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

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.

1. Introduction

Resin materials have been widely used in the field of optical lenses. However, the 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 refined processing of optical lenses [3]. Organic-inorganic composite wear-resistant and hard coating [4] were widely used for its high hardness, good adhesion, good wear resistance 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] introduced silane coupling agent into the wear resistant and hardened coating to improve the adhesion of optical wear resistant coating to the lens, which greatly improved the adhesion of the coating. Organosilicon has greatly improved the wear resistance and hardness of 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, triethoxysilane 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 wear resistance. M. Langlet et al. [12,13] prepared a silicon-titanium sol composite hard coating by controlling the hydrolysis conditions of tetrabutyl titanate, a phthalate coupling agent [14], and a silane coupling agent. The coating can be cured at 110 °C to obtain a transparent hard coating with hardness of 6H. M. R. Mohammadi et al. [15] prepared a high-hardness organic-inorganic composite coating with a transmission rate of 97.8% and a refractive index of 1.65 by sol-gel method using titanium isopropoxide, tetrabutyl silicate and hydroxypropyl cellulose as raw materials.

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) for 30 min to remove the oil stains on the surface. Then the substrates were cleaned ultrasonically 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 an oven at 80 °C. After surface treatment, the substrate was fixed by a lifting machine and then immersed in the hard liquor.

The surface-treated substrate was fixed with a puller and immersed in a hardening solution. After 5 min of immersion, the base material of the lens is lifted at a lifting speed of 1 cm/min at a uniform speed until all the base materials of the lens float out of the liquid surface. The substrate coated with hardener is dried at room temperature for 24 h, and then put into the curing oven for drying. The heating rate of the curing furnace is 0.25 °C/min, the curing time is 4 h when the temperature is raised 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

Using the NEXUS-670 Fourier infrared Raman spectrometer produced by Nicolet company, the structure of sol-gel and hard coating was tested and characterized; Nano Zs type nanoparticle size and potential analyzer produced by Malvern company of UK was used to characterize the size of sol particles; The S-4800 field emission scanning electron microscope produced by Hitachi, Tokyo, Japan and Agilent 5500 atomic force microscope produced by Agilent Co., Ltd., Santa Clara, CA, USA were used to observe the surface morphology of the film; The transmittance of the hard coating was measured by lambda950 solid UV spectrophotometer; The refractive index of the coating was measured by m-2000UI ellipsometry produced by J.A. Woollam company, Lincoln, NE, USA; According to GB/T6739-2006, the hardness of the film was measured; According to GB/T1732-1993, GB/T1733-1993, GB/T9265-2009 and GB/T9286-1998, the impact resistance, water resistance, alkali resistance, high and low temperature resistance and adhesion performance of hardened liquid applied to the surface of resin lens were tested.

3. Results

3.1. The Effect of Ti:Si Ratio on the Hardness of Si-Ti Composite Coatings

Seven groups of experiments were designed with Ti:Si ratio of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3 and 0:1. The solid content, H₂O: (silicon and titanium) and acid content of the system were 25%, 2:1 and 0.2%, respectively. The effect of Ti:Si ratio on the hardness of the coatings was studied, as shown in Figure 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 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 Figure 2.

Figure 2 shows the infrared spectra of pure silica sol, pure titanium sol and SiO₂-TiO₂ composite sol. In the spectrum of pure silica sol, the peak at 3307.89 cm⁻¹ is belong to the stretching vibration of residual -OH that come from the incomplete hydrolysis condensation of siloxane. In the figure, at 1075.82 cm⁻¹ [27] was the Si-O stretching vibration peak in the silica sol, and at 448.70 cm⁻¹ was the Si-O-Si stretching vibration absorption peak. In the infrared spectrum of titanium sol, 3238.80, 1530.25 and 660.70 cm⁻¹ [28] was the residual -OH stretching vibration peak, the Ti-O bond stretching vibration absorption peak and the Ti-O-Ti stretching vibration absorption peak, respectively. In the IR spectrum of composite sol, a new peak appears at 935.60 cm⁻¹, which was the stretching vibration absorption peak of Si-O-Ti. The organic-inorganic hard coating solution of SiO₂-TiO₂ sol was prepared by sol-gel method. The complete hydrolysis of tetrabutyl silicate needs more than 24 h, while the hydrolysis of titanium isopropoxide is completed within 1 h. Due to the great difference between the two hydrolysis rates, the complexing agent acetylacetone was introduced in the reaction process to control the hydrolysis rate of titanium isopropoxide and equimolar water was added for pre-hydrolysis of tetrabutyl silicate. Thus, the hydrolysis rates of the two precursor were matched. A large number of Si-O-Ti bonds were formed in the coatings, which made a crossed structure between nano-TiO₂ and 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.

It can be seen from the figure that the hardness of the coating system increases first and then decreases with the decrease of system acidity. This is because the acid is used as a catalyst in the hydrolysis of TBS and TTIP. The larger the concentration of the acid, the faster the condensation rate of the reaction system, but the hydrolysis reaction which proceeds simultaneously with the increase of the acid concentration slowed down. Therefore, in the reaction process, it was the hydrolysis reaction that controlled the reaction rate to form a denser gel liquid component with a lower degree of crosslinking. The hydrolysis and condensation reactions were carried out simultaneously and the two were competing reaction processes. Different hydrolysis rates allow the production of nanoparticles with different particle sizes and also different degrees of cross-linking, which affect the hardness of the hard coating. Particle sizes of the hardened liquids prepared at different acidity were tested for comparison analysis, as shown in Figure 4.

The particle size of gel particles increased with the decrease of the amount of acid in the system. When the acid content was 0.3%, the hydrolysis rate of the gel system was much smaller than that of the condensation rate, and the gel particles condensed into smaller nanoparticles. At the same time, the pH value of the system was lower than the isoelectric points of the nano silica and nano titanium dioxide in the system, which made 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 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 nanoparticles was about 200 nm. The particle size of nano particles has great effects on the hardness of the coatings. Small particles cannot have a good cross-linking with the coupling agent and film-forming components in the system. While, big particles create great roughness and porosity of the coatings. Both affect the hardness of the coatings. Combined with Figure 3, the optimum acid content is 0.2% and the average particle size of the nano particles is about 100 nm. The sectional SEM photo of hardened coating with acid content of 0.2% proved that the partials dispersed uniformly in the coating without agglomeration and significant phase separation, as shown in Figure 5.

3.3. Effect of Solid Content on the Performances of Coatings

Solid content is a key factor of coating performance. Hardness of the coatings under different solid content were researched, as shown in Figure 6. The solid content of the system was controlled as 35%, 30%, 25%, 20%, 15%, 10% and 5%, respectively, with the ratio of water to TBS and TTIP 2:1, TTIP: TBS 1:2 and acid content 0.2%.

It can be seen from Figure 6 that when the solid content of the coating was 35%, the hardness of the coating reached 6H, and the hardness of the coating decreased with the decrease of the solid content of the coating. When the solid content dropped to 5%, the 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 effective film-forming component decreased When the solid content in the coating reduced. Thus, a porous structure was formed and was not enough to provide effective protection for the lens.

At the same time, the film-forming property and stability of coating solution with different solid content were shown in

Table 1. Coating liquid storage and film forming performance.

Sample	Solid Content (%)	Gel State	Film Forming Property	Storage Period (d)
1	35	Deep yellow	edge cracking	≤15
2	30	Yellow	edge cracking	≤60
3	25	Light yellow transparent	Good	≥120
4	20	Light yellow transparent	Good	≥120
5	15	Light yellow transparent	Good	≥120
6	10	Light yellow transparent	Good	≥120
7	5.0	Colorless transparent	Good	≥120

.

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

3.4. Influence of Crosslinker Content on Coating Properties

Due to the poor heat-resistant performance of resin lens, higher heating temperature will damage the structure of the lens substrate, accelerate the aging of the lens substrate. Higher curing temperature will also damage the surface of the lens with dura layer, resulting in yellowing aging and other phenomena. In order to improve the hardness of the coating at lower curing temperature, aluminum acetylacetonate was introduced in the preparation of hardening solution. The effects of different crosslinker contents (0.1%, 0.5%, 1%, 1.5%, 2%, 4%) on the structure and properties of the coating were studied. The infrared spectrum was shown in Figure 7.

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 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 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 with the increase of crosslinking degree. At 2930.99 cm⁻¹, there is an asymmetric stretching vibration absorption peak of –CH₂. At 2873.63 cm⁻¹ –CH₃, the absorption peak of symmetric stretching vibration indicates that organic film-forming materials have been successfully introduced into the hard coating. At 1084.02 cm⁻¹, there is a stretching vibration absorption peak of –Si–O. At 933.39 cm⁻¹, there is a stretching vibration absorption peak of –Si–O–Ti. At 666.39 cm⁻¹, there is the stretching vibration absorption peak of –Ti–O–Ti. The first two peaks are very obvious, while the latter is weakened. This indicates that the organic-inorganic composite coating with –Si–O–Ti bond as the skeleton structure has been successfully prepared by controlling the hydrolysis conditions and step-by-step hydrolysis method. It can also be seen from the figure that when the amount of cross-linking agent accounts for 1.5% of the total amount, the structure of the hard coating has basically not changed.

The hardness of the coating with different content of crosslinker was measured as shown in Figure 8 below. Samples 1–6 were coatings with different crosslinking agents, and sample 7 was coatings without crosslinking agents.

It can be seen from the figure that the hardness of the coating without cross-linking agent was 4H. After adding cross-linking agent, the hardness of the coating showed an upward trend. From 4H to 7H, the cross-linking degree of the system increases greatly and the system forms a closer cross-linking structure because of the promotion of cross-linking agent, so that the hardness of the coating increases greatly. When the content of cross-linking agent reached 2% and 3%, the hardness of the coating decreased. This is due to the excessive introduction of aluminum acetylacetonate, which destroys the film-forming performance of the coating, and the high degree of cross-linking makes the film-forming components aggregate, resulting in the destruction of the dense structure of the coating and the decrease of the hardness.

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 shown in Figure 9.

It can be seen from the comparative spectrum of thermal weightlessness that the weight loss was caused by the volatilization and decomposition of the residual small molecule solvent in the coating in the range of 0–100 °C, and the weight loss in the range of 100–600 °C was caused by the decomposition of the film-forming substance in the coating, Among them, the thermal weightlessness between 100–300 °C was caused by the thermal decomposition of the organic film-forming substance hydroxypropylcellulose in the coating, and the weight loss in the range of 300–500 °C was caused by the thermal decomposition of the film-forming substance in the coating. It is caused by the decomposition of organic film-forming substances crosslinked with inorganic nanoparticles. With the increase of aluminum acetylacetonate content from 0.5% to 2.0%, the residual weight of the coatings at 600 °C were 55.12%, 56.01%, 56.85% and 52.6%, respectively, and the fastest temperature value of thermal weight loss rate was also advanced. This is because the addition of aluminum acetylacetonate makes the inorganic nanoparticles in the coating form a new and more closely cross-linked structure, which further improves the heat resistance and thermal decomposition temperature of the coating, and further improves the hardness of the coating. However, when the content of aluminum acetylacetonate in the coating is 2%, the heat resistance of the coating is better in the temperature range of 100–200 °C, but the fastest thermal weight loss temperature of the coating is 318.5 °C, and the residual solid content of the coating is only 52.6% at 600 °C. Compared with the decomposition temperature of 330 °C when the content of aluminum acetylacetonate is 0.5%, 1%, 1.5%. Comprehensive analysis shows that the best addition of aluminum acetylacetonate was 1.5%. At this time, the heat resistance of the coating was the best, and the hard coating with high hardness could be prepared at a lower curing temperature. The addition of too much aluminum acetylacetonate would 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

The best ratio of each component was selected, i.e., 0.2% of acid, 25% of solid and 1.5% of cross-linking agent. By adjusting the ratio of silicon to titanium content to 1:1, 1:2, 1:3, 1:5 and 0:1, the refractive index of the coating could be adjusted controllably, respectively numbered as sample 1, sample 2, sample 3, sample 4 and sample 5. The refractive index of the coating was tested by ellipsometer, as exhibited in Figure 10.

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 was 1.57 when the ratio of silicon to titanium was 1:3. With the further increase of titanium content, the refractive index of the coating rose to 1.76.

The transmittance of the coating was measured for the six groups of samples with different solid content. The transmittance diagram of the coating was shown in Figure 11 below.

It can be seen from Figure 11 that the light transmittance of the coating with different Si/Ti ratio was more than 90% in the visible light area, illustrating that the inorganic organic composite coating prepared by synchronous polymerization has better light transmittance.

3.6. Microstructure of SiO₂/TiO₂ Composite Sol Hard Coating

The microstructure of the hard coating with different Si/Ti ratio was characterized. The surface morphology and RMS roughness of the coating were measured by AFM, as shown in Figure 12.

According to the analysis of the atomic force photos of the coating, the flatness of the coating surface was excellent. The surface roughness of the coating changed at the range of 3.27–4.55 nm with the shifty content of nano-TiO₂ in the film-forming material, as shown in Figure 13.

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 tetrabutyl silicate and titanium isopropoxide, which made the nano-TiO₂ and nano-SiO₂ of the coating form a close cross-linking structure. The organic materials in the coating well coated inorganic nanoparticles with the addition of tetrabutyl silicate decreased, making the coating flatter.

3.7. The Application of SiO₂/TiO₂ Composite Sol Hard Coating on the Surface of Resin Lens

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 the best ratio of other conditions. The impact resistance of resin lens samples after film curing was tested, as shown in Figure 14.

It can be seen from the figure that when the ratio of silicon to titanium was 1:1, 1:2, 1:3 and 1:5, the impact resistance of the coating was good, and the sample with the ratio of 0:1 appears film fragmentation. This is because the content of titanium dioxide in the film-forming system is too high, and the film-forming performance of Ti-O-Ti cross-linked structure is worse than that of Ti-O-Si and the film has low flexibility and high brittleness after curing, so it is broken.

The high and low temperature test results of the coating were shown in Figure 15. When the ratio of silicon to titanium is 1:1, the molecular chain was frozen under the impact of low temperature, the internal stress increased, and the coating was broken due to the cross-linking between nanoparticles in the film-forming component. When the ratio of silicon to titanium is 1:2, 1:3, 1:5, 0:1, the cross-linking structure of Ti-O-Si in the coating reduced the internal stress of the coating and the performance of the coating was still good under the impact of high and low temperature with the decrease of the amount of tetraethoxysilane in the coating.

The water resistance, alkali resistance and adhesion of the coating were studied by selecting three groups of Si-Ti with excellent impact resistance and high and low temperature performance. The results showed that the coating has good performance and no bad phenomena such as cracking, peeling, blistering, etc. It can meet the use requirements of hard coating on the lens surface. The test results were exhibited in

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

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

.

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.