograph, Fig. 9A shows a stormy structure wile; a little fibrillar structure is observed on the fractured surface in Fig 9B. A wavy form is observed from Fig. 9C and finally Fig. 9D shows a classical fracture zone which involves a fibrillar structure. Gungor and coworkers also found the fibrillar fracturing which could be due to the structure of soybean oil-based oligomer. By increasing the amount of sol-gel content increases fiber size and gives more ordered fibers [33]. Furthermore, the absence of silicone in the hybrid coating structure causes fluctuates in the morphology; fibrillar form was changed to wavy structure. Considering the obtained results, it is worth mentioning that silica particles are not dispersed homogenously at the molecular level in the hybrid system.

Physical and mechanical properties of the hybrid film coatings
The film properties included solvent resistance, hardness, gel content and adhesion properties.
To measure the solvent resistance of coatings, the MEK rubbing test was performed. According to Table 2, after 450 double rubs, the prepared hybrid coatings were unaffected and the chemical resistance was found to be excellent for all samples due to the observed the same results (450) double rubs after three separate measurements. Almost the similar results were found in the work of Bayramoglu et al [34].
The measurement of hardness is a quick, reliable means of quantifying the mechanical properties and performance of coatings. Hardness is one of the most important parameters affecting the mechanical properties of coatings especially scratch and abrasion resistance. Although surface smoothness, temperature and film thickness must be controlled carefully [33], crosslink density is primer structural parameter that plays a major role in the determination of hardness [34]. According to Jianbing et al. and also Karatas, et al [35,36]. the coating hardness increases by increasing the sol-gel content. Whereas the condensation of silanol results in the formation of inorganic silica network [37,38]. The resistance of coatings to scratch effects can be measured with the scratch hardness test by pencil hardness vehicle. It is equipped with a calibrated set of drawing leads ranging from 6B (the softest) to 6H (the hardest). The pencil that does not scratch the coating is specified as "pencil scratch hardness". As can be seen from Table 2, the obtained results of pencil hardness test of the hybrid coatings showed that the pencil scratch hardness of the samples without carboxylic acid-containing alkoxysilane (TSTA, TSPA and TMIS) coatings was 4H. With the addition of carboxylic acid, the pencil scratch hardness increased to 5H. Xiongfa Yang et al. also found a pencil hardness of 5 H for cured silicone material [39].
The gel content is an important factor and has a direct relationship with the ultimate properties of UV-cured hybrid films [40,41]. As shown in Table 2, the gel content of all the films which contained TSTA, TSPA and/or TMIS was between 85 and 90% whereas for samples without carboxylic acid-containing alkoxysilane films, this value was between 75 and 80%. The difference in the gel content could be attributed to the contribution of the alkoxysilanes groups on the silica surface to the conversion of polymerization.
Furthermore, the high values could be ascribed to highly cross-linked coating material. It 20 is worth mentioning that the photoinitiator causes slightly yellowish color in the cured films but they are generally transparent appearance.
The physical and chemical interactions between the coating and the substrate could be a logical reason for adhesion. Weak adhesion means poor performance [42]. DIN 53151 standard tests were applied to determine adhesion properties of the coatings. According to this method, adhesion is divided by the two numbers o (which represents a weak adhesion) to 5 (which represents an acceptable adhesion). Table 2 shows that almost excellent adhesion was observed on polycarbonate panels according to the obtained measurements varied between 3.5 and 5. This phenomenon can be ascribed to the good interaction between the hybrid coatings and polycarbonate panels.

Conclusions
In the present research, three novel carboxylic acid-containing alkoxysilanes have been synthesized and used in the preparation of alkoxysilanes based hybrid coatings by sol-gel technique. From this study, the following conclusions can be made: TSTA, TSPA and TMIS have been prepared successfully as novel carboxylic acids with a functional group, -COOH bond of acid. 1 H-NMR and FT-IR analysis showed that TSTA, TSPA and TMIS segments are present in the structures of silane-based acids and siliconated/methacrylated soybean oil (SMSO). TSTA, TSPA and TMIS improve the solvent resistance of hybrid coatings. Furthermore, the thermal stability of hybrid films is improved by both sol-gel content and carboxylic acid-containing alkoxysilanes. The adhesion of coating onto polycarbonate panels is excellent and the surface resistance to scratch improved with TSTA, TSPA and/or TMIS enrichment in the coating.