Crosslinking, Mechanical Properties, and Antimicrobial Activity of Photocurable Diacrylate Urethane/ZnO-Ag Nanocomposite Coating

In this article, ZnO-Ag nanohybrids were chemically synthesized in the aqueous medium by reducing silver nitrate with sodium borohydride NaBH4. These nanohybrids were then homogeneously dispersed into the diacrylate urethane/1,6-hexanediol diacrylate resin system at a content of 2 wt%. The structural morphology, mechanical resistances, and crosslinking of the as-prepared nanocomposite coating (nanocoating) were evaluated. The antimicrobial characteristic was tested by keeping track of the lag-log growth phase of E. coli bacteria in the coating existence among cell cultures. The obtained data indicated that the nanohybrids added into the UV curing diacrylate urethane matrices had significantly increased the abrasion resistance, relative hardness, and conversion of the acrylate groups of the nanocoating. In addition, the antibacterial test revealed that the nanocoating had good antibacterial property against E. coli, whereas for the pure coating (without ZnO-Ag nanoparticles), there was no antibacterial activity observed.


Introduction
UV curable acrylate resin-based paints exhibit many advantages, such as transparency, moisture resistance, chemical resistance, and being environmentally friendly, since they do not contain the organic solvents and can be processed at normal temperatures. Consequently, they have been widely applied to various finish surfaces, such as steel or wooden floors [1][2][3][4].
Nowadays, nanomaterials attract strong research and application interest. The addition of nanoscale additives to the polymer matrix significantly enhanced the properties of the material [5][6][7][8][9][10][11][12]. Enhancing the properties of coatings with antibacterial function has become a social demand; thus, there is an attempt to mix inorganic nanoparticles (like Ag, ZnO, and TiO 2 ) as antibiotics into paint [13][14][15][16][17][18][19]. Among those, Ag nanoparticles showed the highest antimi-crobial effectiveness against most bacteria. In order to improve the antimicrobial activity of nano-Ag, Ag particles are often attached to nanosized transition metal oxides (such as Fe 3 O 4 and MnO 2 nanoparticles). In the previously published paper, we reported the antimicrobial activity of Fe 3 O 4 -Ag hybrid nanoparticles which was clearly more efficient than that of the single Ag nanoparticles [20][21][22][23]. The plausible explanation for this observation is that (1) Ag + ions were released faster from the hybrid nanoparticles and (2) the ionization of nano-Ag was accelerated by the presence of Fe 3+ ions. Moreover, the absorbed radiation range of the nanohybrids can shift to the UV and visible light region [20].
However, the addition of nanoparticles (as nanofillers) can affect the curing reaction of thermoset resin systems depending on the intrinsic composition of the nanoparticles. In case of the epoxy matrix, Fe 3 O 4 nanoparticles can act as a bridging link molecule, leading to reduce the total free mass and increase the cross-linking density [24,25]. Herein, to explore the healing dynamics of the epoxy/amine system, Fe 3 O 4 nanoparticles played a role as nanocontainers for carrying functional groups such as the carboxylic acid [26], amino [27,28], or hydroxyl [27,29]. The authors found that reaction between carboxylic acid and amine functional groups of curing agent could impact hardener activation [26]. In contrast, hydroxyl and amine groups could increase the amount of thermal curing by gaining access to the epoxy groups [27]. It was reported that nano-SiO 2 might cause an incomplete crosslinking stage of clearcoat based on acrylicmelamine resin, reducing the crosslinking density of the neat clearcoat, and enhancing the weathering durability of the clearcoats. One possible explanation is that nano-SiO 2 absorbed the UV light, thus protecting the clearcoat from the impact of weathering degradation [30]. In our previous works [31][32][33] when examining the curing reaction of acrylic polyols with isocyanate, we found that isocyanate groups are also involved in reaction with OH groups of nano-SiO 2 to form a tight inorganic-organic hybrid material. For UV curing systems, the degree of influence of additives depends not only on the content but also on their ability to absorb UV rays and photocatalytic activity. Fe 3 O 4 -Ag hybrid nanoparticles, despite a strong UV absorption, their low content (0.1 wt%) might not significantly affect the kinetics of curing reaction [23]. The organic UV absorber T384 reduced the efficiency of double bonds while nanoparticles absorb UV anatase TiO 2 and ZnO increased the conversion of the acrylate groups of the system due to strong photocatalytic activity [34].
In this work, ZnO-Ag nanohybrids were chemically synthesized in the aqueous medium by reducing silver nitrate with sodium borohydride NaBH 4 . Their structural morphology had been characterized. These nanohybrids were then homogeneously dispersed into the diacrylate urethane/1,6hexanediol diacrylate resin system. The photocrosslinking polymerization process of the system of acrylate urethane resin and 1.6 hexanediol diacrylate diluent in the presence of ZnO-Ag nanohybrids had been investigated by using various measurements, such as the quantitative infrared spectra, coating hardness, and coating gel fraction. In addition, morphology, abrasion resistance, and antimicrobial activity of nanocomposite coating were also evaluated.

2.2.
Synthesis of ZnO-Ag Nanohybrids. ZnO-Ag nanohybrids are chemically synthesized in the aqueous medium by reduc-ing silver nitrate with sodium borohydride NaBH 4 [11]. Firstly, nano-ZnO (1.5 g) is added in 300 ml of distilled water, then dispersed homogeneously with aid of the ultrasonication 35 kHz for 60 min. Next, 30 ml of AgNO 3 solution (1.67 mg/ml) is dripped into the above-prepared dispersion of ZnO nanoparticles by further sonication for 40 min. The mixture is then placed into the 500 ml glass flask for the next synthesis step. To fabricate ZnO-Ag nanohybrids, NaBH 4 solution formed by dissolving 0.024 g of reductant into 50 ml water is then dripped (1 drop/s) to the reaction flask. The mixed solution is stirred (120 rpm for 60 min) at 4°C. The nanoparticles were obtained by repeatedly adding fresh distilled water and then centrifuging the solution at 10,000 rpm for 5 min for several times until the total removal of the residual precursors and agents.

Preparation of Nanocomposite
Coating. ZnO-Ag nanohybrids at the 2 wt% content of HDDA and E284 are added into HDDA, then ultrasonicated during 3 hrs. Thereafter, this mixture is mixed with E284 and I.184 by mechanical stirring in the Ika RW16 Basic Mixer (England) for 30 min. The ratio of HDDA/E284/I.184 was 45/55/3.
The paint films are prepared on different surfaces with varied sizes depending on investigation purposes. In detail, for IR spectral analysis, paint layers with a 25 μm thickness are coated on KBr pellets. For gel fraction examination and morphology observation, the coatings were made on Teflon sheets with the size of 100 × 100 × 10. The coatings were deposited on glass plates (100 × 100 × 3 mm) for testing the abrasion resistance and relative hardness.
The coating samples are exposed to UV light (250 mW/cm 2 ) using the FUSION UV system (F300S, USA) at 25°C in air by passing several times with a 5-40 m/s rate of web. The time of UV exposure can calculate from the web rate, for example, UV exposure time of 0.15 s corresponds to a web pass at 40 m/s.

FTIR Spectrum Analysis.
A decrease of characteristic IR absorption bands of acrylate groups during the crosslinking reaction is quantitatively studied according to the method described in previous papers [23,34] by using a FTIR spectroscopy Nicolet iS10 (USA).

Gel Fraction Analysis.
The gel fraction (GEL) of coatings is analyzed according to ASTM D 2765. The cured samples are carried out in a Soxhlet tool in CH 3 COCH 3 for 24 h [23]. The GEL is determined by the following formula: where m t is the weight of insoluble part which was heated at 50°C until completely dry and m 0 was the weight of initial coating (before analysis).

Study on Hardness and Abrasion
The abrasion resistance of the films is determined according to the abrasive falling methods (ASTM D968). Each sample was tested 3 times, and the illustrated results were the average values.

Microstructural Examination.
The morphology of the self-synthesized ZnO-Ag nanohybrids has been analyzed by transmission electron microscopy (JEM 1010, JEOL Ltd., Tokyo, Japan). The crystallinity and crystalline phase of the fabricated nanoparticle structures were examined using an X-ray diffractometer (XRD, Philips X'Pert MPD) with CuKα radiation (0.1540 nm) in the range of 20°-60°.
The morphology of the nanocomposite coating has been studied using Field Emission Scanning Electron Microscopy S 4800 (FE-SEM) (Hitachi, Japan).

Antimicrobial
Assay. For antimicrobial assay, E. coli strain DH5α is received from Invitrogen (USA); LB medium including yeast extract and Bacto tryptone are ordered from Merck (Germany). E. coli strain DH5α is inoculated into 2 ml of LB medium and shacked overnight at 200 rpm while the temperature remained 37°C. Another cell suspension at 1% v/v cultured cell suspension: LB medium is prepared by inoculating the cells into 100 ml of fresh LB medium. The incubation underwent the same conditions as presented above. Once the value of optical density value (λ = 600 nm, OD 600 ) was 0.3, the coatings are placed into incubation-flagon and kept cultivating. The suspension of cultured cell suspension is then collected at various time scales (from 30 to 300 min) after adding either UVAU or UVAU/ZnO-Ag nanocoatings (with the same size of 100 × 100 × 0:03 mm); then, their OD 600 values were determined. All experiments were conducted in triplicate for determining the mean value [19,23].

Results and Discussions
3.1. Study on Characteristics of ZnO-Ag Nanohybrids. The structural morphology of pure ZnO nanoparticles (ZnO) and ZnO-Ag nanohybrids is characterized by TEM and XRD analyses and shown in Figures 2 and 3. Figure 2 shows that the used commercial ZnO nanoparticles were cylindrical in shape and have dimensions of about 30-70 nm in width and about 100-200 nm in length; Ag nanoparticles are spherical in shape with a size of about 10-30 nm attached to ZnO nanoparticles. Because these two types of particles are different in shape and size, it is easy to see the presence of both types of particles on TEM image.
The crystalline phase structure of the self-synthesized ZnO-Ag nanohybrids is analyzed by the X-ray diffraction method. XRD diffraction diagrams of ZnO-Ag nanohybrids and pure ZnO nanoparticles are presented in Figure 3.    . Apart from the spectral lines that characterize the hexagonal crystal structure of pure ZnO compounds, no other peaks or spectra of the impurity were observed. On the XRD diffraction pattern of ZnO-Ag hybrid nanoparticles, the characteristic diffraction peaks of ZnO remain the same, which shows that Ag ions are not substituted for ZnO in the crystal lattice of ZnO. In addition, on the XRD pattern of ZnO-Ag nanohybrid nanoparticles, diffraction peaks appear at angular positions corresponding to the lattice planes (111), (200), (220), and (311) in the face center cubic structure of Ag (JCPDS No. 04-0783). Thus, the method of using NaBH 4 reducing agent to reduce Ag+ ions to Ag atoms is described by the following reaction equation:

Study on Crosslinking and Characteristics of Photocuring
Acrylate Urethane/ZnO-Ag Nanocoating 3.2.1. Conversion of the Acrylate Groups. Using the IR spectral method to study the crosslinking of the curing resin system is one of the most simple and effective methods. It allows to assess both the rate of polymerization reaction and the conversion of the reacted functional groups [1,23,34]. In the current paper, the photocrosslinking polymerization reaction of the coating base on E.284 resin, HDDA diluent, and I.184 photoinitiator in the presence of 2 wt% ZnO-Ag nanohybrids is investigated by analyzing the changes of IR absorption density of acrylate double bonds. IR spectra of the photocurable diacrylate urethane/1,6-hexanediol diacrylate/1-hydroxy-cyclohexyl-phenyl-ketone systems without (UVAU) and with 2 wt% ZnO-Ag nanohybrids (UVAU/ZnO-Ag) before and after UV exposure of 4.8 s are presented in Figure 4. Figure 4 shows that after 4.8 s exposed to UV, IR bands at 1632, 1409, 984, and 811 cm -1 which are assigned to the double bonds of acrylate groups in E.284 and HDDA molecules decreased. Among those, the absorption band at 811 cm -1 is chosen to study numerically the transformation of the acrylate groups throughout the crosslinking process of the coatings because of its most clearly decrease. The calculated data are presented in Figure 5.
As shown in Figure 5, the conversion of the acrylate groups occurs rapidly in the first 0.15 s and then slows down. The acrylate double bond conversion at the coating containing 2 wt% ZnO-Ag nanohybrids was higher than that at neat coating (without ZnO-Ag nanohybrids). After 4.8 s exposed to UV light, most of the acrylate double bonds were covered by 93.53 and 95.82% for neat and nanocomposite coatings, respectively. Thus, ZnO-Ag nanoparticles enhanced the transformation of acrylate double bonds.

Variation of Gel Fraction.
To have further information of the photopolymerization reaction of the system in the presence of ZnO-Ag nanohybrids, the GEL of the coatings was determined. Figure 6 visualizes the obtained results. Figure 6 indicates that, after 0.3 s of crosslinking, the GEL of the films without and with 2 wt% the nanohybrids appeared. The GEL increased rapidly in the first 2.4 s; after then, it slowed down. In the presence of 2 wt% of the nanohybrids, the GEL of the film was increased in comparison with that of the neat film. After 4.8 s of the crosslinking, the GEL of the films got the highest value of approximately 95.3 (without the nanohybrids) and 96.7% (with 2 wt% ZnO-Ag nanohybrids). Thus, 2 wt ZnO-Ag nanohybrids increased the GEL of the films.

Hardness Evaluation.
For the thermoset resin system, the hardness of the coating increased with the crosslinking density, and thus, it is possible to study the polymerization reaction by keeping track of the growth of the coatings' hardness. Figure 7 represents the relative hardness of coatings varied over time range of 0-4.8 s. It is clear that the hardness values rose rapidly in the first 1.2 s of the reaction before slowing down. The hardness of the coating containing 2 wt% ZnO-Ag nanohybrids was higher than that of the neat coating (without the nanohybrids). After 4.8 s of crosslinking reaction, the hardness of the coating and ZnO-Ag hybrid-reinforced coating reached the highest value of around 0.72 and 0.76, respectively. The incorporation of the nanoparticles in the coating matrix increased slightly the coating hardness.
It is well known that the outstanding characteristic of a photoinitiator is susceptible to UV light [2,27]. When being exposed to UV light in FUSION UV equipment, the I 184 photoinitiator is destructed into free radicals which then attack the acrylate group, initiating the crosslinking polymerization. At first (in the first 0.15 s of UV exposure), the content of photoinitiators is high (about 3%) and the curing system is relatively flexible (low viscosity) so the reaction of the acrylate double bonds is inconsiderably impacted by UV absorbers of ZnO-Ag nanohybrids. However, concentration of the photoinitiators and the acrylate groups reduce rapidly by time, and thus, the conversion of the acrylate groups  Adsorption Science & Technology slowed down. In the presence of 2 wt% ZnO-Ag nanohybrids, the conversion of the acrylate groups is affected by photocatalytic characteristic of nano ZnO-Ag hybrid structure. This effect is similar to the case of photocatalyst nanoparticles (A-TiO 2 , ZnO) [34][35][36][37]. The difference is that due to the hybridization with Ag, the photocatalytic activity of ZnO is reinforced. When absorbing UV energy, an electron jumps from the valence band to the conduction band, resulting in the formation of an electron-hole pair. These electron and positive hole pairs reacted with hydroxyl functional groups, water and oxygen molecules attached to the nanoparticles' surface, generating free radicals ( ⋅ OH) [38][39][40] which could also initiate the crosslinking polymerization reaction of acrylate double bonds, leading to the increase of its conversion as well as the increase in the GEL and relative hardness of the film.

Study on Abrasion
Resistance of the Paint Film. Abrasion resistance of the films without (UVAU) and with 2 wt% ZnO-Ag nanohybrids (UVAU/ZnO-Ag) after 4.8 s of crosslinking is presented in Figure 8.

5
Adsorption Science & Technology resistance of the photocurable acrylate urethane/ZnO-Ag nanocoating, its FE-SEM images are analyzed and indicated in Figure 9.
As can be seen from Figure 9, the coating possessed tight structures with no defects or cracks. Additionally, the nanohybrids are well-dispersed into the coating with a size of about 40-250 nm. The abrasion resistance of the nanocoating is enhanced by tough ZnO-Ag nanohybrids which had high hardness. This is also the reason for the increase in the relative hardness of the nanocoating in Figure 7. 3.3. Antimicrobial Activity of the Nanocoating. The growth rate of E. coli bacteria in liquid culture without (control) or with the presence of the coating films not containing (UVAU) or containing ZnO-Ag nanohybrids (UVAU/ZnO-Ag) during 300 min (5 h) is presented in Figure 10.
As shown in Figure 10, after 5 h of test, the growth rate of E. coli bacteria was only~91% in the culture with added UVAU/ZnO-Ag coating (containing 2 wt% ZnO-Ag nanohybrids), whereas it was 98.5% in the culture with the presence of UVAU films (not containing 2 wt% ZnO-Ag nanohybrids). These results has proved that the UVAU/ZnO-Ag nanocomposite film has obviously stronger antibiotic effect against E.

Conclusion
ZnO-Ag nanohybrids were chemically synthesized in the aqueous medium by reducing silver nitrate with sodium borohydride NaBH 4 . Their structural morphology had been characterized by FE-SEM, TEM, and XRD. These nanohybrids (2 wt%) were then homogeneously dispersed into the diacrylate urethane/1,6-hexanediol diacrylate resin system. Photopolymerization, morphology and mechanical properties, and antimicrobial activity of UV curing acrylate urethane coating in the presence of 2 wt% ZnO-Ag nanohybrids were investigated. The analysis results show that ZnO-Ag nanohybrids are dispersed well in polymer matrix, with a size of about 40-250 nm. The nanohybrids enhance significantly both the conversion of acrylate groups, gel fraction, relative hardness, abration resistance, and antimicrobial activity of the coating. After 4.8 s crosslinking reaction, the conversion of acrylate groups, gel fraction, abration resistance, and hardness of the paint film increases from 93.53 to 95.82%, from 95.3 to 96.7%, from 0.72 to 0.76, 6 Adsorption Science & Technology and from 95.28 to 121.54 l/mil, respectively. After 5 h of the antimicrobial test, the growth rate of E. coli bacteria is onlỹ 91% in the culture with the presence of the paint films containing 2 wt% ZnO-Ag nanohybrids, while it is 98.5% in the culture with the presence of the films not containing the nanohybrids.

Data Availability
All the data and supporting materials are included within the article.

Conflicts of Interest
The authors declare no conflicts of interest.