1 Introduction

UV-curing nanocomposites^{[1, 2]} are a new type of materials that incorporated inorganic nanoparticles into traditional UV-curing materials.Due to the fast speed of UV-curing, conventional UV-curing materials^[3-5] had the defects of large shrinkage stress, poor wear resistance and small hardness after curing.This restricted its application in some areas.In comparison, UV-curing nanocomposites can not only maintain the advantages of traditional UV-curing materials, but also effectively improve these defects, so it has great practical value.

Nanoparticles had large specific surface areas and high surface energy.This, plus the presence of large quantities of hydroxyl radicals at their surfaces, made silica nanoparticles thermodynamically instable and likely to agglomerate^{[6, 7]}. This can lead to the dispersion of nano-SiO₂ in the polymer and polymer interface bonding was not good, prone to phase separation.Therefore, in order to apply it to the polymer matrix, it was necessary to modify the surface of the nano-SiO₂ to improve the lipophilicity of the surface. At present, the surface modification methods of nano-SiO₂ were mainly:alcohol, acid, surfactant, graft polymerization, emulsion polymerization, silane coupling agent and so on^[8-10]. Because of the simple process and low production cost of the silane coupling agent graft modification method, it was more common in industrial application.

In this study, nano-SiO₂ was surface-treated with cetyltrimethylammonium bromide and modified by silane coupling agent KH-570 to prepare modified nano-SiO₂.Then the modified and unmodified nano-SiO₂ particles were doped into the synthesized prepolymer PGEA to prepare UV-curing nanocomposites.Then the scanning electron microscopy (SEM) was used to observe the distribution of nanoparticles in the system. Meanwhile, the surface morphology of the cured films was observed by atomic force microscopy (AFM). Finally, the mechanical properties of the composite after curing were studied.

2 Experimental 2.1 Materials

Phenyl glycidyl ether (PGE) was purchased by Shenzhen Tengda Sheng Trading Co., Ltd. Acrylic acid (AA) was purchased by Sinopharm Chemical Reagent Co., Ltd. Acrylic acid, triphenylphosphine and p-methoxyphenol were purchased by Sinopharm Chemical Reagent Co., Ltd.γ-Methacryloxypropyltrimethoxysilane (KH-570) was purchased by Nanjing Silicone Chemical Co., Ltd. Cetyltrimethylammonium bromide was purchased by Tianjin Damao Chemical Reagent Factory. 1-hydroxycyclohexyl phenyl ketone (Irgacure 184) was purchased by Changsha Tianyu Chemical Co., Ltd. Nanometer SiO₂ was purchased by Shanghai Jing Chun Reagent Co., Ltd.

2.2 Instrementation

External light curing machine (INTELLI-RAY 400) was purchased by the Shenzhen Hui Shuo Electrical Company. Differential scanning calorimetry - thermogravimetric analyzer (SDT Q600) was purchased by the United States TA Company. The scanning electron microscope (S-3000N) was purchased by Hitachi, Japan.Atomic force microscope (FM-Nainoview 6800) was purchased by the Shanghai Biotechnology Company.

2.3 Synthesis of Phenyl Glycidyl Ether Acrylate (PEGA)

PEGA was synthesized in our laboratory^[10].The main reaction equation for the preparation of PGEA is shown in Scheme 1.

The specific synthesis method was as follows: Took appropriate amount of phenyl glycidyl ether, catalyst triphenylphosphine, and polymerization inhibitor p-hydroxy anisole, added the three into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser and a dropping funnel, and mixed thoroughly.Then the oil bath were heated to 8095 ℃, used the dropping funnel in accordance with the rate of dropwise addition every two seconds to drop acrylic acid, and the temperature of the reaction system must be controlled below 95 ℃.After dropping, the reaction system was maintained at the temperature range for 10 minutes.Then heated to 100115 ℃ gradually and stirred for 45 hours to complete the reaction.The reaction was completed until the acid value of the product was less than 5 mg KOH/g.

2.4 Surface Modification of Nano-SiO₂ 2.4.1 Modified Principle

Hydrolyzed KH-570 was polycondensed with the hydroxyl groups on the surface of the nano-SiO₂ molecules, next the organic matrix was bridged with nano-SiO₂, then the modification was completed.Mechanism of KH-570 modified nano-SiO₂ is shown in Scheme 2^[11-13].

2.4.2 Modified Experimental Method

(1) The nano-SiO₂, an appropriate amount of Cetyltrimethylammonium bromide(CTMAB) in ethanol was added into a round-bottomed flask, refluxed and stired at 80 ℃.Four hours later, the supernatant was removed by centrifugation, added absolute ethanol to wash, placed in a vacuum oven, dried at 100 ℃ for 12 h.

(2) The dried nano-SiO₂ and 100 mL of ethanol were placed into a clean beaker, added a certain amount of KH-570 ethanol solution(2%, 4%, 6%, 8%, 10%, 12%, 14% of the silica mass fraction, respectively).Then transferred to an ultrasonic cell disrupter and sonicated for 30 minutes.

(3) The above solution was transferred to a three-necked flask.After reacting at a certain temperature for a period of time, cooled, centrifuged, washed and dried in a vacuum oven at 50 ℃ for 24 h.Thus, the product of KH-570 modified nano-SiO₂ was prepared and labeled as Nano-SiO₂-KH570, and unmodified nano-SiO₂ labeled as nano-SiO₂.

2.4.3 The Determination of Lipophilic Degree

Lipophilic degree (LD) is the value of lipophilicity of the modified nano-SiO₂, and its size can be used as a standard to judge the modification effect^{[14, 15]}. The larger the LD value, the better the modification.

Weighed 1.0 g experimental modified nano-SiO₂, placed in 50 mL of pure water, and methanol was added thereto until the powder completely infiltrated, the volume of methanol was recorded V₁ (mL). LD of Nano-SiO₂-KH570 can be calculated in the follwing form.

2.4.4 Preparation of Nano-SiO₂ Composite Materials

According to the formula, weighed appropriate amount of synthetic prepolymer PGEA, adding 4% UV184.Then added the modified and unmodified nano-SiO₂ with mass fraction of 0%, 1%, 2%, 3%, 4%, 5% and 6% respectively, and stired well.Next, the mixture was sonicated for 10 minutes with an ultrasonic oscillator to make the nano-SiO₂ uniformly dispersed.In this way, UV-curing nanocomposites were obtained.

3 Results and Discussion 3.1 The effect of Reaction Temperature on the Lipophilic Degree of Nano-SiO₂-KH570

Figure 1 showed the effect of reaction temperature on the lipophilicity of Nano-SiO₂-KH570. As can be seen from the figure, the LD value increased first and then decreased with the increase of the reaction temperature, and the optimum reaction temperature was 80 ℃.This was because, when the reaction temperature was too low, the energy required for the reaction can not be provided. With the increase of temperature, the contact probability between KH-570 and nano-SiO₂ increased, and the LD value increased gradually.However, when the temperature was too high, KH-570 will be decomposed on the one hand, on the other hand, the collision between the nano-SiO₂ itself will increase and agglomeration will occur, leading to the decrease of LD value.

3.2 The Effect of Reaction Time on the Lipophilic Degree of Nano-SiO₂-KH570

Figure 2 showed the effect of reaction time on the LD of Nano-SiO₂-KH570. As can be seen from the figure, with the increase of reaction time, the LD value first increased and then decreased, and the optimum reaction time was 4 hours.This was because, with the passage of time, KH-570 continuously reacted with the hydroxyl groups on the surface of the nano-SiO₂, the LD value gradually increased and reached the maximum value at 4 hours after the reaction.Continued to extend the reaction time, the reaction will not continue because of the steric hindrance effect of KH-570 attached to the surface of the reacted nano-SiO₂.In addition, due to the intermolecular physical adsorption, the coupling agent gradually formed surface micelles^{[16, 17]}, and the LD value decreased instead.

3.3 Scanning Electron Microscope Analysis

Weighed appropriate amount of formulated UV-curing nanocomposite coatings, wherein the modified nano-SiO₂ content were 3%, 5%, respectively, unmodified nano-SiO₂ content were 3%, 5%.The four groups of composite coatings were cured first, then removed the cured film, observed by scanning electron microscopy. The scan results were shown in Figure 3.

As can be seen from the four SEM pictures (Figure 3): when the same content of modified and unmodified nano-SiO₂ were added to the UV-curing material, it was found that the surface of the modified nano-SiO₂ coatings was more smooth than the unmodified. This was because the modified nano-SiO₂ can be more evenly dispersed in the resin matrix, while the unmodified nano-SiO₂ was prone to agglomeration.When the addition amount was 3%, The modified nano-SiO₂ was dispersed uniformly in the resin matrix with little agglomeration, while the unmodified nano-SiO₂ was more agglomerated in the resin matrix.When the addition amount was 5%, the modified nano-SiO₂ partially re-agglomerated in the resin matrix, the particle size had become the micron level, and unmodified nano-SiO₂ agglomeration in the resin matrix had been very serious, so the surface of the film was very rough.This showed that the more nano-SiO₂ content was not better.Excessive amount of nano-SiO₂ would degrade the properties of the UV-curable material and thus lose the properties of the nano-SiO₂.

3.4 Atomic Force Microscopy Analysis

Weighed appropriate amount of formulated UV-curing nanocomposite coating, wherein the modified nano-SiO₂ and unmodified nano-SiO₂ content of 4%, the two groups of nanocomposite coating were cured first, then removed the cured film, observed by atomic force microscope(AFM).The result was shown in Figure 4 and Figure 5.

Figure 4 and Figure 5 were the AFM topography image and phase image of the modified and unmodified nano-SiO₂ UV-curable materials with a content of 4%.Comparing the topography image (a) of the two, it was found that the surface of the cured film added with modified nano-SiO₂ was more smoother, and the average surface roughness of the cured film was 54.1 nm.The surface of the cured film added with unmodified nano-SiO₂ appears to be relatively rough, with an average surface roughness of 124.3 nm.Comparing the phase image (b) of the two, it can be seen that the nano-SiO₂ particles dispersed in the PEGA.Nano-SiO₂ particles in Figure 4 was dispersed uniform relatively, while the Nano-SiO₂ particles in Figure 5 was more concentrated, indicating that the dispersion of the modified nano-SiO₂ was better.

3.5 Tensile Strength of the UV-curing Nanocom-posites

Weighed appropriate amount of formulated UV-curing nanocomposite coating, cured, the curing samples were taken for tensile test after curing. The results were shown in Figure 6 and Figure 7.

As can be seen from Figure 6 and Figure 7:when unmodified nano-SiO₂ was added to the UV-curing material, it was found by testing that the higher the content thereof, the lower the tensile strength and elongation at break of the cured material.When the modified nano-SiO₂ was added, the test showed that with the increase of the content of nano-particles, the tensile strength and the elongation at break showed the trend of increasing first and then decreasing.The experimental results showed that the tensile strength and elongation at break of the cured material reached the maximum values of 10.08 MPa and 95.38% when the content of modified nano-SiO₂ was 4%.The reason was that the nano-SiO₂ modified by KH-570 had better dispersibility in UV-curing materials. When UV-curing was carried out, the modified nano-SiO₂ can form a network three-dimensional structure interwoven with UV-organisms.Tensile stress after curing will be rapidly transmitted to more molecular chains through the network cross-linking points, so that the micro-cracks on the surface of the material were not easily diffused, it can show better tensile properties.However, when added excessive modified nano-SiO₂, the probability of nano-particle agglomeration would increase greatly.When stretched after curing, stress concentration would occur, and the tensile properties of the cured material will deteriorate.

3.6 Flexibility of the UV-curing Nanocomposites

Weighed appropriate amount of formulated UV-curing nanocomposite coating, cured, the curing samples were taken for flexibility test after curing in accordance with GB/T 1731-1993. The results were shown in Table 1.

In Table 1, when the modified and unmodified nano-SiO₂ added to UV-curing materials, the flexibility of the cured film will be reduced. However, when the mass fraction of modified nano-SiO₂ was 1%4%, bending tests on the cured film of the UV-curing material revealed that all cured films could passed through the shaft 7, indicating that all have good flexibility.The reason was that during the curing process of the UV-curing material, the modified nano-SiO₂ not only played a role of filling but also can participate in the curing reaction and formed a chemical bond with the UV-organism, firmly connected with the UV-curing material.So it will not have too much impact on the flexibility of the UV-curing material.But the unmodified nano-SiO₂ will have serious agglomeration, resulting in poor flexibility of UV-curing materials.

3.7 Pencil Hardness of the UV-curing Nanocom-posites

Weighed appropriate amount of formulated UV-curing nanocomposite coating, cured, the curing samples were taken for hardness test after curing in accordance with GB/T 6739-1996. The result was shown in Table 2.

As can be seen from Table 2, both of the cured film pencil hardness had increased when the modified and unmodified nano-SiO₂ were added to UV-curing material. However, when the modified and unmodified nano-SiO₂ were added in the same amount, the UV-curing material containing the modified nano-SiO₂ had a higher pencil hardness. The reason was that the modified nano-SiO₂ can interact with the UV-organism to enhance the hardened material, thereby having higher pencil hardness.

4 Conclusion

The steps of the experiment were:silane coupling agent KH-570 was used to modify the surface of inorganic nano-SiO₂ at first; then, it was filled into the synthesized photosensitive prepolymer PGEA.The SEM observation showed that the modified nano-SiO₂ dispersed well in the UV-curing system.Observing by atomic force microscopy, it was found that the surface of the film after curing was smooth and uniform.And adding the appropriate amount of modified nano-SiO₂ can effectively enhance the hardness, flexibility, impact strength and tensile mechanical properties of the UV-curing material.In comprehensive consideration, the addition of modified nano-SiO₂ was best at four percent.