Preparation and Optical Application of Sio2-Tio2 Composite Hardening Coatings with Controllable Refractive Index by Synchronous Polymerization

$Preparation and Optical Application of Sio2-Tio2 Composite Hardening Coatings with Controllable Refractive Index by Synchronous Polymerization$

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Article Preparation and Optical Application of SiO2-TiO2 Composite Hardening Coatings with Controllable Refractive Index by Synchronous Polymerization

Weiping Du 1,†, Shuting Cai 1,†, Yang Zhang 1 and Huifang Chen 1,2,*

1 College of Materials Science and Engineering, Donghua University, Shanghai 201620, China; [email protected] (W.D.); [email protected] (S.C.); [email protected] (Y.Z.) 2 State Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China * Correspondence: [email protected] † These authors contributed equally to this article.

Abstract: The homogeneous SiO2-TiO2 composite sols were prepared by organic-inorganic synchronous polymerization with titanium isopropoxide and tetrabutyl silicate as precursor. The organic-inorganic composite hard coating with Si-O-Ti as the framework was prepared by adding compound crosslinkers (up-401) and 3-Methacryloxypropyltrimethoxysilane (KH-560). The structure of the coating and the hardened film were characterized by infrared spectrum, scanning electron microscopy, atomic force microscopy, particle size analyzer and thermogravimetry. The refractive index, transmittance and hardness of the hardened film were measured by ellipsometry, UV-Vis spectrophotometer and hardness tester. By adjusting the ratio of Si/Ti and optimizing the reaction conditions, the hardness of the hardened film could reach 6H, and the refractive index could be adjusted from 1.55 to 1.76. At the same time, the application of hard coatings on the surface of optical lens were studied. Citation: Du, W.; Cai, S.; Zhang, Y.; Chen, H. Preparation and Optical Keywords: resin lens; high refractive index; hard coating; nano titanium dioxide; sol-gel method

Application of SiO2-TiO2 Composite Hardening Coatings with Controllable Refractive Index by Synchronous Polymerization. 1. Introduction Coatings 2021, 11, 129. https:// Resin materials have been widely used in the ﬁeld of optical lenses. However, the doi.org/10.3390/coatings11020129 defects of resin lenses also seriously affect the service life and optical properties of the lenses [1,2]. Therefore, various complex surface treatment technologies are applied to the re- Academic Editor: Aivaras Kareiva ﬁned processing of optical lenses [3]. Organic-inorganic composite wear-resistant and hard Received: 6 January 2021 coating [4] were widely used for its high hardness, good adhesion, good wear resistance Accepted: 21 January 2021 Published: 25 January 2021 and excellent optical propertie [5]. The sol-gel method is very useful to the development of wear resistant and hard coating on the surface of optical lens [6,7]. Schmidt et al. [8] intro-

Publisher’s Note: MDPI stays neutral duced silane coupling agent into the wear resistant and hardened coating to improve the with regard to jurisdictional claims in adhesion of optical wear resistant coating to the lens, which greatly improved the adhesion published maps and institutional afﬁl- of the coating. Organosilicon has greatly improved the wear resistance and hardness of iations. the coating, and has been widely used in the subsequent preparation of hard coating [9]. T. Iwamoto et al. [10] used titanium isopropoxide, tetraethyl orthosilicate, triethoxysi- lane and phthalate coupling agent as raw materials to prepare the organic-inorganic hard coating of SiO2-TiO2 sol composite [11]. The results showed that when the content of titanium in the inorganic network accounts for 30%, the coating has higher hardness and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. wear resistance. M. Langlet et al. [12,13] prepared a silicon-titanium sol composite hard This article is an open access article coating by controlling the hydrolysis conditions of tetrabutyl titanate, a phthalate coupling ◦ distributed under the terms and agent [14], and a silane coupling agent. The coating can be cured at 110 C to obtain a conditions of the Creative Commons transparent hard coating with hardness of 6H. M. R. Mohammadi et al. [15] prepared a Attribution (CC BY) license (https:// high-hardness organic-inorganic composite coating with a transmission rate of 97.8% and creativecommons.org/licenses/by/ a refractive index of 1.65 by sol-gel method using titanium isopropoxide, tetrabutyl silicate 4.0/). and hydroxypropyl cellulose as raw materials.

Coatings 2021, 11, 129. https://doi.org/10.3390/coatings11020129 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 129 2 of 15

Although much research [16,17] has been done on the hardening coatings, there are still many disadvantages, such as the adhesion of the hard coating to the resin lens substrate is low [18], the refractive index of the coating is not adjustable [19], and the storage stability of the sol system is poor. In this paper, the precursor gel particles were prepared by controlling the hydrolysis conditions using titanium isopropoxide and tetrabutyl silicate as precursors, and then tetraisopropyl bi (dioctylphosphate) titanate (UP-401) and γ-(2,3- glycidoxy) propyltrimethoxysilane (KH-560) were added to prepare an organic-inorganic composite hard coating solution [20]. Compared with the traditional sol-gel method, the synchronous polymerization method used in this paper has the advantage of controlling the hydrolysis rate of two systems with great different hydrolysis rate. By adding a complexing agent, acetylacetone, into titanium isopropoxide and pre-hydrolyzing tetrabutyl silicate, the synchronization polymerization of the two system was achieved [21]. Hydrolyzed to form a stable Si-O-Ti bond cross-linked structure, which overcomes the problems of uncontrollable particle size of nano sol and ineffective cross-linking of nano particles caused by traditional sol-gel direct mixing method, and achieves the purpose of improving coating hardness [22,23]. The addition of phthalate crosslinker, diisopropyldi (dioctyl phosphite oxy) titanate and hydroxypropyl cellulose can improve the coating’s refractive index [24]. The addition of hydroxypropyl cellulose not only improves the coating’s adhesion [25], but also gives the coating excellent impact resistance. During the preparation process, we also explored the effects of different hydrolysis conditions, raw material ratio and curing conditions on the stability and hardness of the sol-gel system, and developed a series of transparent and hard coating solutions with controllable refractive index and excellent comprehensive properties. The application of hardener on the surface of resin lens was also studied in detail [26].

2. Materials and Methods 2.1. Experimental Reagents

The following materials were used as purchased. Titanium isopropanol (Ti (OCH(CH3)2)4, 99.0%) was obtained from Aladdin Reagent Co., Ltd., China. Acetylacetone (C5H8O2) AR, 1- Propanol ((C3H8O), 99.5%), Tetrabutyl silicate (99%), Ethanol ((C2H6O), 99.5%), Aluminum acetylacetonate (C15H21AlO6), 98%), γ- -glycidyloxypropyltrimethoxysilane ((C9H20O5si), 97%) and Isopropyl alcohol (C3H8O) AR were obtained from McLean Reagent Co., Ltd., China. Nitric acid (HNO3) GR was obtained from chemical test of Sinopharm group Agent Co., Ltd., China. Grade I of pyrophosphate titanate coupling agent (up-401) was obtained from Nanjing YOUPU Chemical Co., Ltd., China. Hydroxypropyl cellulose (C36H70O19) was obtained from m.w.100000 McLean Reagent Co., Ltd., China. Sodium hydroxide (NaOH) AR was obtained from Sinopharm Chemical Reagent Co., Ltd., China.

2.2. Preparation of Sol-Gel Composite Hard Coating Solution At room temperature, proper amount of titanium isopropoxide (TTIP), n-propanol, and acetylacetone were mixed for 1 h to form solution A. Tetrabutyl silicate (TBS) was mixed with ethanol for 30 min and equimolar deionized water was dropped. After stirring for 3 h, solution B was formed. Solution A and B were blended and stirred for 2 h. Subsequently, a certain proportion of up-401 and KH-560 were added. Finally, deionized water, ethanol, concentrated nitric acid and ethanol solution of hydroxypropyl cellulose were added drop by drop in a certain proportion, and continuously stirred for 24 h. A series of SiO2-TiO2 composite hard coating were prepared by adjusting the total mass fraction of acid, the amount of water, the solid content of the system and the ratio of TiO2:SiO2.

2.3. Hardening Liquid Curing Process The surfaces of substrates were cleaned by acid washing method. The glass slide or resin lens substrate should be soaked in piranha cleaning solution (30% H2O2 + 70% H2SO4) Coatings 2021, 11, 129 3 of 15 for 30 min to remove the oil stains on the surface. Then the substrates were cleaned ultra- sonically for 30 min and washed with deionized water and anhydrous ethanol alternately for 5 times to remove the acid residues and sundries. Finally, the substrates were dried in analternately oven at 80 for°C .5 After times surface to remove treatment, the acid the residues substrate and was sundries. fixed by a Finally, lifting machin the substratese and thenwere immersed dried in anin oventhe hard at 80 liquor.◦C. After surface treatment, the substrate was fixed by a lifting machineThe surface and then-treated immersed substrate in the was hard fixed liquor. with a puller and immersed in a hardening solution.The After surface-treated 5 min of immersion, substrate the was base fixed material with a of puller the lens and is immersed lifted at a in lifting a hardening speed ofsolution. 1 cm/min After at a uniform 5 min of speed immersion, until al thel the base base material materials of theof the lens lens is liftedfloat out at a of lifting the liquid speed surface.of 1 cm/min The substrate at a uniform coated speed with until hardener all the baseis dried materials at room of the temperature lens float out for of 24 the h, liquidand thensurface. put into The substratethe curing coated oven withfor drying. hardener The is driedheating at roomrate of temperature the curing for furnace 24 h, andis 0.25 then °Cput/min, into the the curing curing time oven is for4 h drying.when the The temperature heating rate is ofraised the curingto 60 °C furnace, and then is 0.25 the ◦curingC/min, timethe is curing 3H when time the is 4 temperature h when the temperature is raised to 120 is raised °C. to 60 ◦C, and then the curing time is 3H when the temperature is raised to 120 ◦C. 2.4. Characterization of Sol-Gel and Hard Coating 2.4. Characterization of Sol-Gel and Hard Coating Using the NEXUS-670 Fourier infrared Raman spectrometer produced by Nicolet company, theUsing structure the NEXUS-670 of sol-gel and Fourier hard coating infrared was Raman tested spectrometer and characterized; produced Nano by Zs Nicolet type nanoparticlecompany, the size structure and potential of sol-gel analyzerand hard produced coating by was Malvern tested company and characterized; of UK was Nanoused to Zs characterizetype nanoparticle the size sizeof sol and particles; potential The analyzer S-4800 field produced emission by scanning Malvern electron company microscope of UK was producedused to characterizeby Hitachi, Tokyo, the size Japan of sol and particles; Agilent The 5500 S-4800 atomic field force emission microscope scanning produced electron by Agilentmicroscope Co., Ltd produced, Santa Clara, by Hitachi, CA, USA Tokyo, were Japan used andto observe Agilent the 5500 surface atomic morphology force microscope of the produced by Agilent Co., Ltd., Santa Clara, CA, USA were used to observe the surface film; The transmittance of the hard coating was measured by lambda950 solid UV spectro- morphology of the film; The transmittance of the hard coating was measured by lambda950 photometer; The refractive index of the coating was measured by m-2000UI ellipsometry solid UV spectrophotometer; The refractive index of the coating was measured by m- produced by J.A. Woollam company, Lincoln, NE, USA; According to GB/T6739-2006, the 2000UI ellipsometry produced by J.A. Woollam company, Lincoln, NE, USA; According hardness of the film was measured; According to GB/T1732-1993, GB/T1733-1993, to GB/T6739-2006, the hardness of the film was measured; According to GB/T1732-1993, GB/T9265-2009 and GB/T9286-1998, the impact resistance, water resistance, alkali resistance, GB/T1733-1993, GB/T9265-2009 and GB/T9286-1998, the impact resistance, water resis- high and low temperature resistance and adhesion performance of hardened liquid applied tance, alkali resistance, high and low temperature resistance and adhesion performance of to the surface of resin lens were tested. hardened liquid applied to the surface of resin lens were tested.

3.3. Results Results 3.1.3.1. The The Effect Effect of ofTi:Si Ti:Si Ratio Ratio on on the the Hardness Hardness of of Si Si-Ti-Ti Composite Composite Coatings Coatings SevenSeven groups groups of of experiments experiments were were designed with Ti:SiTi:Si ratioratio of of 1:0, 1:0, 3:1, 3:1, 2:1, 2:1, 1:1, 1:1, 1:2, 1:2, 1:3 1:3and and 0:1. 0:1. The The solid solid content, content, H2 HO:2O: (silicon (silicon and and titanium) titanium) and and acid acid content content of the of systemthe system were were25%, 25%, 2:1 and 2:1 0.2%,and 0.2%, respectively. respectively. The effectThe effect of Ti:Si of ratioTi:Si onratio the on hardness the hardnes of thes coatingsof the coat- was ingsstudied, was studied, as shown as inshown Figure in1 .Figure 1.

FigureFigure 1. 1.HardnessHardness of ofcoatings coatings with with different different Ti:Si Ti:Si ratio ratio (the (the Ti:Si Ti:Si ratio ratio in insample sample 1– 1–77 is is1:0, 1:0, 3:1, 3:1, 2:1, 2:1, 1:1,1:1, 1:2, 1:2, 1:3, 1:3, 0:1) 0:1)..

It can be seen from the figure that the hardness of the coating increases first and then decreases with the increase of silica content in the coating, and the maximum hardness can reach 7H. The increase of the coating’s hardness is due to two reasons. Firstly, Nano-SiO2 has higher hardness, which was covered by the film former and increases the density of coatings. When the SiO2 content is 3/4, the hardness of the coating reaches the highest Coatings 2021, 11, x FOR PEER REVIEW 4 of 15

It can be seen from the figure that the hardness of the coating increases first and then decreases with the increase of silica content in the coating, and the maximum hardness can reach 7H. The increase of the coating’s hardness is due to two reasons. Firstly, Nano- Coatings 2021, 11, 129 SiO2 has higher hardness, which was covered by the film former and4 of 15increases the density of coatings. When the SiO2 content is 3/4, the hardness of the coating reaches the highest value. Secondly, the increased hardness of the coatings is due to the cross-linked Si-O-Ti value. Secondly, the increased hardness of the coatings is due to the cross-linked Si-O-Ti bond formed by the co hydrolysis of nano-TiO2 and nano-SiO2 in the coating, as shown in bond formed by the co hydrolysis of nano-TiO2 and nano-SiO2 in the coating, as shown in FigureFigure2 .2.

FigureFigure 2.2.Infrared Infrared spectrum spectrum of gel solution.of gel solution.

Figure2 shows the infrared spectra of pure silica sol, pure titanium sol and SiO 2-TiO2 −1 compositeFigure sol. 2 In shows the spectrum the infrared of pure silica spectra sol, the of peak pure at 3307.89 silica cmsol, pureis belong titanium to the sol and SiO2-TiO2 stretching vibration of residual -OH that come from the incomplete hydrolysis condensation −1 compositeof siloxane. In sol. the ﬁgure,In the at spectrum 1075.82 cm− of1 [27 pure] was silica the Si-O sol, stretching the peak vibration at 3307.89 peak in the cm is belong to the stretchingsilica sol, and vibration at 448.70 cm of− 1residualwas the Si-O-Si -OH stretchingthat come vibration from absorption the incomplete peak. In the hydrolysis condensa- tioninfrared of siloxane. spectrum of In titanium the figure, sol, 3238.80, at 1075.82 1530.25 andcm 660.70−1 [27 cm] was−1 [28 the] was Si the-O residualstretching vibration peak -OH stretching vibration peak, the Ti-O bond−1 stretching vibration absorption peak and the inTi-O-Ti the silica stretching sol, vibration and at absorption 448.70 cm peak, respectively.was the Si In-O the-Si IR s spectrumtretching of compositevibration absorption peak. Insol, the a new infrared peak appears spectrum at 935.60 of cm titanium−1, which was sol, the 3238.80 stretching, 1530.25 vibration absorptionand 660.70 peak cm−1 [28] was the re- sidualof Si-O-Ti. -OH The stretching organic-inorganic vibration hard coating peak, solution the Ti- ofO SiO bond2-TiO stretching2 sol was prepared vibration by absorption peak andsol-gel themethod. Ti-O- TheTi completestretching hydrolysis vibration of tetrabutyl absorption silicate needspeak more, respectively. than 24 h, while In the IR spectrum of the hydrolysis of titanium isopropoxide is completed within 1 h. Due to the great difference compositebetween the twosol, hydrolysisa new peak rates, appears the complexing at 935.60 agent acetylacetonecm−1, which was was introduced the stretching in vibration ab- sorptionthe reaction peak process of toSi control-O-Ti. the The hydrolysis organic rate-inorganic of titanium hard isopropoxide coating and solution equimolar of SiO2-TiO2 sol was preparedwater was addedby sol- forgel pre-hydrolysis method. The of complete tetrabutyl silicate. hydrolysis Thus, the of hydrolysistetrabutyl rates silicate of needs more than the two precursor were matched. A large number of Si-O-Ti bonds were formed in the 24coatings, h, while which the made hydrolysis a crossed structure of titanium between isopropoxide nano-TiO2 and nano-SiOis completed2 particles within in 1 hour. Due to thethe great coatings, difference and greatly between improved thethe hardness two hydrolysis of the coating. rates, the complexing agent acetylacetone was3.2. The introduced Effect of Acid iContentn the on reaction the Hardness process of the Coating to control the hydrolysis rate of titanium iso- propoxideThe effect and of acidequimolar content 0.3%, water 0.2% was and added 0.1% on for the hardnesspre-hydrolysis of the coating of tetrabutyl were silicate. Thus, theexplored, hydrolysis respectively, rates as of shown the intwo Figure precursor3. The solid were content matched. was 25%, theA large ratio of number water of Si-O-Ti bonds to silicon alcohol and titanium alcohol was 2:1, and the ratio of titanium to silicon content were formed in the coatings, which made a crossed structure between nano-TiO2 and was 1:2. nano-SiO2 particles in the coatings, and greatly improved the hardness of the coating.

3.2. The Effect of Acid Content on the Hardness of the Coating The effect of acid content 0.3%, 0.2% and 0.1% on the hardness of the coating were explored, respectively, as shown in Figure 3. The solid content was 25%, the ratio of water to silicon alcohol and titanium alcohol was 2:1, and the ratio of titanium to silicon content was 1:2.

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Figure 3. Hardness comparison of different acid content coatings. FigureFigure 3. 3. HardnessHardness comparison comparison of of different different acid acid content content coatings coatings.. It canIt can be beseen seen from from the the figure ﬁgure that that the the hardness hardness of the of the coating coating system system increases increases first ﬁrst It can be seen from the figure that the hardness of the coating system increases first andand then then decreases decreases with with the the decrease decrease of ofsystem system acidity. acidity. This This is isbecause because the the acid acid is is used used as as a and then decreases with the decrease of system acidity. This is because the acid is used as a catalystcatalyst in in the the hy hydrolysisdrolysis of TBS andand TTIP.TTIP The. The larger larger the the concentration concentration of theof the acid, acid, the the faster a catalyst in the hydrolysis of TBS and TTIP. The larger the concentration of the acid, the fasterthe the condensation condensation rate rate of theof the reaction reaction system, system, but but the the hydrolysis hydrolysis reaction reaction which which proceeds pro- faster the condensation rate of the reaction system, but the hydrolysis reaction which pro- ceedssimultaneously simultaneously with with the the increase increase of of the the acid acid concentration concentration slowed slowed down.down. Therefore, in ceeds simultaneously with the increase of the acid concentration slowed down. Therefore, in ththee reaction reaction process, process, it it was was the the hydrolysis hydrolysis reaction reaction that that controlled controlled the the reaction reaction rate rate to to in the reaction process, it was the hydrolysis reaction that controlled the reaction rate to formform a denser a denser gel gelliquid liquid component component with with a lower a lower degree degree of ofcrosslinking. crosslinking. The The hydrolysis hydrolysis form a denser gel liquid component with a lower degree of crosslinking. The hydrolysis andand condensation condensation reactions reactions were were carried carried out out simultaneously simultaneously and and the the two two were were compe competingting and condensation reactions were carried out simultaneously and the two were competing reactionreaction processes. processes. Different Different hydrolysis hydrolysis rates rates allow allow the the production production of nanoparticles of nanoparticles with with reaction processes. Different hydrolysis rates allow the production of nanoparticles with differentdifferent particle particle sizes sizes and and also also different different degrees degrees of of cross cross-linking,-linking, which affect thethe hardnesshard- different particle sizes and also different degrees of cross-linking, which affect the hard- nessof of the the hard hard coating. coating. Particle Particle sizes sizes of of the the hardened hardened liquids liquids prepared prepared at at different different acidity acidity were werenesstested tested of the for for hard comparison comparison coating. analysis, Particle analysis, sizes as shownas ofshown the in hardened Figurein Figure4. liquids 4. prepared at different acidity were tested for comparison analysis, as shown in Figure 4.

Figure 4. Particle size of coating solution with different acid content. Figure 4. Particle size of coating solution with different acid content. FigureThe 4. Particle particle size size of coating of gel particlessolution with increased different with acid the content decrease. of the amount of acid in theThe system. particle When size of the gel acid particles content increased was 0.3%, with the the hydrolysis decrease of rate the of amount the gel of system acid in was themuch systeThem. smaller particle When than thesize acid thatof gel ofcontent particles the condensation was increased 0.3%, the with rate, hydrolysis the and decrease the rate gel particlesof the amountgel condensed system of acid was into in muchthesmaller syste smallerm. nanoparticles. When than thatthe acid of At the thecontent condensation same was time, 0.3%, the rate, pHthe and valuehydrolysis the of gel the particlesrate system of the was condensed gel lower system than into was the smallermuchisoelectric nanoparticles. smaller points than of thatAt the the nano of thesame silica condensation time, and the nano pH titanium rate, value and of dioxide the system gel in particles the was system, lower condensed which than the made into isoelesmalleranctric anti-particle nanoparticles. points of layer the nano around At the silica thesame and gel time,particles. nano the titanium pH The value repulsive dioxide of the forcein system the of system, the was anti-particle lower which than made layers the isoeleresultedctric inpoints small of size the ofnano the silica gel particles, and nano such titanium as 20 dioxide nm for in 0.3% the system, acid content. which Onmade the contrary, as the acidity of the system decreased, the particle size of the system increased

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Coatings 2021, 11, 129 an anti-particle layer around the gel particles. The repulsive force of the anti-particle6 lay- of 15 an anti-particle layer around the gel particles. The repulsive force of the anti-particle layers resulted in small size of the gel particles, such as 20 nm for 0.3% acid content. On the ers resulted in small size of the gel particles, such as 20 nm for 0.3% acid content. On the contrary, as the acidity of the system decreased, the particle size of the system increased contrary, as the acidity of the system decreased, the particle size of the system increased obviously. When the acid content in the system was 0.1%, the particle size of inorganic obviously.obviously. WhenWhen thethe acidacid contentcontent inin thethe systemsystem waswas 0.1%,0.1%, thethe particleparticle sizesize ofof inorganicinorganic nanoparticles was about 200 nm. The particle size of nano particles has great effects on the nanoparticlesnanoparticles was about 20 2000 n nm.m. The The particle particle size size of of nano nano part particlesicles has has great great effects effects on onthe hardness of the coatings. Small particles cannot have a good cross-linking with the cou- thehardness hardness of the of thecoatings. coatings. Small Small particles particles cannot cannot have have a good a good cross cross-linking-linking with with the cou- the pling agent and film-forming components in the system. While, big particles create great couplingpling agent agent and and film ﬁlm-forming-forming components components in the in system. the system. While, While, big particles big particles create create great roughness and porosity of the coatings. Both affect the hardness of the coatings. Com- greatroughness roughness and porosity and porosity of the of coatings. the coatings. Both affect Both affectthe hardness the hardness of the ofcoatings. the coatings. Com- bined with Figure 3, the optimum acid content is 0.2% and the average particle size of the Combinedbined with withFigure Figure 3, the3, optimum the optimum acid acidcontent content is 0.2% is 0.2% and the and average the average particle particle size of size the nano particles is about 100 nm. The sectional SEM photo of hardened coating with acid ofnano the particles nano particles is about is about 100 n 100m. The nm. sectional The sectional SEM SEMphoto photo of hardened of hardened coating coating with with acid content of 0.2% proved that the partials dispersed uniformly in the coating without ag- acidcontent content of 0.2% of 0.2% proved proved that thatthe thepartials partials dispersed dispersed uniformly uniformly in the in thecoating coating without without ag- glomeration and significant phase separation, as shown in Figure 5. agglomerationglomeration and and significant signiﬁcant phase phase separation, separation, as as shown shown in in Figure Figure 5. 5.

Figure 5. Section electron micrograph of 0.2% acid coating. FigureFigure 5.5. SectionSection electronelectron micrographmicrograph ofof 0.2%0.2% acidacid coating.coating. 3.3. Effect of Solid Content on the Performances of Coatings 3.3.3.3. EffectEffect ofof SolidSolid ContentContent onon thethe PerformancesPerformances ofof CoatingsCoatings Solid content is a key factor of coating performance. Hardness of the coatings under SolidSolid contentcontent isis aa keykey factorfactor ofof coatingcoating performance.performance. HardnessHardness ofof thethe coatingscoatings underunder different solid content were researched, as shown in Figure 6. The solid content of the differentdifferent solidsolid contentcontent werewere researched,researched, asas shownshown inin FigureFigure6 .6. The The solid solid content content of of the the system was controlled as 35%, 30%, 25%, 20%, 15%, 10% and 5%, respectively, with the systemsystem waswas controlledcontrolled asas 35%,35%, 30%,30%, 25%,25%, 20%,20%, 15%,15%, 10%10% andand 5%,5%, respectively,respectively, withwith thethe ratio of water to TBS and TTIP 2:1, TTIP: TBS 1:2 and acid content 0.2%. ratioratio ofof waterwater toto TBSTBS andand TTIPTTIP 2:1,2:1, TTIP:TTIP: TBSTBS 1:21:2 andand acidacid contentcontent 0.2%.0.2%.

Figure 6. Hardness diagram of coating with different solid content. Figure 6. Hardness diagram of coating with different solid content. Figure 6. Hardness diagram of coating with different solid content. It can be seen from Figure6 that when the solid content of the coating was 35%, the It can be seen from Figure 6 that when the solid content of the coating was 35%, the hardnessIt can of be the seen coating from reached Figure 6 6H,that and when the the hardness solid content of the coatingof the coating decreased was with35%, thethe hardness of the coating reached 6H, and the hardness of the coating decreased with the decreasehardness ofof thethe solidcoating content reached of the 6H, coating. and the When hardness the solidof the content coating dropped decreased to with 5%, thethe hardness of the coating decreased for 2H. This is because a dense stiffening layer was formed on the lens surface when the solid content was high, which provided effective

protection for the lens and signiﬁcantly increased the hardness of the coatings. While, the CoatingsCoatings 20212021,, 1111,, x 129 FOR PEER REVIEW 7 7of of 15 15

decreaseeffective of film-forming the solid content component of the decreased coating. WhenWhen thethe solid solid content content in dropped the coating to reduced.5%, the hardnessThus, a porous of the structurecoating decreased was formed for and 2H. was This not is enoughbecause toa providedense stiffening effective layer protection was formedfor the lens.on the lens surface when the solid content was high, which provided effective protectionAt the for same the lens time, and the significantly film-forming increased property the and hardness stability of ofthe coating coatings. solution While, with the effectivedifferent film solid-forming content componentwere shown decreased in Table1. When the solid content in the coating reduced. Thus, a porous structure was formed and was not enough to provide effective pro- tectionTable 1. forCoating the lens. liquid storage and film forming performance. At the same time, the film-forming property and stability of coating solution with Solid Content Film Forming Storage Period differentSample solid content were shown in TableGel 1. State (%) Property (d) Table 1. Coating1 liquid storage 35 and film forming Deep performance yellow. edge cracking ≤15 2 30 Yellow edge cracking ≤60 Solid Content Light yellow Film Forming Prop- Storage Period Sample 3 25 Gel State Good ≥120 (%) transparent erty (d) Light yellow 1 435 20 Deep yellow edgeGood cracking ≥≤15120 2 30 Yellowtransparent edge cracking ≤60 Light yellow 3 525 15Light yellow transparent GoodGood ≥≥120 transparent 4 20 Light yellow transparent Good ≥120 Light yellow 6 10 Good ≥120 5 15 Light yellow transparenttransparent Good ≥120 6 10 Light yellow transparentColorless Good ≥120 7 5.0 Good ≥120 7 5.0 Colorless transparenttransparent Good ≥120

It can be seen from the table that when the solid content of the hardening liquid was It can be seen from the table that when the solid content of the hardening liquid increased to 30% or more, the storage period was significantly reduced, and the film-form- was increased to 30% or more, the storage period was signiﬁcantly reduced, and the ﬁlm- ing performance was poor. This is because the gel particles in the coating are easily aggre- forming performance was poor. This is because the gel particles in the coating are easily gated and condensed to form gelation, which induces the cracked thick edge on the coat- aggregated and condensed to form gelation, which induces the cracked thick edge on the ing. Considering the above factors, 25% solid content is optimum. coating. Considering the above factors, 25% solid content is optimum.

3.4.3.4. Influence Inﬂuence of of Crosslinker Crosslinker Content Content on on Coating Coating Properties Properties DueDue to to the the poor poor heat heat-resistant-resistant performance performance of of resin resin lens, lens, higher higher heating heating temperature temperature willwill damage damage the the structure structure of of the the lens lens substrate, substrate, accelerate accelerate the the aging aging of of the the lens lens substrate. substrate. HigherHigher curing curing temperature temperature will will also also damage damage the the surface surface of the of thelens lens with with dura dura layer, layer, re- sultingresulting in yellowing in yellowing aging aging and andother other phenomena. phenomena. In order In orderto improve to improve the hard theness hardness of the coatingof the coating at lower at lowercuring curing temperature, temperature, aluminum aluminum acetylacetonate acetylacetonate was introduced was introduced in the in preparationthe preparation of hardening of hardening solution. solution. The The effects effects of of different different crosslinker crosslinker contents contents (0.1%, (0.1%, 0.5%,0.5%, 1%, 1%, 1.5%, 1.5%, 2%, 2%, 4%) 4%) on on the the structure structure and and properties properties of of the the coating coating w wereere studied. studied. The The infraredinfrared spectrum spectrum was was shown shown in in Figure Figure 77..

FigureFigure 7. 7. InfraredInfrared spectrum spectrum of of coating coating with with different different crosslinking crosslinking agent agent content content..

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The broad peak at 3394.84 cm−1 and the sharp peak at 1529.12 cm−1 are the stretching The broad peak at 3394.84 cm−1 and the sharp peak at 1529.12 cm−1 are the stretching vibration and bending vibration absorption peaks of –OH, respectively. The weaker the vibration and bending vibration absorption peaks of –OH, respectively. The weaker the peak value of –OH in the coating indicates that the more –OH groups are involved in the peak value of –OH in the coating indicates that the more –OH groups are involved in the reaction, the greater the crosslinking degree of the coating. It can be seen from the figure reaction, the greater the crosslinking degree of the coating. It can be seen from the figure that with the increase of crosslinking agent, the peak value of –OH decreases obviously that with the increase of crosslinking agent, the peak value of –OH decreases obviously under the same curing conditions, which indicates that the hardness of coating increases under the same curing conditions, which indicates that the hardness of coating increases with the increase of crosslinking degree. At 2930.99 cm−1, there is an asymmetric stretching with the increase of crosslinking degree. At 2930.99 cm−1, there is an asymmetric stretch- vibration absorption peak of –CH2. At 2873.63 cm−1 –CH3−, 1the absorption peak of symmet- ing vibration absorption peak of –CH2. At 2873.63 cm –CH3, the absorption peak of ricsymmetric stretching stretching vibration vibration indicates indicates that organic that organic film-forming film-forming materials materials have havebeen beensuccess- suc- −1 fullycessfully introduced introduced into intothe hard the hard coating. coating. At 1084.02 At 1084.02 cm cm, there−1, there is a stretching is a stretching vibrati vibrationon ab- −1 sorptionabsorption peak peak of of–Si –Si–O.–O. At At933.39 933.39 cm cm, −there1, there is a is stretching a stretching vibration vibration absorption absorption peak peak of −1 –ofSi –Si–O–Ti.–O–Ti. At At666.39 666.39 cm cm, −there1, there is the is the stretching stretching vibration vibration absorption absorption peak peak of of – –Ti–O–Ti.Ti–O–Ti. The first first two peaks are very obvious, while the latter is weakened. This indicates that the organicorganic-inorganic-inorganic composite composite coating coating with with –Si–O–Ti –Si–O–Ti bond bond as the as theskeleton skeleton structure structure has been has beensuccessfully successfully prepared prepared by controlling by controlling the hydrolysis the hydrolysis conditions conditions and step-by-step and step-by hydrolysis-step hy- drolysismethod. method. It can also It can be seenalso be from seen the from figure the thatfigure when that thewhen amount the amount of cross-linking of cross-linking agent agentaccounts accounts for 1.5% for of1.5% the of total the total amount, amount, the structure the structure of the of hardthe hard coating coating has basicallyhas basically not notchanged. changed. TheThe hardness of of the coating with different conten contentt of crosslinker was measured as shownshown in in Figure Figure 88 below.below. SamplesSamples 1–61–6 werewere coatingscoatings withwith differentdifferent crosslinkingcrosslinking agents,agents, andand sample sample 7 was coatings without crosslinking agents.

FigureFigure 8. 8. HardnessHardness diagram diagram of of coating coating with different crosslinking agent content content..

ItIt can can be be seen from the figure figure that the hardness of the coating without cross-linkingcross-linking agentagent was was 4H. After adding cross cross-linking-linking agent, the hardness of the coating showed an upwardupward trend. FromFrom 4H4H to to 7H, 7H, the the cross-linking cross-linking degree degree of theof the system system increases increases greatly greatly and andthe systemthe system forms forms a closer a closer cross-linking cross-linking structure structure because because of the of promotion the promotion of cross-linking of cross- linkingagent, soagent, that so the that hardness the hardness of the coatingof the coating increases increases greatly. greatly. When When the content the content of cross- of crosslinking-linking agent agent reached reached 2% and 2% 3%,and the3%, hardnessthe hardness of the of coatingthe coating decreased. decreased. This This is dueis due to tothe the excessive excessive introduction introduction of of aluminum aluminum acetylacetonate, acetylacetonate, which which destroys destroys the the film-forming film-form- ingperformance performance of the of the coating, coating, and and the the high high degree degree of cross-linkingof cross-linking makes makes the the film-forming film-form- ingcomponents components aggregate, aggregate, resulting resulting in thein the destruction destruction of theof the dense dense structure structure of theof the coating coat- ingand and the the decrease decrease of the of the hardness. hardness. The weight loss tests for the coatings with different cross-linking agent contents of The weight loss tests for the coatings with different cross-linking agent contents of 0.5%, 1.0%, 1.5% and 2.0% were conducted to compare the change of thermal resistance, as 0.5%, 1.0%, 1.5% and 2.0% were conducted to compare the change of thermal resistance, shown in Figure9. as shown in Figure 9.

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Figure 9. Thermogravimetric diagram of different crosslinker contents.contents.

It can be be seen seen from from the the comparative comparative spectrum spectrum of of thermal thermal weightlessness weightlessness that that the the weight loss was caused by the volatilization and decomposition of the residual small weight loss was caused by the volatilization and decomposition◦ of the residual small mol- moleculeecule solvent solvent in the in thecoating coating in inthe the range range of of0– 0–100100 °CC,, and and the the weight weight loss loss in in the the range of 100–600 ◦C was caused by the decomposition of the film-forming substance in the coating, 100–600 °C was caused by the decomposition of the film-forming◦ substance in the coating, Among them, the thermal weightlessness betweenbetween 100–300100–300 °CC was caused by the thermalthermal decomposition of the organic film-forming substance hydroxypropylcellulose in the coating, decomposition of the organic film-forming substance hydroxypropylcellulose in the coat- and the weight loss in the range of 300–500 ◦C was caused by the thermal decomposition ing, and the weight loss in the range of 300–500 °C was caused by the thermal decompo- of the film-forming substance in the coating. It is caused by the decomposition of organic sition of the film-forming substance in the coating. It is caused by the decomposition of film-forming substances crosslinked with inorganic nanoparticles. With the increase of organic film-forming substances crosslinked with inorganic nanoparticles. With the in- aluminum acetylacetonate content from 0.5% to 2.0%, the residual weight of the coatings at crease of aluminum acetylacetonate content from 0.5% to 2.0%, the residual weight of the 600 ◦C were 55.12%, 56.01%, 56.85% and 52.6%, respectively, and the fastest temperature coatings at 600 °C were 55.12%, 56.01%, 56.85% and 52.6%, respectively, and the fastest value of thermal weight loss rate was also advanced. This is because the addition of temperature value of thermal weight loss rate was also advanced. This is because the ad- aluminum acetylacetonate makes the inorganic nanoparticles in the coating form a new dition of aluminum acetylacetonate makes the inorganic nanoparticles in the coating form and more closely cross-linked structure, which further improves the heat resistance and a new and more closely cross-linked structure, which further improves the heat resistance thermal decomposition temperature of the coating, and further improves the hardness and thermal decomposition temperature of the coating, and further improves the hard- of the coating. However, when the content of aluminum acetylacetonate in the coating isness 2%, of the the heat coating. resistance However, of the when coating the iscontent better of in aluminum the temperature acetylacetonate range of 100–200in the coat-◦C, buting theis 2%, fastest the heat thermal resistance weight of loss the temperature coating is better of the in coating the temperature is 318.5 ◦C, range and the of residual100–200 solid°C , but content the fastest of the thermal coating weight is only loss 52.6% temperature at 600 ◦C. of Compared the coating with is 318.5 the decomposition°C , and the re- temperaturesidual solid content of 330 ◦ofC the when coating the content is only of52.6% aluminum at 600 °C acetylacetonate. Compared with is 0.5%, the decompo- 1%, 1.5%. Comprehensivesition temperature analysis of 330 shows°C when that the the content best addition of aluminum of aluminum acetylacetonate acetylacetonate is 0.5%, was1%, 1.5%. AtComprehensive this time, the heatanalysis resistance shows of that the the coating best wasaddition the best, of aluminum and the hard acetylacetonate coating with highwas 1.5%. hardness At this could time, be the prepared heat resistance at a lower of curingthe coating temperature. was the best, The additionand the hard of too coating much aluminumwith high hardness acetylacetonate could be would prepared destroy at a thelower close curing cross-linking temperature. structure The addition of the coating of too andmuch reduce aluminum the heat acetylacetonate resistance of thewould coating. destroy the close cross-linking structure of the coating and reduce the heat resistance of the coating. 3.5. Optical Properties of Hard Coating Made by Sol-Gel 3.5. OpticalThe best Properties ratio of eachof Hard component Coating Made was selected,by Sol-Gel i.e., 0.2% of acid, 25% of solid and 1.5% of cross-linkingThe best ratio agent. of each By adjusting component the was ratio selected, of silicon i.e. to, titanium0.2% of acid, content 25% to of 1:1, solid 1:2, and 1:3, 1:51.5% and of 0:1,cross the-linking refractive agent. index By adjusting of the coating the ratio could of besilicon adjusted to titanium controllably, content respectively to 1:1, 1:2, numbered1:3, 1:5 and as 0:1, sample the refractive 1, sample index 2, sample of the 3, coating sample could 4 and be sample adjusted 5. The controllably, refractive index respec- of thetively coating numbered was tested as sample by ellipsometer, 1, sample 2, as sample exhibited 3, sample in Figure 4 and 10. sample 5. The refractive index of the coating was tested by ellipsometer, as exhibited in Figure 10.

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Figure 10. Refractive index of coating with different silicon and titanium content (Note: the content ratio of sample 1–5 is 1:1, 1:2, 1:3, 1:5, 0:1). FigureFigure 10. 10. RefractiveRefractive index index of of coating coating with different silicon and titanium content (Note:(Note: the contentcon- tentratio ratio of sample of sample 1–5 is1– 1:1,5 is 1:2,1:1, 1:3,1:2, 1:5,1:3, 0:1).1:5, 0:1). It can be seen from the figure that the refractive index of the coating increased with the increase of inorganic nano titanium dioxide content. The refractive index of the coating ItIt can can be be seen seen from the figure ﬁgure that the refractive index of the coating increased with was 1.57 when the ratio of silicon to titanium was 1:3. With the further increase of titanium thethe increase increase of of inorganic inorganic nano nano titanium titanium dioxide content. The The refractive refractive index of the the coating coating content,was 1.57 the when refractive the ratio index of silicon of the to coating titanium rose was to 1:3. 1.76. With the further increase of titanium was 1.57 when the ratio of silicon to titanium was 1:3. With the further increase of titanium content,The thetransmittance refractive index of the of coating the coating was rosemeasured to 1.76. for the six groups of samples with content, the refractive index of the coating rose to 1.76. differentThe transmittancesolid content. ofThe the tr coatingansmittance was measured diagram forof the coating six groups was of shown samples in with Figure differ- 11 The transmittance of the coating was measured for the six groups of samples with below.ent solid content. The transmittance diagram of the coating was shown in Figure 11 below. different solid content. The transmittance diagram of the coating was shown in Figure 11 below.

Figure 11. Transmittance of different Si Ti ratio coatings (Note: the solid content of samples 1–6 is Figure 11. Transmittance of different Si Ti ratio coatings (Note: the solid content of samples 1–6 is 30%, 25%, 20%, 15%, 10% and 5% 5%,, respectively) respectively).. Figure 11. Transmittance of different Si Ti ratio coatings (Note: the solid content of samples 1–6 is 30%, 25%, 20%, 15%, 10% and 5%, respectively). ItIt can can be be seen seen from from Figure Figure 1111 thatthat the the l lightight transmittance transmittance of of the the coating coating with with different different Si/Ti ratio ratio was moremore thanthan 90% 90% in in the the visible visible light light area, area, illustrating illustrating that that the inorganicthe inorganic organic or- It can be seen from Figure 11 that the light transmittance of the coating with different ganiccomposite composite coating coating prepared prepared by synchronous by synchronous polymerization polymerization has better has light better transmittance. light trans- Si/Ti ratio was more than 90% in the visible light area, illustrating that the inorganic or- mittance. ganic3.6. Microstructure composite coating of SiO 2prepared/TiO2 Composite by synchronous Sol Hard Coating polymerization has better light transmittance.The microstructure of the hard coating with different Si/Ti ratio was characterized. 3.6. Microstructure of SiO2/TiO2 Composite Sol Hard Coating The surface morphology and RMS roughness of the coating were measured by AFM, as The microstructure of the hard coating with different Si/Ti ratio was characterized. 3.6.shown Microstructure in Figure 12 o.f SiO2/TiO2 Composite Sol Hard Coating The surface morphology and RMS roughness of the coating were measured by AFM, as The microstructure of the hard coating with different Si/Ti ratio was characterized. shown in Figure 12. The surface morphology and RMS roughness of the coating were measured by AFM, as shown in Figure 12.

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(A)

(B)

(C)

(D)

Figure 12. Cont.

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(E)

Figure 12. AFM of different Si Ti ratio(E coatings) ((A–E) in turn correspond to the SiO2-TiO2 content ratio of 1:1, 1:2, 1:3, 1:5, 0:1). FigureFigure 12. 12.AFMAFM of different of different Si Ti Si ratio Ti ratio coatings coatings ((A–E (() Ain– turnE) in correspond turn correspond to the SiO to the2-TiO SiO2 content2-TiO2 content ratioratio of 1:1, of 1:1, 1:2, 1:2, 1:3, 1:3,1:5, 1:5,0:1). 0:1). According to the analysis of the atomic force photos of the coating, the flatness of the coatingAccordingAccording surface to the to was theanalysis analysis excellent. of the of atomic the The atomic forcesurface force photos photosroughness of the of coating, the of coating, thethe flatness coating the ﬂatness of thechanged of the at the range coating surface was excellent. The surface roughness of the coating changed at the range ofcoating 3.27– surface4.55 n wasm with excellent. the The shifty surface content roughness of of nano the coating-TiO2 changedin the film at the-forming range of material, as of 3.27–4.55 nm with the shifty content of nano-TiO2 in the film-forming material, as shown3.27–4.55 in nmFigure with the13. shifty content of nano-TiO2 in the ﬁlm-forming material, as shown shownin Figure in Figure 13. 13.

Figure 13. Roughness of coatings with different titanium content.

The roughness of the coating decreased with the increase of nano titanium dioxide content. The structure of Si-O-Ti was formed in the coating after the co hydrolysis of tetrabutylFigure 13. silicateRoughness and titanium of coatings isopropoxide, with different which titanium made content. the nano-TiO2 and nano-SiO2 Figure 13. Roughness of coatings with different titanium content. of the coating form a close cross-linking structure. The organic materials in the coating well coatedThe roughness inorganic nanoparticles of the coating with decreased the addition with the of tetrabutyl increase of silicate nano titaniumdecreased, dioxide makingcontent.The the coating Theroughness structure flatter . of of the Si-O-Ti coating was formeddecreased in the with coating the after increase the co of hydrolysis nano titanium of dioxide content.tetrabutyl The silicate structure and titanium of Si isopropoxide,-O-Ti was which formed made in the the nano-TiO coating2 and after nano-SiO the 2 coof hydrolysis of 3.7.the The coating Application form of a SiO close2/TiO cross-linking2 Composite Sol structure. Hard Coating The organicon the Surface materials of Resin in theLens coating well tetrabutyl silicate and titanium isopropoxide, which made the nano-TiO2 and nano-SiO2 coatedThe application inorganic nanoparticlesof Si Ti composite with hard the liquor addition on the of tetrabutylsurface of resin silicate lens decreased, was studied making byothef adjusting the coating coating the ﬂatter. ratio form of TBS a andclose TTIP cross to 1:1,-linking 1:2, 1:3, 1:5structure. and 0:1, respectively, The organic and select-materials in the coating ingwell the best coated ratio inorganicof other conditions. nanoparticles The impact withresistance the of addition resin lens samples of tetrabutyl after film silicate decreased, 3.7. The Application of SiO /TiO Composite Sol Hard Coating on the Surface of Resin Lens curingmaking was thetested, coating as shown flatter in2 Figure. 2 14. The application of Si Ti composite hard liquor on the surface of resin lens was studied by adjusting the ratio of TBS and TTIP to 1:1, 1:2, 1:3, 1:5 and 0:1, respectively, and selecting 3.7.the The best Application ratio of other of conditions. SiO2/TiO The2 Composite impact resistance Sol Hard of Coating resin lens on samples the Surface after ﬁlmof Resin Lens curingThe was application tested, as shown of Si inTi Figure composite 14. hard liquor on the surface of resin lens was studied by adjusting the ratio of TBS and TTIP to 1:1, 1:2, 1:3, 1:5 and 0:1, respectively, and selecting the best ratio of other conditions. The impact resistance of resin lens samples after film curing was tested, as shown in Figure 14.

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FigureFigureFigure 14. 14.14. Impact ImpactImpact resistance resistanceresistance of ofof hardened hardened hardened coating coating (Note: (Note: AA, A,, BB, B,, CC, C,, DD, D,, E E are are sample sample 1:1,1:1, 1:1, 1:2,1:2, 1:2, 1:3,1:3, 1:3, 1:5,1:5, 1:5, 0:1, 0:1in0:1, sequence). ,in in sequence) sequence). .

ItItIt can cancan be bebe seen seenseen from fromfrom the thethe figure figurefigure that thatthat when whenwhen the thethe ratio ratioratio of ofof silicon siliconsilicon to toto titanium titaniumtitanium was waswas 1:1, 1:1,1:1, 1:2, 1:2,1:2, 1:31:31:3 and andand 1:5, 1:5,1:5, the thethe impact impactimpact resistance resistanceresistance of ofof the thethe coating coatingcoating was waswas good, good,good, and andand the thethe sample sample with withwith the thethe ratio ratio ofofof 0:1 0:10:1 appears appearsappears film filmfilm fragmen fragmentation.fragmentation.tation. This ThisThis is isis because becausebecause the thethe content contentcontent of ofof titanium titaniumtitanium dioxide dioxidedioxide in inin the the filmfilm-formingfilm-forming-forming system systemsystem is isis too tootoo high, high,high, and andand the the film film-formingfilm-forming-forming performance performanceperformance of ofof Ti Ti-O-TiTi-O-O-Ti-Ti cross cross-linkedcross-linked-linked structurestructurestructure is isis worse worseworse than thanthan that thatthat of ofof Ti Ti-O-SiTi-O-O-Si-Si and andand the thethe film filmfilm has hashas low lowlow flexibility flexibilityflexibility and andand high highhigh brittleness brittlenessbrittleness afterafterafter cur curing,curing,ing, so soso it itit is isis broken. broken.broken. TheTheThe high highhigh and andand low lowlow temperature temperaturetemperature test testtest results resultsresults of ofof the thethe coating coatingcoating were werewere shown shownshown in inin Figure FigureFigure 15.1515. . WhenWhenWhen the the ratio ratio ofof of siliconsilicon silicon toto to titaniumtitanium titanium is is is 1:1, 1:1, 1:1, the the the molecular molecular molecular chain chain chain was was was frozen frozen frozen under under under the the impactthe im- im- pactofpact low of of low temperature, low temperature, temperature, the internalthe the internal internal stress stress stress increased, increased, increased, and and theand coatingthe the coating coating was was was broken broken broken due due todue the to to thecross-linkingthe cross cross-linking-linking between between between nanoparticles nanoparticles nanoparticles in thein in the film-formingthe film film-forming-forming component. component. component. When When When the the the ratio ratio ratio of ofsiliconof silicon silicon to to titaniumto titanium titanium is is 1:2,is 1:2 1:2 1:3,, ,1:3 1:3 1:5,, ,1: 1:5,5, 0:1, 0:1, 0:1, the the the cross-linkingcross cross-linking-linking structurestructure structure ofof of Ti-O-SiTi Ti-O-O-Si-Si in in the the coating coating reducedreducedreduced the the internal internal internal stress stress stress of of ofthe the the coating coating coating and and and the the theperformance performance performance of of the the of coating coating the coating was was still still was good good still undergoodunder underthe the impact impact the impact of of high high of and and high low low and temperature temperature low temperature with with the the with decrease decrease the decrease of of the the ofamount amount the amount of of tetra- tetra- of ethoxysilanetetraethoxysilaneethoxysilane in in the the in coating. coating. the coating.

FigureFigureFigure 15. 15.15. High HighHigh and andand low lowlow temperature temperaturetemperature test test diagram diagram of of coating coating (Note: (Note: the the order order of of lens lens place place placementmentment is is thethethe same samesame as asas Figure FigureFigure 1414 14))..) .

TheTheThe water water resistance, resistance, alkali alkali alkali resistance resistance resistance and and and adhesion adhesion adhesion of of ofthe the the coating coating coating were were were studied studied studied by by selectingbyselecting selecting three three three groups groups groups of of Si Si- ofTi-Ti with Si-Tiwith excellent withexcellent excellent impact impact impactresistance resistance resistance and and high high and and and high low low andtemper- temper- low aturetemperatureature performance. performance. performance. The The results results The showed showed results that that showed the the coating coating that the has has coating good good performance performance has good performance and and no no bad bad phenomenaandphenomena no bad such phenomenasuch as as cracking, cracking, such peeling, peeling, as cracking, blistering, blistering, peeling, etc. etc. It blistering, It can can meet meet etc.the the use Ituse can requirements requirements meet the use of of hardrequirementshard coating coating on ofon the hardthe le le coatingnsns surface. surface. on theThe The lens test test surface.results results were Thewere testexhibited exhibited results in werein Table Table exhibited 2. 2. in Table2.

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Table 2. Water resistance, alkali resistance and adhesion performance of coating.

Si Ti ratio Water Resistance Test Alkali Resistance Test Adhesion Grade No blistering, wrinkling, No blistering, pulverization, 1:2 0 cracking and falling off softening and falling off No blistering, wrinkling, No blistering, pulverization, 1:3 0 cracking and falling off softening and falling off No blistering, wrinkling, No blistering, pulverization, 1:5 0 cracking and falling off softening and falling off

4. Conclusions The hydrolysis rate of Ti sol and Si sol was regulated by the method of organic- inorganic synchronous polymerization, then the complexing agent acetylacetone and deionized water were introduced, respectively. The cross-linked and well mixed Si-O-Ti bonded sol was obtained. Finally, the Si-Ti organic-inorganic hybrid hardening liquid was prepared. By adjusting the ratio of silica sol and titanium dioxide sol, the refractive index of coating liquid can be adjusted in the range of 1.56–1.76, the hardness can reach 6 h, and the transmittance can be more than 90%. When the silicon content in the coating is high, the molecular chain of the coating will freeze under the impact of low temperature, the internal stress will increase, and the coating will easily break; when the Ti content is high, the impact resistance of the coating will be reduced. The inorganic organic transparent coating prepared by this method has a bright future in the ﬁne processing of resin lens surface.

Author Contributions: Conceptualization, H.C.; investigation, Y.Z.; writing—original draft preparation, S.C.; writing—review and editing, W.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key Research and Development Program of China, grant number 2016YFB0302300. The APC was funded by 2016YFB0302300. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The study did not report any data. Acknowledgments: This work was financially supported by the National Key Research and Devel- opment Program of China (2016YFB0302300). Conflicts of Interest: The authors declare no conflict of interest.

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