Tetraethyl orthosilicate (TEOS) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. 3-methacryloxypropyltrimethoxyl silane (MPS) was received from Sinopharm Chemical Reagent Co., Ltd. The commercial hexa-functional aliphatic urethane-acrylate (UA) oligomer named as “ Ebecryl 1290” and trimethylolpropane tris-acrylate (TMPTA) serving as reactive diluent were supplied by Cytec Industries Inc. The photo-initiator 1-hydroxy-cyclohexyl phenyl ketone (Irgacure 184) was provided by Ciba Specialty Chemicals. A commercially available nano-SiO₂ named as Aerosil® R 7200 with a primary particle size distribution in the range of 13-50 nm was kindly supplied by Evonik Degussa GmbH. All the materials in this study were used as received without further purification.

2.2.1. In-situ modification of FSP-SiO₂

The in-situ modification of SiO₂ was carried out in a modified flame FSP reactor. An illustration of the experimental reactor is shown in Fig. 1. FSP burner was enclosed by a 45-cm long tube with 5-cm inner diameter, above which a 5-cm long tube with 12 internal section outlets (1 mm i.d. each) was placed, through which flowed cooling air (0.5 m³/h). Another 15-cm long tube with a side branch was placed above the cooling tube. An internal gas-assist nozzle (i.d. = 1 mm, 60 cm above the FSP burner) was put into that side tube, spraying 3 mL/min of MPS solution dispersed by 0.25 L/min N₂ at 1.5 × 10⁵ Pa (1.5 bar) pressure drop (45° from the vertical) into the tube. Finally a 30-cm long tube for surface modification of silica was presented. The final products were collected by filter cloth.

Fig. 1.

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	Fig. 1. Illustration of experimental reactor for in-situ surface modification of FSP-made SiO2

The TEOS solution in ethanol (0.5 mol/L) was fed at 3 mL/min into the FSP burner and dispersed into drops by O₂ (0.3 m³/h) with 1.0 × 10⁵ Pa (1.0 bar) pressure drop at the nozzle tip. The MPS solution in ethanol/water (9:1 by volume, 0.04 mol/L, pH = 4.5) was pre-activated at 60 ° C for 1 h to ensure complete hydrolyzation of MPS. Hydrophobic SiO₂ was prepared by spraying MPS solution (3 mL/min) directly into the freshly formed hydrophilic SiO₂ particles.

2.2.2. Ex-situ modification of FSP-silica

For comparison, ex-situ modified SiO₂ was synthesized by means of wet chemistry route. The pure SiO₂ was prepared by direct spraying TEOS solution into fame reactor, under the same condition as g-SiO₂ without spraying MPS solution. The ex-situ surface modification was performed as follows: 2.0 g of as-prepared pure SiO₂ was dispersed into 200 mL solution of ethanol and water (94.5:5.5 by volume) via sonication for 30 min. Acetic acid was added to adjust the pH value of the solution to 4.5. 0.2 g MPS (10 wt% of pure SiO₂) was gradually added to the dispersed solution, and the solution was stirred at 65 ° C for 24 h. Finally, the suspension was centrifuged and washed with ethanol for 3 times and dried.

2.2.3. Coating formulation and film preparation

Three types of SiO₂ were introduced in the nanocomposite coatings: the hydrophobic SiO₂ in-situ FSP-modified by MPS (marked as g-SiO₂), the SiO₂ ex-situ treated in ethanol/water by MPS (marked as l-SiO₂), and the hydrophobic pyrogenic SiO₂ named as Aerosil® R7200 (marked as r-SiO₂) for comparison.

Proper amount of SiO₂ nanopowder was added into TMPTA, followed by stirring and sonication for 1.5 h with ultrasonic cell disruptor. The resultant clear blends were used as the masterbatch in the following step. Because of residual-OH groups on the surfaces of SiO₂, the viscosity of the masterbatch increases dramatically with the content of SiO₂, causing great difficulty in later operation. Maximum filler content existed for these masterbatches: 25 wt% for g-SiO₂ and 20 wt% for l-SiO₂. Further increase of nanosilica content was impractical for processing. Masterbatch of r-SiO₂ with 25 wt% particle content was also prepared for comparison.

The SiO₂ masterbatch was added with proper amount of UA and TMPTA to obtain dispersions with varying SiO₂ contents. The compositions of all the mixtures produced are outlined in Table 1 The amount of the photoinitiator remained constant and equal to 4 wt% of the mixture. After addition of the photoinitiator, the dispersions were intensively stirred and degassed under reduced pressure and then spin-coated onto PC substrates (1.5 mm in thickness) at a rotation speed of 900 rpm for 6 seconds and 1800 rpm for 15 seconds. All PC plates were washed with ethanol before use. The wet films were subsequently UV-cured for 180 seconds using a hot embossing system equipped with UV unit, with the UV light intensity of about 1.5 mW/cm² at a wavelength of 365 nm. The thickness of the coatings is in the range of 40-50 µ m. For simplicity, “ 15g-SiO₂” represents the coating with 15 wt% g-SiO_2.

Table 1.

Table 1.

Table 1. Compositions of coating samples with various nano-silica contents Sample Masterbatch (wt%) UAd (wt%) TMPTAd (wt%) Irgacure 184 (wt%) Neat coating — 33.3 66.7 4 5 wt% g-SiO2 33.3 g-25a 30.0 36.7 4 10 wt% g-SiO2 50.0 g-25 28.3 21.7 4 15 wt% g-SiO2 66.7 g-25 26.7 6.6 4 5 wt% l-SiO2 33.3 l-20b 30.0 36.7 4 10 wt% l-SiO2 50.0 l-20 28.3 21.7 4 5 wt% r-SiO2 33.3 r-25c 30.0 36.7 4 10 wt% r-SiO2 50.0 r-25 28.3 21.7 4 15 wt% r-SiO2 66.7 r-25 26.7 6.6 4 a g-25 is the masterbatch containing 25 wt% g-SiO2 and 75 wt% TMPTA. b l-20 is the masterbatch containing 20 wt% l-SiO2 and 80 wt% TMPTA. c r-25 is the masterbatch containing 25 wt% r-SiO2 and 75 wt% TMPTA. d In final coating formulations, the weight ratio of UA/TMPTA maintained 1/2.

Table 1. Compositions of coating samples with various nano-silica contents

Thermogravimetric analysis (TGA) was taken by TGA/SDTA851e analyzer from room temperature to 800 ° C with a heating rate of 10 ° C/min under nitrogen. Transmission electron microscopy (TEM) images were directed by a JEM-1400 with in-column filter at 120 kV of acceleration voltage type transmission electron micrograph. The specific surface area (SSA) of silica was determined using a surface area and porosity analyzer (ASAP 2010N) by Brunauer-Emmett-Teller (BET).

The nano-indentation test was conducted on a TriboIndenter system (standard diamond Berkovich tip, triangular pyramid shape and curvature radius of 200 nm). A constant maximum penetration load of 10 mN was chosen for all samples. The applied load was controlled according to a pre-determined loading function, which consists of a 20-s linear loading to 10mN, followed by a 5-s dwelling, and finally a 20-s linear unloading. The displacement of indenter was monitored during the loading and unloading process.

The nano-scratch test was also carried out; a conical diamond tip of 60° cone angle was used. A pre-scan was performed on a 15 µ m × 15 µ m square area to get the initial topography of the sample. The load applied on the tip increased linearly to 0.8 mN in 5 s, maintained for 3 s, and then moved the tip by 10 µ m in 30 s, maintained for 3 s, and finally decreased the normal load to 0 mN in 5 s. The displacement of the indenter was constantly monitored during the test. At last, the post-scan was performed on the worn sample surface through the image processing process.

The pencil hardness of coatings on the PC plate was tested according to the standard GB/T 6739-1996. The visible light transmittance of the coatings was measured by a UV-vis-NIR scanning spectrophotometer (CARY 500) over a range of 300-800 nm. The worn surfaces of the nano-coatings were examined by optical microscopy. The haze values of the coatings were determined using a haze meter (WGT-S, Shanghai Precision and Scientific Instrument) according to GB/T 2410-2008.

TGA curves of g-SiO₂ and l-SiO₂ are shown in Fig. 2(a). The obvious weight loss in the range of 300-600 ° C was attributed to thermal decomposition of organic MPS molecules. By comparison of the curves, higher grafting content of 8.6 wt% was presented for g-SiO₂ than l-SiO₂ of 2.8 wt%. Full hydrolyzation of MPS in ethanol/water before spraying and high grafting temperature helped to obtain higher grafting content of g-SiO₂. The g-SiO₂ exhibited strong hydrophobic property than l-SiO₂ (the inset in Fig. 2 (a)). The TEM pictures of SiO₂ particles are shown in Fig. 2 (b). The g-SiO₂ particles dispersed well in acetone and exhibited typical network structure. While for l-SiO₂, the network structure was destroyed, l-SiO₂ particles dispersed in small aggregations. Direct surface modification of freshly formed silica particles in in-situ process could avoid further aggregation to a certain extent, which gives this process the advantage of obtaining fine dispersion of silica particles in organic solvent. The SSA of pure flame-made SiO₂ was 192 m²/g and reduced to 111 m²/g after in-situ modification due to the present of MPS on the surface.

Fig. 2.

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	Fig. 2. Characterization of SiO2 particles: (a) TG results of g-SiO2 and l-SiO2 particles (the inset: the hydrophobic property test); (b) TEM pictures of g-SiO2, l-SiO2 and pure SiO2 in acetone.

3.2.1. Visible light transmittance

Transparency of the films presents a key parameter for the use of the prepared coatings. The transparency of coatings with comparable thickness in the range from 30 to 40 µ m was characterized by UV-vis spectrophotometry (Fig. 3(a)). All films loaded with nano-silica were highly transparent. The light transmittance decayed obviously with increasing content of l-SiO₂, while in the case of g-SiO₂, the addition of g-SiO₂ hardly reduced the visible light transmittance of coatings even with 15 wt% silica, which is different from the behavior of traditional flame-made SiO₂ reported in the literature^[16]. In fact, g-SiO₂ and l-SiO₂ were obtained from the same silica (FSP-made SiO₂) and modifier (MPS), and therefore, it can be assumed that the variations in optical transmittance arise from their difference in modification process rather than other factors. Fig. 4 shows the TEM images of SiO₂ nanocomposite coatings. It can be easily observed that the dispersion state of g-SiO₂ was notably different from l-SiO₂. l-SiO₂ particles aggregated severely with the aggregation size of even 200-400 nm in coat matrix, whereas g-SiO₂ dispersed homogeneously on nanoscale without large aggregations. Large size of aggregations of nano-fillers in coatings causes significant light-scattering, declining the coating optical transparency. In-situ FSP modified SiO₂ exhibited advantage in obtaining homogeneous filler dispersion in coatings over ex-situ treated one, which plays a great role in optical transparency.

Fig. 3.

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	Fig. 3. Optical qualities of coatings: (a) optical transmittance of coatings; (b) haze values (the inset: the digital photos of neat coating (left), 10 wt% g-SiO2 coating and 10 wt% l-SiO2 coating (right)). In a typical example for simplicity, “ 15g-SiO2” represents the coating with 15 wt% g-SiO2.

Fig. 4.

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	Fig. 4. TEM images and schematic illustrations of dispersion state of silica in composite coatings embedded with 10 wt% g-SiO2 and 10 wt% l-SiO2.

3.2.2. Haze values

The haze values can be used to evaluate the clarity of transparent coatings. As shown in Fig. 3(b), g-SiO₂ loaded coating gave the lowest haze values among all films, and the haze values tended to be insensitive to the content of SiO₂, which is also different from the observation reported elsewhere^[16]. For l-SiO₂ and r-SiO₂ samples, the haze value increased significantly with SiO₂ content. With the SiO₂ content of 15 wt%, the haze value was 1.85% for g-SiO₂ nanocomposite coating, while it increased to 5.72% for r-SiO₂. The haze value of l-SiO₂ coating with only 10 wt% filler was 2.78%, higher than g-SiO₂ coating with 15 wt% silica. The advantage of in-situ FSP modification in the haze of nano-coatings can be attributed to the same reason described in optical transmittance measurement. Higher grafting content of MPS on SiO₂ for g-SiO₂ facilitates the dispersion of g-SiO₂ and stronger interfacial interaction^{[20] and [21]}, thus providing lower haze value of final coatings.

3.3.1. Nano-indentation measurement

Nano-indentation test is an effective technique in characterizing surface mechanical properties of thin films or coatings^[22]. The typical loading-unloading curves of neat coating and coatings filled with 10 wt% g-SiO₂ or 10 wt% l-SiO₂ are shown in Fig. 5(a). The surface hardness (H) and elastic modulus (E) of the coatings obtained from the Oliver and Pharr methodology are given in Fig. 5(b)^[23].

Fig. 5.

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	Fig. 5. Loading-unloading curves of coatings (a) and corresponding surface mechanical properties (b) in nano-indention test.

As presented in Fig. 5(a), the maximum indentation depth of neat coating is about 1167 nm under the maximum load of 10 mN, and it decreased to around 1069 nm and 1080 nm for coatings with 10 wt% g-SiO₂ and 10 wt% l-SiO₂, respectively, indicating that the incorporation of inorganic SiO₂ particles enhanced the coating resistance to the penetration by the indenter, and g-SiO₂ composite coating exhibited slightly higher penetration resistance than l-SiO₂ filled one. In comparison to the maximum indentation depth above 1000 nm, all three coatings showed much lower residual depth, around 300 nm. The addition of nano-SiO₂ hardly decreased the recovery ability of the coating matrix. Actually the percentage of elastic recover (Re) of g-SiO₂ coating increased slightly in comparison to neat coating and l-SiO₂ composite coating. As shown in Fig. 5(b), both coating hardness and modulus could be enhanced with the incorporation of nano-SiO₂. At the filler content of 10 wt%, the surface hardness rose from 418 MPa for neat coating to 494 and 475 MPa for g-SiO₂ and l-SiO₂ filled coatings, respectively, and elastic modulus from 5.02 GPa to 5.91 and 5.95 GPa; SiO₂ particles made the coating much stiffer.

Scheme illustration of indentation test is presented in Fig. 6(a). It is recognized that the mechanical properties of nano-fillers and the interface combination play a great role in the final composite performance. The hardness and modulus of nano-SiO₂ are much higher than those of the coating matrix, thus enhancing the hardness and modulus of composite coatings. Moreover, modified SiO₂ could act as a load-transferring agency, which could effectively enhance the maximum standing load applied on the coating, thereby increasing the ability against indentation. It could also be seen in Fig. 5 that the g-SiO₂ composite coating exhibited higher indentation hardness than l-SiO₂ composite coating at the filler content of 10 wt%. As previously proven, the MPS grafting content of g-SiO₂ is much higher than that of l-SiO₂, indicating that more MPS molecules could connect silica with the coating matrix at the interfaces after photo curved. When being indented, a better load transference effect could occur in g-SiO₂ embedded coating, offering higher indentation hardness.

Fig. 6.

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	Fig. 6. Illustration of indentation (a) and scratch (b) test on neat coating and silica-loaded coating.

3.3.2. Nano-scratch measurement

The scratch resistance of composite coatings can be estimated by friction coefficient in nano-scratch test (Fig. 7). It can be seen that the average friction coefficient of neat coating and coatings with 10 wt% g-SiO₂ and 10 wt% l-SiO₂ are 0.74, 0.63 and 0.66, respectively. Here the average friction coefficient was taken from the stable stage of the friction coefficient curves. About 15% decrease in average friction coefficient was achieved for coatings with g-SiO₂, and 11% decrease for coatings with l-SiO₂.

Fig. 7.

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	Fig. 7. Friction coefficient curves of the coatings in nano-scratch test.

The worn surfaces of coating samples are presented in Fig. 8. The residual depths were much lower than penetration depths (not shown), which can be attributed to the deformation recovery of the coatings. The g-SiO₂ hybrid coating gives the lowest residual depth, indicating the best scratch resistance on nanoscale. The SiO₂ filler enhanced the scratch resistance of the coatings, and g-SiO₂ offered a slightly better enhancement than l-SiO₂. Besides, the surface morphologies of the films after SiO₂ incorporation were also changed, as observed in Fig. 8. The neat film is almost smooth, and the incorporation of SiO₂ causes the surface roughness of the films to increase. The bright spot in the images of SiO₂ embedded coatings resulted from the aggregation of SiO₂ in the vicinity of coat surface. The coating filled with l-SiO₂ was rougher than g-SiO₂ composite coating, which could be explained from the severe aggregation of l-SiO₂ in the coating revealed in Fig. 4.

Fig. 8.

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	Fig. 8. Worn surfaces of the coating samples before and after nano-scratch test: (a) neat coating, (b) with 10 wt% g-SiO2 nanoparticles, and (c) with 10 wt% l-SiO2 nanoparticles.

The illustration of nano-scratch test is presented in Fig. 6(b). As concluded in nano-indentation test, the addition of silica particles make the coatings harder, which accordingly means lower indentation depth and smaller contact area, so the real contact area decreases; besides, as shown in Fig. 8, the roughness of the coating increases after adding silica particles, the hump resulting from the existence of silica could prevent the scratch tip from contacting with more flatter coating surface. As a result, the coatings filled with silica particles present lower friction coefficient. The g-SiO₂ composite coating gives higher indentation hardness in comparison with l-SiO₂ coating, which means smaller contact area of coating surface and indenter; as a result, g-SiO₂ composite coating shows lower friction coefficient.

3.3.3. Pencil hardness

The scratch hardness of the cured coatings was also measured by pencil hardness test; the results are reported in Fig. 9(a). For better comparison, r-SiO₂ was also presented. The pencil hardness was HB for neat coating, and it greatly enhanced to 2H for coating with 15 wt% g-SiO₂ and 15 wt% r-SiO₂. Because of dramatically increased coating viscosity for l-SiO₂, the maximum content of l-SiO₂ was 10 wt%, with the highest pencil hardness of H. The pencil hardness showed significant improvement with increasing nano-SiO₂ content for all three types of coatings. The improvement of scratch hardness can be attributed to the increase of coating modulus and load-transferring effect due to the presence of the hard modified silica nanoparticles^[20]. Besides, the strong interconnection of modified SiO₂ with coating matrix might prevent the cracks from propagating when being scratched, which helped to improve the scratch resistance. For these three kinds of silica particles, at the same particle content, the nanocomposite coatings exhibited the same value of pencil hardness; different surface modifications of nano-silica did not affect the pencil hardness values greatly.

Fig. 9.

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	Fig. 9. Pencil hardness of the coating samples as a function of the nano-silica content (a) and worn surfaces of coatings after pencil hardness test (b).

The worn surfaces after pencil hardness test are shown in Fig. 9(b). The neat coating showed severe worn surfaces with numerous grooves after H grade pencil scratched. Comparatively, the nanocomposite films loaded with 10 wt% g-SiO₂ or 10 wt% l-SiO₂ presented smooth surface without grooves, indicating improved scratch hardness. When the content of silica increased to 15 wt% for g-SiO₂, the nanocomposite coating can stand 2H grade pencil without grooves.