Characterization of silicon acrylic resin containing silica nanoparticles as candidate materials for antifouling and anticorrosion properties in seawater

2.1 Reagents

Methyl acrylate (MA) (98%) andacrylicacid (AA) (99%) were obtained from Tianjin Guangfu Fine chemical research institute (Tianjin, China). Methyl methacrylate (MMA) (98%), styrene (ST) (99.5%) and hydroxyethyl methyl acrylate (HEMA) (99%) were from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). PDMS (99%) from Dow Corning Chemicals, USA and hexamethylenediisocyanate biuret (HDI biuret) was purchased from MYAY Reagent (Zhejiang, China) and Bayer Material Science Co., Ltd. (China), respectively. While ethyl acetate (99.5%) and butyl acetate (99%) which were used as mixed solvents was provided by Tianjin Fuyu Fine Chemical Reagent Co., Ltd. (Tianjin, China). An organic flatting agent BYK-331 (polyether-modified dimethylpolysiloxane copolymer) and defoamer BYK-052 (polyvinyl butyl ether in Stoddard solvent) were supplied by BYK Chemie (Wesel, Germany). Benzoperoxide (BPO) and (3-Aminopropyl) triethoxysilane (APTES, 99%) which served as free radical initiator and modifying agent were purchased from Aladdin Chemistry Co., Ltd. (China). All the reagents were employed as received without further purification.

2.2 Synthesis of silicone-acrylic resin

Silicone-acrylic resin copolymer was prepared by free radical polymerization in a 250 mL four neck round glass reactor equipped with a reflux condenser, mechanical stirrer, nitrogen gas inlet and thermometer. Initially, 42.82 g of ethyl acetate, butyl acetate and PDMS mixture were added into the reactor and heated to 75 °C with continuous stirring at the rate of 300 rpm under nitrogen atmosphere. 30 min later, the reactor was heated to 105 °C, then the acrylic monomer mixture (34.92 g) mixed with evocating agent (0.377 g) were added into the reactor at a constant speed using a peristaltic pump. The polymerization reaction was conducted at 105 °C under azeotropic distillation until complete conversion of the acrylic monomer to yield a light opalescent transparent viscous solution. The composition of the medium resin and its physical characteristics are shown in Table 2.

2.3 Synthesis of modified SiO₂ nanoparticles

Many nanoparticles are used in coating, such as nano SiO₂, nano TiO₂, nano ZnO, nano CaCO₃, and so on. The SiO₂ nanoparticles are widely used in polymer composites coating because the industrial production of nano SiO₂ is currently the largest in the world. Moreover, adding nano SiO₂ to the coating can form a shielding effect, resulting in enhance the anti-UV aging performance of the coating. Also, nano SiO₂ has a three-dimensional mesh structure, which can form a mesh structure when the coating is dry, increasing the strength and gloss of the coating.

The amino-functionalized SiO₂ was obtained through surface modification by using APTES ((3-Aminopropyl) triethoxysilane) as the silane coupling agent. 1 g of nanoparticles (10–80 nm) were dried in a vacuum drying oven under 100 °C for 12 h in advance and then dissolved in 22 mL of nitrogen methylpyrrolidone (NMP) under ultrasonic dispersion for 0.5 h. Then 5 mL of APTES and NMP mixed liquors were added into the beaker by dripping slowly under ultrasonic condition. After that, all the reactants were transferred into a three round bottom flask for 2 h under nitrogen atmosphere. After the reaction progress was completed, the modified SiO₂ nanoparticle was separated by a centrifuge. After the modification, the as prepared SiO₂ suspension exhibits unique properties such as higher molecular weight, less polar components and better hydrophobicity. Therefore, the suspension exhibits enhanced performance when dispersed in the organic polymers.

2.4 Preparation of the SiO₂/silicon acrylic nanocomposite coating

The preparation of the SiO₂/silicon acrylic nanocomposite coating was carried out in a 250 mL beaker at room temperature under ultrasonic condition. Initially the organosilicon acrylic resin and the curing agent HDI biuret were added with a ratio of 6:1. After 15 min, 0.5 wt% BYK-052 and 0.2 wt% BYK-331 were introduced into the reaction system to improve surface leveling, thus organosilicon acrylate was acquired. At the same time, 4.0 wt% modified nano-SiO₂ nanoparticle was dispersed into the mixture. After the mixing process was carried out under ultrasonic condition for 0.5 h, the nanocomposite coating with wonderful dispersion was obtained.

2.5 Preparation of sample plates

Three rectangular aluminum plates (5 × 3 cm × 1 mm) were polished with abrasive sandpaper, followed by rinsing with deionized water and degreased with methanol. Then paint free plate was placed as blank sample and was named S1. Another two plates with coating on one side were deposited in a drying cabinet for 2 h before usage. The sample with silicone acrylic coating was named S2, while the nano-SiO₂ (4.0 wt%) silicone acrylic coating was named S3. Each sample was painted with a brush and dried at room temperature for about one week to form a stable film with approximate 1 mm thickness.

2.6 Accelerated corrosion test in simulated seawater

To test the anticorrosion properties of the nanocomposite coating, all the samples were periodically immersed in the simulative marine environment. The simulated seawater is composed of 2.5% NaCl, 1.1% MgCl₂, 0.40% Na₂SO₄ and 0.16% CaCl₂ (Yamada et al. 2013). Before the simulated seawater periodic immersion experiment, the SEM analysis of the three samples was conducted to observe the surface morphology. Then, all samples were immerged into the simulated seawater for 8 h and exposed in the air for 16 h for a cycle and the experiments were carried out for 14 cycles (Cai and Liu 2006). After the experiment, three samples were tested again with scanning electron microscope. The corrosion resistance of the coating was detected by the surface morphology of the three samples before and after the experiments.

3.1 Analysis of infrared spectra of silicone acrylic resin

The FT-IR spectra of silicon acrylic resin are showed in Figure 2. The peaks appearing around 3520 cm⁻¹and 3025 cm⁻¹ are the –OH and –CH telescopic vibration. The absorption bands at 2945 cm⁻¹ can be assigned to the –CH₂ symmetric stretching vibration (Smith 1998). The absorption bands at 1730 cm⁻¹ is attributed to the CdbndO asymmetrical stretching vibration (Andrews 1992). The absorption band at 1451 cm⁻¹ is associated with –CH₃ bending vibration (Azémard et al. 2014). The characteristic peaks at 1261 and 988 cm⁻¹ are ascribed to the –COOH stretching vibration (Kuanet al. 2005). The absorption bands at 1074 and 517 cm⁻¹ are attributed to the Si–O asymmetrical stretching vibration (Ammar et al. 2016a). The absorption bands at 841 cm⁻¹ is related to C–Si telescopic vibration (Ammar et al. 2016a). Finally, the absorption bands at 760 and 700 cm⁻¹ are corresponds to the vibrations of aromatic groups (Su and Chang 2003).

Figure 2:

FT-IR spectra of silicon acrylic resin.

To prove that PDMS is involved in cross-linking reactions, the comparative experiments are designed. Experimental procedure is the same as section 2.2. The resins were synthesized with adding PDMS (labeled as Resin 1) and without PDMS (labeled as Resin 2). From the gel permeation chromatography analysis to Resin 1 and Resin 2, it can be known from Table 3 that all molecular weight (including weight-average molecular weight, number-average molecular weight, higher average molecular weight and peak molecular weight) of Resin 1 is larger than those of Resin 2. Combined with FT-IR analysis, it can be concluded that PDMS participated in the cross-link reaction.

3.2 FT-IR spectra of nano-SiO₂ before and after modification with KH-550

The FT-IR spectra of nano-SiO₂ before and after modification with KH-550 are displayed in Figure 3. In the IR spectra of the nano-SiO₂ before modification with KH-550 (Figure 3a), the peaks around 3447 cm⁻¹ can be assigned to the Si-OH telescopic vibration. The absorption bands at 1087 cm⁻¹ are attributed to Si–O–Si antisymmetric stretching vibration. The absorption bands at 780 cm⁻¹ can be assigned to the Si–O–Si symmetrical stretching vibration. There are also characteristic absorption bonds of Si–O–Si bending vibration at 487 cm⁻¹. These groups are also seen in the FT-IR spectra of nano-SiO₂ after modification with KH-550 (Figure 3b). However, the intensity of absorption bands at 3438 cm⁻¹are obviously enhanced because of the simultaneous appearance of –NH₂ and Si–OH absorption peaks. In addition, new characteristic peaks are also appeared on the spectral line b. The peak around 2982 cm⁻¹ is –CH₂ stretching vibration. The absorption bands at 1397 cm⁻¹ are associated with C–N stretching vibration. The absorption bands at 1172 cm⁻¹ are C–C stretching vibration. The absorption bands at 864 cm⁻¹ are C–Si stretching vibration. All the presented data indicates the successful modification of nano-SiO₂ by KH-550.

Figure 3:

FT-IR spectra of nano-SiO₂ (a) before modification with KH-550, (b) after modification with KH-550.

3.3 Test of glass transition temperature

The glass transition temperature (Tg) is one of the characteristics of polymer materials, and the transformation temperature of non-stereotyped polymer from glass state to high-bounce state is the minimum temperature of free movement of the non-stereotyped polymer macromolecule chain segment, which has an important influence on the performance and processing process of polymer materials. In general, Tg value for seawater applications should be controlled at 55 to 60 °C.

The differential scanning calorimetry was used to obtain the glass transition temperature of the silicone acrylic resin to determine the phase diagrams and mixtures of polymer systems. The test was carried out under nitrogen flow at atmospheric pressure in a temperature range of 20 to 500 °C. In this temperature range three characteristic temperatures can be detected. According to Figure 4a, the glass transition temperature of organosilicon acrylic resin is in the range of 50–66 °C (relatively flat exothermic section). This is due to the resin appeared transition from the glassy state to the high elastic state in the temperature range. It can be seen from TG-DTG curve that the resin does not appear weight loss within the temperature range, and the exothermic section analysis shows that the glass transition temperature of the resin is about 58 °C. The endothermic peak appears at 164 °C. Under this temperature, the silicone acrylic resin copolymers show a weight loss of 1.8% (Figure 4b), which is due to the solvent evaporation (Zhang et al. 2016). The sample weight drops from 91.5% to 7.5%, a drop of 84% occurs between 327 and 447 °C, among which a conspicuous endothermic peak appears at 390 °C. This is because most polymers begin to break down at this temperature range. It can be concluded that the resin copolymers exert some thermal instability and indicate an endothermic small shift around 58 °C, which is assigned to glass transition temperature (Tg).

Figure 4:

(a) Differential scanning calorimetry and (b) TG-DTG of silicon acrylic resin.

3.4 Contact angle (CA) measurements of applied resins

The CA to water can be used to evaluate the wetting of the solid surface. The greater the CA, the stronger the hydrophobic. In this work, the water CA measurements were introduced to investigate the hydrophobic character of the developed coatings (Ammar et al. 2016b). The CAs of four droplets of distilled water at different points of each sample was recorded by using JC2000 CA instrument. 5 μL water droplet was gently deposited on the sample surface and capturing the image was occurred directly in terms of measuring the static CA. The reported CA values were the averages of four measurements with less than 2° as a measurement error. The multiple averaging method is adopted to reduce CA measurement error. The CA of silicon acrylic resin coatings and nanocomposite coatings are measured in four different positions respectively. Figure 5 depicts CA values of silicon acrylic resin coating plate (Figure 5a) and nanocomposite coating plate (Figure 5b). It is well known that nano-SiO₂ is easy to agglomerate because of its high surface energy and strong surface polarity. In this experiment, nanometer SiO₂ was modified to reduce its surface energy, prevent its agglomeration and improve its affinity of organic phase. The experimental results showed that the CA values of silicon acrylic resin coating plate and nanocomposite coating plate were 108.5° and 110°, respectively. According to the experimental results, the surface of the coating can be maintained to some extent by the introduction of modified nano-SiO₂ in silicon acrylic resin. The similar observations were previously reported by researchers introducing SiO₂ nanoparticles into polymeric matrix (Wang et al. 2011). It is known that the antifouling effect of the coating display only the CA at above 98°. According to the CA test results, both silicon acrylic resin coating and nanocomposite coating possess the characteristics of preventing marine biofouling.

Figure 5:

Contact angle (CA) of (a) silicon acrylic resin coating plate and (b) nanocomposite coating plate.

3.5 SEM image of coatings against seawater corrosion

SEM micrographs of all prepared sample plates show the efficiency of coatings against seawater corrosion (Figure 6). It was found that the sample plate S1 was badly corroded by simulated seawater with a cavity over the surface (S1 in Figure 6), and the sample plate S3 was slightly corroded and the sample plate S2 was scarcely corroded. Obviously, paint plays a key role in the resistance of seawater corrosion (S3 comparing to S1). In addition, the anticorrosion properties of the coating were significantly enhanced with the addition of nano-SiO₂ (S3 comparing to S2). According to the SEM images of all the sample plates, it can be concluded that the nanocomposite coatings have excellent corrosion resistance to seawater.

Figure 6:

SEM images of three sample plates before and after the accelerated corrosion test.

3.6 Influence of silicon monomer content on film performance

The influence of the silicon monomer content on the wettability of the resulting coated surfaces was shown in Figure 7. The incorporation of silicone monomer into the acrylic resin results in an increase in the CA at 104° compared to the uncoating (CA at 89°), which shows that PDMS within the polymeric coating has the ability to enhance hydrophobicity. It is because the PDMS possesses a significant low surface energy and the ability to produce hydrophobic surfaces. This leads to changes in the chemical composition due to the presence of the (–Si–O–Si–) silicone. However, from the experimental results, it was found that the CA was not increasing further when the content of the PDMS exceeds a certain value. This consequence can be ascribed to the saturation of the grafting of PDMS on the main chain of the polymer. Finally, the mass of the PDMS selected was 2.82 g.

Figure 7:

The water CA values of different PDMS content in acrylic resin. (1) 0 g PDMS (2) 0.94 g PDMS (3) 1.88 g PDMS (4) 2.82 g PDMS (5) 3.76 g PDMS.

The antifouling tests were not performed yet, but we are planning to measure the antifouling properties in future.