Assessment of Some Characteristics and Properties of Zirconium Dioxide Nanoparticles Modified with 3-(Trimethoxysilyl) Propyl Methacrylate Silane Coupling Agent

Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 100000, Vietnam Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 100000, Vietnam Faculty of Chemistry, Hanoi National University of Education, No. 136 Xuan +uy Road, Cau Giay District, Hanoi 100000, Vietnam NTT Institute of High Technology, Nguyen Tat +anh University, 300A Nguyen Tat +anh, District 4, Ho Chi Minh 700000, Vietnam


Introduction
Zirconium dioxide (ZrO 2 ) or zirconia nanoparticles have been used in many technological fields such as catalysis, sensors, dielectric materials, polymeric nanocomposites, metallic nanocomposites, coating, semiconductor, or optical materials, thanks to their advantages including high strength, good natural color, high transparency and chemical stability, transformation toughness, thermal stability, chemical resistance, anticorrosion, and microbial resistance. ere are several available methods for producing zirconia nanoparticles, consisting of hydrolysis, sol/gel, hydrothermal, pyrolysis, microwave plasma, or thermal treatment [1][2][3][4].
ere are few studies on surface modification of zirconia nanoparticles to increase the dispersion of zirconia nanoparticles in polymer matrices [12][13][14].
e ZrO 2 nanoparticles modified with 3-methoxysilyl propyl amine significantly improved the mechanical properties of epoxy resin [12,13]. Yan et al. modified ZrO 2 nanoparticles using N-(2-aminoethyl)-c-aminopropylmethyl dimethoxy silane coupling agent and investigated the effect of nanometer ZrO 2 content and silane coupling agent on the friction and wear properties of bismaleimide (BMI) nanocomposites [14]. e presence of ZrO 2 nanoparticles contributed to the decrease in the frictional coefficient and the wear rate of the nanocomposites. e modified ZrO 2 nanoparticles were dispersed in a polymer matrix better than untreated ZrO 2 nanoparticles, leading to the better tribological performance of nanocomposites containing modified ZrO 2 nanoparticles. Xu et al. ex situ synthesized functionalized ZrO 2 nanoparticles from zirconium (IV) isopropoxide isopropanol complex in benzyl alcohol and 3-(trimethoxysilyl) propyl methacrylate and applied them in UV curable poly (urethane-acrylate) (PUA) coating [5]. e coating was completely transparent when using 20 wt.% of ZrO 2 . e mechanical and thermal properties of PUA coating were improved significantly with the presence of functionalized ZrO 2 nanoparticles. Sayılkan et al. modified the surface of ZrO 2 nanoparticles with 2-acetoacetoxyethyl methacrylate for optical purposes [3]. e modified ZrO 2 nanoparticles have a size of 6.22 nm (transmission electron microscope) and 14.7 nm (particle size analysis). e surface modification also increases the stabilization of ZrO 2 nanoparticles [15].
Emulsion acrylic resin is widely used in daily life and industry because it has valuable properties such as high UV resistance, high aesthetics, weather resistance, and working well for various materials. In addition, it is environmentally friendly, low cost, and thus widely used in water-based paints and coatings for interior applications [16][17][18][19]. e evaluation of the effect of modified ZrO 2 nanoparticles on the abrasion resistance of emulsion acrylic resin can contribute to developing the paint system based on acrylic and nanoadditives in life sciences and technologies.
It can be recognized that the surface modification of ZrO 2 nanoparticles is necessary to improve their dispersibility in a polymer matrix. However, there are few reports related to the assessment of the efficiency with grafting of silane coupling agent on the surface of ZrO 2 nanoparticles [20]. e purpose of this work is to evaluate some characteristics, properties including grafting efficiency, functional groups, thermal stability, size distribution, and water stability of ZrO 2 nanoparticles modified with 3-(trimethoxysilyl) propyl methacrylate silane. Moreover, the effect of silane content as well as modified ZrO 2 nanoparticle content on abrasion resistance of emulsion acrylic coating has been also tested and discussed.

Modification of ZrO 2 Nanoparticles.
e procedure for modification of ZrO 2 nanoparticles by MSPMS was carried out based on different reports [11,12,21,22] as follows: first, MSPMS was hydrolyzed in 100 mL of ethanol solution for 30 minutes at 50°C. Next, 5 g of ZrO 2 nanoparticles was added into above solution and magnetic stirred continuously for 2 hours at 50°C. e solution was then homogenized on a T25 Ultra-Turrax digital high-speed homogenizer (IKA, Germany) for 30 minutes at a speed of 15,000 rpm. After that, the solid part was obtained by centrifuging and washing with ethanol 3-4 times before drying in a vacuum oven at 70°C to obtain modified ZrO 2 nanoparticles (abbreviated by m-ZrO 2 ). e weight ratio of silane and ZrO 2 nanoparticles and designation of the samples are presented in Table 1.

Characterization.
e functional groups of unmodified and modified ZrO 2 nanoparticles were analyzed by infrared (IR) spectroscopy, which means by a Nicolet iS10 ( ermo Scientific, USA) in 400-4000 cm −1 wavenumbers, 8 cm −1 resolutions, and 32 scans. e thermogravimetric (TG) analysis of the unmodified and modified ZrO 2 nanoparticles was carried out using a DTG 60H (Shimadzu, Japan) at a heating speed of 10°C/min in air from room temperature to 800°C. Field emission scanning electron microscopy (FESEM) images of unmodified and modified ZrO 2 nanoparticles were taken using S4800 FESEM (Hitachi, Japan). e dynamic light scattering (DLS, Zetasizer SZ-100, Horiba, Japan) was used to determine the size distribution and zeta potential of unmodified and modified ZrO 2 nanoparticles. e samples were dispersed in distilled water, and DLS spectra were recorded at 25°C.

Preparation of Acrylic Resin Coatings Containing Unmodified and Modified ZrO 2 Nanoparticles and Evaluation of eir Abrasion
Resistance. e acrylic resin coatings were prepared according to the following steps: first, the unmodified or modified ZrO 2 nanoparticles were dispersed in distilled water by a TPC-15H ultrasonic tank for 30 minutes at room temperature with the nanoparticles/water ratio of 1/ 10 w/v (mixture A). Next, Texanol and other paint additives were added into acrylic resin at the additives/resin ratio of 1.5/100 (w/w) on an IKA RW16 stirrer with a speed of 400 rpm for 15 minutes at room temperature (mixture B). e mixture A was mixed with mixture B on an IKA RW16 stirrer with a speed of 600 rpm for 15 minutes at room temperature before ultrasonicating in the Branson sonifier 450 device for 5 minutes. e coating samples were made by the Erichsen film applicator thickness wiper (model 360) at wet film thickness of 120 μm on glass. e abrasion resistance test of the coating samples has been performed using the falling sand abrasion method according to ASTM D968-15. e ElektroPhysik MiniTest 600 machine was used to measure the thickness of coatings. e volume of abrasive sand per unit coating thickness was the abrasion resistance of coating, expressed in L/mil (1 mil � 25 μm) as where V is the volume of sand (L), and d is the coating thickness (mil). e dispersion of ZrO 2 nanoparticles in the acrylic resin matrix was evaluated by the field emission scanning electron microscopy (FESEM) method on a S4800 FESEM (Hitachi, Japan).

TG Analysis and Grafting Efficiency of Silane on ZrO 2
Nanoparticles. Figures 1 and 2 show the TG and DTG diagrams of unmodified and modified ZrO 2 nanoparticles. e weight loss of unmodified ZrO 2 is zero, confirming that there is no presence of hydroxyl groups on the surface or inside the structure of ZrO 2 nanoparticles [23]. From the TG diagrams, it can be seen that the modified ZrO 2 (m-ZrO 2 ) nanoparticles lost the weight in the range temperature of 200°C-500°C. ere is a broad peak on the DTG diagrams of m-ZrO 2 samples corresponding to the degradation of organosilane grafted on the surface of ZrO 2 nanoparticles [21]. As varying the MSPMS content, the maximum degradation temperatures of m-ZrO 2 samples were changed (Table 2); however, the onset degradation temperatures of all m-ZrO 2 samples are similar.
e grafting efficiency of MSPMS to ZrO 2 nanoparticles was estimated by the TG method [20,21] and is given in Table 2. Because the surface of ZrO 2 nanoparticles is hydrophobic, the grafting efficiency of MSPMS to ZrO 2 nanoparticles is low, lowest value of 3.4% for m-ZrO 2 -15 and highest value of 13.0% for m-ZrO 2 -3. As increasing the silane content, the grafting efficiency of MSPMS to ZrO 2 nanoparticles was decreased.
is exhibits that the organosilane was residue in the modification process. In the investigated MSPMS contents, 3% of MSPMS is the most suitable for modifying ZrO 2 nanoparticles with a high effectiveness.

IR Spectra of Unmodified and Modified ZrO 2
Nanoparticles.
e IR spectra of unmodified and m-ZrO 2 nanoparticles are shown in Figure 3. A strong band can be seen with the peaks at 570 cm −1 and 670 cm −1 characterized for Zr-O-Zr stretching vibration of ZrO 2 nanocrystals [13,[24][25][26]. ere is no new peaks appeared in IR spectra of m-ZrO 2 nanoparticles, suggesting that the surface modification does not have any influence on the vibration of ZrO 2 nanocrystals. It is difficult to observe the vibration of functional groups in MSPMS on m-ZrO 2 nanoparticles. is may be due to the high hydrophobic surface of ZrO 2 leading to difficult formation of bonding between organosilane and ZrO 2 nanoparticles. Moreover, the content of silane grafted on the surface of ZrO 2 nanoparticles is quite small causing an effect on the appearance of organic groups in MSPMS on IR spectra of m-ZrO 2 nanoparticles. erefore, only a small peak at 1120 cm −1 which is assigned to the stretching vibration of Si-O-Zr in the IR spectrum of m-ZrO 2 -5 nanoparticles [12,13] can be seen, as shown in Figure 4.

Morphology of Unmodified and Modified ZrO 2
Nanoparticles. As observation from the FESEM images of unmodified and m-ZrO 2 nanoparticles in Figure 5, the ZrO 2 nanoparticles are in spherical shape and have size in range from 50 nm to 150 nm. e tendency to agglomerate ZrO 2 nanoparticles can be observed due to the affinity of nanoparticles. e modification process has a negligible effect on morphology of ZrO 2 nanoparticles.

Size Distribution and Zeta Potential of Unmodified and Modified ZrO 2 Nanoparticles.
e size distribution of unmodified and m-ZrO 2 nanoparticles is shown in Figure 6. e size distribution of m-ZrO 2 nanoparticles at different contents of MSPMS is not systematic. is may be due to agglomeration of m-ZrO 2 nanoparticles occurred when using the high content of MSPMS. Moreover, the modification process of ZrO 2 nanoparticles with MSPMS also increases the hydrophobic of nanoparticles [22,27], leading to a difficult dispersion of m-ZrO 2 nanoparticles in water. e polydispersity index (PI) of all tested samples is higher than 0.3, corresponding to a broad size distribution of nanoparticles as reported by Danaei et al. [28]. e difference between particle sizes obtained in the FESEM method and DLS method is caused by the different dispersions of unmodified and m-ZrO 2 nanoparticles. For FESEM analysis, the nanoparticles were in solid and taken FESEM images, while for DLS analysis, the ZrO 2 nanoparticles were dispersed in distilled water before taking size distribution. e nature of ZrO 2 nanoparticles is hydrophobic; thus, they were dispersed difficultly in water, leading to the bigger size of the ZrO 2 nanoparticles. e zeta potential of unmodified ZrO 2 (u-ZrO 2 ) nanoparticles and m-ZrO 2 -3 nanoparticles shown in Figure 7 demonstrates the u-ZrO 2 nanoparticles have a positive charge surface (21.4 mV) while m-ZrO 2 -3 nanoparticles have a negative charge surface (−12 mV). e modification process that caused the change in the charge on the surface of ZrO 2 nanoparticles from the positive region to the negative region may be due to the conjugation effect of C�C-C�O bond in MSPMS grafted onto the surface of ZrO 2 nanoparticles to form the negative charge on oxygen atom. From the above results, it can be recognized that the ZrO 2 nanoparticles were modified successfully with MSPMS.      Journal of Chemistry ese zeta potential values also suggest that unmodified and modified ZrO 2 nanoparticles are relative stable in water.

Application of Unmodified and Modified ZrO 2
Nanoparticles for Emulsion Acrylic Resin Coating 3.5.1. Abrasion Resistance. e effect of MSPMS content on the abrasion resistance of acrylic based coating containing 2 wt.% of nanoparticles was evaluated and is given in Table 3.
e acrylic resin and acrylic/u-ZrO 2 coating has a low abrasion resistance, 83.60 ± 4.47 L/mil and 77.50 ± 4.08 L/mil, respectively. In this case, u-ZrO 2 nanoparticles are not able to improve the abrasion resistance of acrylic coating due to the less dispersion of u-ZrO 2 nanoparticles in acrylic matrix, leading to the agglomeration of ZrO 2 nanoparticles and the formation of defect in structure of coating, causing the decrease in the abrasion resistance of acrylic coating.
Using m-ZrO 2 nanoparticles makes a remarkable enhancement in abrasion resistance of coating containing because m-ZrO 2 nanoparticles dispersed more regularly in acrylic matrix. e best improvement in abrasion resistance of coating was observed for acrylic/m-ZrO 2 -3 coating, an increase of 62.33% as compared to acrylic resin coating.
To assess the influence of m-ZrO 2 nanoparticle content on the abrasion resistance of acrylic coating, the coatings based on acrylic resin and different contents of m-ZrO 2 -3 nanoparticles were prepared. It can be seen that the abrasion resistance of coating containing m-ZrO 2 -3 nanoparticles was increased as increasing the content of m-ZrO 2 -3 from 0.5 to 2 wt.% and then decreased at the 5 wt.% of m-ZrO 2 -3 nanoparticles.
is reduction can be caused by the agglomeration of m-ZrO 2 -3 nanoparticles when using at high content. From obtained results, the suitable content of MSPMS silane for modification is 3 wt.% and of m-ZrO 2 -3 nanoparticles in acrylic resin coating is 2 wt.% (Table 4).      e FESEM images of the cross-surface of acrylic coating containing unmodified or modified ZrO 2 nanoparticles are shown in Figure 8. It can be seen that the unmodified ZrO 2 nanoparticles were agglomerated in acrylic resin matrix, while the ZrO 2 nanoparticles modified with 3 wt.% MSPMS were dispersed regularly in acrylic resin. e interaction of carbonyl and vinyl groups in MSPMS on the surface of modified ZrO 2 nanoparticles with carbonyl groups in acrylic resin leading to the MSPMS plays a role of binder for acrylic resin and ZrO 2 nanoparticles [29], resulting in the good dispersion of modified ZrO 2 nanoparticles in acrylic resin. anks to the good dispersion of m-ZrO 2 -3 nanoparticles in the acrylic resin, the abrasion resistance of the coating was improved as discussed above.

Conclusions
In conclusion, ZrO 2 nanoparticles were modified successfully with 3-(trimethoxysilyl) propyl methacrylate silane (MSPMS). e modification process does not affect morphology and functional groups but cause the change in surface charge and thermal behavior of ZrO 2 nanoparticles. e grafting efficiency of silane to ZrO 2 nanoparticles reached 13.0% when using 3 wt.% MSPMS for modification. e content of organosilane and content of modified ZrO 2 nanoparticles have an effect on the abrasion resistance of acrylic resin coating. e modified ZrO 2 nanoparticles improved significantly the abrasion resistance of acrylic resin coating, especially, when using 2 wt.% ZrO 2 nanoparticles modified with 3 wt.% MSPMS. is is the initial result to open up the prospects for the application of modified ZrO 2 nanoparticles in coatings based on emulsion acrylic resin or other polymers.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.