Melamine resin / nanosilica composite microspheres, a preparation method thereof, and a light diffuser and an optical material

Abstract

The application discloses melamine resin / nano-silica composite microspheres, a preparation method thereof, and a light diffuser and optical material, and relates to the technical field of organic-inorganic composite materials. The method uses branched nano-silica and large-particle-size spherical nano-silica as a balling aid, and utilizes melamine and formaldehyde to polymerize in an aqueous phase to prepare the composite microspheres, without using a dispersant or an alcohol organic solvent. The obtained microspheres have a rough surface morphology similar to a litchi shell, and exhibit excellent high-haze and high-transmittance performance when used as the light diffuser. The preparation method is simple, low in cost, suitable for industrial production, and can be widely used in the fields of LED lighting and liquid crystal display.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of organic-inorganic composite materials, and particularly relates to a melamine resin / nano-silica composite microsphere, a preparation method thereof, and a light diffuser and an optical material. BACKGROUND

[0002] Melamine resin is a thermosetting resin synthesized by condensation of melamine and formaldehyde in the presence of a catalyst. It has the advantages of heat resistance, chemical corrosion resistance, high mechanical strength, and high hardness, and is widely used in the tableware, kitchenware, wood processing, electrical, and mechanical industries. In recent years, melamine resin microspheres have attracted attention due to their potential application value, and the synthesis of microspheres with controllable particle size and simple process has become a research hotspot. The existing literature reports two main methods for preparing melamine resin microspheres. The first method uses a dispersing agent, such as polyvinyl alcohol (PVA). Liu Yajun et al. prepared melamine resin microspheres (MF microspheres) with a low crosslinking degree and a particle size of 12 μm by using PVA as a dispersing agent and adjusting the pH value of the reaction system, reaction time, reaction temperature, and dispersant dosage. Hui Liu et al. also prepared MF microspheres with a particle size of 1.2 μm by using PVA as a dispersing agent. Although this method can make the microsphere size uniform, the dispersing agent is difficult to completely remove, which affects the performance of the microspheres, and the reaction concentration is relatively low, only 3%-5%. The second method uses an alcohol-water mixed solution as a dispersion medium. For example, Yang Yixiang prepared MF microspheres with a particle size of 0.5-5.2 μm by using an alcohol-water mixed solution as a medium and adjusting the alcohol-water ratio, catalyst dosage, reaction time, and temperature. Lin Chaolan et al. prepared MF microspheres with a particle size of 39 μm by using a methanol-water solution as a medium and changing the alcohol-water ratio, monomer molar ratio, formic acid dosage, and reaction temperature. This method requires the use of alcohol organic solvents, which increases the production cost, and the reaction concentration is still relatively low, about 4%-5%. Therefore, it is of great significance to develop a preparation method without dispersing agents and alcohol solvents and with a relatively high reaction concentration.

[0003] In the application of melamine resin microspheres, there are several patent literatures reporting related technologies. Chinese patent (CN106140039A) forms a precursor by polymerization of melamine, formaldehyde and urea, and then removes the solvent to obtain hollow melamine resin microspheres with a particle size of 90-140 μm, which are suitable for adsorbent materials and catalyst carriers. Chinese patent (CN109675507A) uses F127 ethanol dispersion and glacial acetic acid to prepare μm-level melamine resin spheres, which are applied to new energy electronic materials. Chinese patent (CN112642411A) prepares porous melamine resin microspheres containing amidoxime groups and amino groups by a water-in-air template method, which are used for metal ion adsorption. Chinese patent (CN104356594A) dopes hydroxyl silicone oil in the synthesis to prepare silicone / melamine resin composite particles, which are applied to drug carriers and other fields. The microspheres prepared by the above methods are all spherical structures with smooth surfaces, and there is no report on melamine resin microspheres with rough surfaces and similar lychee shell morphology.

[0004] Light diffuser is an important additive in optical materials, which is often used in transparent resins such as polycarbonate and polymethyl methacrylate (PMMA). By enhancing light scattering and refraction, the light diffuser can improve the haze and diffuse point or linear light source into surface light source, making the light soft and comfortable, reducing the sense of glare and glare, and being widely used in LED lighting lamps and liquid crystal display screens. An ideal light diffuser needs to achieve high haze while maintaining high light transmittance. Existing light diffusers are divided into inorganic and organic types. Inorganic light diffusers such as calcium carbonate and silica powder have low cost, but the light transmittance is significantly reduced because the light cannot penetrate, and the performance is poor due to easy agglomeration. Organic light diffusers such as PMMA microspheres and silicone resin microspheres can improve the haze by interface refraction while maintaining high light transmittance, but the preparation process is complex and the cost is high.

[0005] Therefore, the present application aims to provide a novel preparation method, which uses cheap melamine and formaldehyde as raw materials, uses branched nanosilica and spherical nanosilica as ball-forming aids, synthesizes melamine resin / nanosilica composite microspheres with lychee shell morphology, and applies them to the field of light diffusers to achieve the goal of low cost and high performance. SUMMARY

[0006] In view of the deficiencies of the prior art, the present application aims to provide a melamine resin / nanosilica composite microsphere, a preparation method thereof, and a light diffuser and optical material.

[0007] To achieve the above-mentioned purpose, the present application realizes the following technical solutions:

[0008] A preparation method of melamine resin / nanosilica composite microspheres, comprising the following steps:

[0009] a) mixing deionized water, melamine, formaldehyde aqueous solution, branched nanosilica sol and spherical nanosilica sol uniformly to obtain a mixture;

[0010] b) adjusting the pH value of the mixture to 7.5-8.0 with ammonia water;

[0011] c) heating the mixture to 70-80 DEG C, stirring for 20-40 min to obtain a transparent reaction product;

[0012] d) heating to 80-90 DEG C, adjusting the pH value to 5.0-6.0 with nitric acid to initiate polymerization reaction, reacting for 2-8 h, and post-treating to obtain the composite microspheres.

[0013] The branched nanosilica sol has an average length of 50-100 nm and a diameter of 10-20 nm; the spherical nanosilica sol has an average particle size of 80-120 nm.

[0014] The formaldehyde aqueous solution has a concentration of 20-45 wt%.

[0015] The mass ratio of melamine to formaldehyde aqueous solution is 1:(1.5-2.5).

[0016] The mass ratio of the total mass of silica in the branched nanosilica sol and the spherical nanosilica sol to melamine is (0.1-0.5):1.

[0017] A melamine resin / nanosilica composite microsphere is prepared by the above method, and the composite microsphere has a rough surface morphology like a lychee shell.

[0018] A light diffuser is prepared by the above method.

[0019] An optical material comprises a transparent resin and the light diffuser.

[0020] The transparent resin is one of polymethyl methacrylate and polycarbonate.

[0021] The application provides a preparation method of lychee-shaped melamine resin / nanosilica composite microspheres and application of the composite microspheres as a light diffuser.

[0022] Compared with the prior art, the application does not need to use dispersants or alcohol organic solvents, the reaction concentration can reach 5-15 wt%, which is significantly higher than 3-5 wt% of the traditional method, and the production cost and process complexity are reduced. The obtained composite microspheres have a morphology similar to that of litchi shells, and exhibit excellent high haze and high transmittance when used as a light diffuser, and are suitable for optical material fields such as LED lighting and liquid crystal display screens.

[0023] The advantages of the application include:

[0024] (1) Simple and green process: no dispersants or organic solvents are used, reducing subsequent processing steps and being environmentally friendly;

[0025] (2) Low cost: using cheap melamine and formaldehyde as main raw materials, the economy is better than that of organic light diffusers;

[0026] (3) Excellent performance: the composite microspheres have high haze and high transmittance, meeting the needs of optical materials;

[0027] (4) Unique morphology: having a litchi shell-shaped rough surface, improving the light scattering effect. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 TEM photograph of branched nanosilica sol used for Example 2;

[0029] Figure 2 TEM photograph of spherical nanosilica sol used for Example 2;

[0030] Figure 3 SEM photograph of composite microspheres prepared in Example 1;

[0031] Figure 4 SEM photograph of composite microspheres prepared in Example 2;

[0032] Figure 5 SEM photograph of composite microspheres prepared in Example 3;

[0033] Figure 6 SEM photograph of composite microspheres prepared in Comparative Example 1;

[0034] Figure 7 SEM photograph of composite microspheres prepared in Comparative Example 2;

[0035] Figure 8 SEM photograph of composite microspheres prepared in Comparative Example 4. DETAILED DESCRIPTION

[0036] The present application is illustrated by way of example and other advantages and benefits will be apparent from this disclosure which teaches an improved method for producing a melamine resin / nano-silica composite microsphere. The present application can be implemented or applied in other different embodiments and various modifications and changes can be made with respect to the details of the present application without departing from the spirit of the present application. It should be noted that the following examples and features in the examples can be combined with each other without conflict. It should also be understood that the terms used in the examples of the present application are intended to describe specific embodiments and are not intended to limit the scope of protection of the present application. The test methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or according to the conditions recommended by the manufacturer.

[0037] When a numerical range is given in the examples, it should be understood that, unless otherwise specified by the present application, each numerical range has two endpoints and any number between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present application are consistent with the understanding of the present technology by those skilled in the art and the description of the present application. Any method, equipment and material of the prior art similar or equivalent to the method, equipment and material used in the examples of the present application can be used to implement the present application.

[0038] Example 1

[0039] In a beaker, 155 g of deionized water, 10 g of melamine, 25 g of formaldehyde aqueous solution (20 wt%), 2 g of branched nano-silica sol (average length 50 nm, diameter 10 nm, solid content 30 wt%, pH = 9.5) and 1.4 g of spherical nano-silica sol (particle size 80 nm, solid content 30 wt%, pH = 9.5) were added after stirring to uniformity. The pH of the mixture was adjusted to 7.5 with ammonia water. The mixture was transferred to a three-necked flask, and the water bath was heated to 70°C, stirred at 300 r / min, and reacted for 40 min to obtain a transparent reaction product. Then the temperature was increased to 80°C, and nitric acid (2M) was added dropwise to adjust the pH to 5.0. After 2 min, the system gradually turned white, and the stirring reaction was continued for 8 h. After cooling to room temperature, the precipitate was separated by centrifugation and washed with deionized water 3 times (35 ml each time), and dried at 150°C to constant weight to obtain melamine resin / nano-silica composite microspheres with a rough surface morphology like lychee fruit shells.

[0040] Example 2

[0041] In a beaker, 155 g of deionized water, 10 g of melamine, 20 g of formaldehyde aqueous solution (37 wt%) were added, and after stirring, 6.3 g of branched nano-silica sol (average length 75 nm, diameter 15 nm, solid content 30 wt%, pH = 9.5) and 2 g of spherical nano-silica sol (particle size 100 nm, solid content 30 wt%, pH = 9.5) were added. The pH of the mixture was adjusted to 7.7 with ammonia water. The mixture was transferred to a three-necked flask, and the water bath was warmed to 75°C, and stirred at 300 r / min for 30 min to obtain a transparent reaction product. Then the temperature was raised to 85°C, and nitric acid (2M) was added dropwise to adjust the pH to 5.5, and after 2 min the system gradually turned white, and the stirring was continued for 5 h. After cooling to room temperature, the precipitate was separated by centrifugation, washed with deionized water 3 times (35 ml each time), and dried at 150°C to constant weight to obtain melamine resin / nano-silica composite microspheres with a lychee shell-like rough surface morphology.

[0042] Example 3

[0043] In a beaker, 155 g of deionized water, 10 g of melamine, 20 g of formaldehyde aqueous solution (37 wt%) were added, and after stirring, 6.3 g of branched nano-silica sol (average length 75 nm, diameter 15 nm, solid content 30 wt%, pH = 9.5) and 2 g of spherical nano-silica sol (particle size 100 nm, solid content 30 wt%, pH = 9.5) were added. The pH of the mixture was adjusted to 7.7 with ammonia water. The mixture was transferred to a three-necked flask, and the water bath was warmed to 75°C, and stirred at 300 r / min for 30 min to obtain a transparent reaction product. Then the temperature was raised to 85°C, and nitric acid (2M) was added dropwise to adjust the pH to 5.5, and after 2 min the system gradually turned white, and the stirring was continued for 5 h. After cooling to room temperature, the precipitate was separated by centrifugation, washed with deionized water 3 times (35 ml each time), and dried at 150°C to constant weight to obtain melamine resin / nano-silica composite microspheres with a lychee shell-like rough surface morphology.

[0044] Comparative Example 1

[0045] In a beaker, 155 g of deionized water, 10 g of melamine, 20 g of formaldehyde aqueous solution (37 wt%) were added, and after stirring, 8.3 g of branched nano-silica sol with an average length of 75 nm and a diameter of 15 nm, a solid content of 30 wt%, and a pH of 9.5 were added. The pH of the mixture was adjusted to 7.7 with ammonia water. The mixture was transferred to a three-necked flask, and the water bath was warmed to 75°C with stirring at 300 r / min for 30 min to obtain a transparent reaction product. Then, the temperature was raised to 85°C, and nitric acid (2M) was added dropwise to adjust the pH to 5.5. After 2 min, the system gradually turned white, and the stirring was continued for 5 h. After cooling to room temperature, the precipitate was separated by centrifugation and washed with deionized water three times (35 ml each time). The product was dried at 150°C to constant weight to obtain melamine resin / nano-silica composite microspheres with a rough surface morphology like a lychee shell.

[0046] Comparative Example 2

[0047] In a beaker, 155 g of deionized water, 10 g of melamine, 20 g of formaldehyde aqueous solution (37 wt%) were added, and after stirring, 8.3 g of branched nano-silica sol with an average length of 75 nm and a diameter of 15 nm, a solid content of 30 wt%, and a pH of 9.5 were added. The pH of the mixture was adjusted to 7.7 with ammonia water. The mixture was transferred to a three-necked flask, and the water bath was warmed to 75°C with stirring at 300 r / min for 30 min to obtain a transparent reaction product. Then, the temperature was raised to 85°C, and nitric acid (2M) was added dropwise to adjust the pH to 5.5. After 2 min, the system gradually turned white, and the stirring was continued for 5 h. After cooling to room temperature, the precipitate was separated by centrifugation and washed with deionized water three times (35 ml each time). The product was dried at 150°C to constant weight to obtain melamine resin / nano-silica composite microspheres with a rough surface morphology like a lychee shell.

[0048] Comparative Example 3

[0049] In a beaker, 167.6 g of deionized water, 4.2 g of melamine, and 8.1 g of formaldehyde (37% aqueous solution) were added, and the pH of the mixture was adjusted to 7.7 with ammonia water. The mixture was transferred to a three-necked flask, and the water bath was warmed to 75°C with stirring for 30 min to obtain a transparent product. The temperature was raised to 85°C, and nitric acid was added dropwise to adjust the pH to 5.8. The system turned white, but during the continuous reaction, clumps appeared on the inner wall of the flask and the stirring paddle, and a powder product could not be obtained.

[0050] Comparative Example 4

[0051] In a beaker, 158 g of deionized water, 4.2 g of melamine, and 8.1 g of formaldehyde (37% aqueous solution) were added, and after stirring, 10 g of 10 wt% PVA (type 1788) aqueous solution was added. The pH was adjusted to 7.7 with ammonia water. The subsequent steps were the same as in Example 1, and a white powder product was obtained.

[0052] (1) Morphology characterization

[0053] The products prepared in Examples 1-3 and Comparative Examples 1, 2, 4 were characterized by transmission electron microscopy (TEM, JEM-1400, JEOL Ltd.). The results are shown in Figs. 5, 6, 7, and 8, which show that the composite microspheres prepared in Examples 1-3 have a rough surface structure similar to that of a lychee fruit shell, while the microspheres prepared in Comparative Examples 1 and 2 have a smoother surface. Figure 3 、 4

[0054] Figure 1 and Figure 2 are TEM photographs of the branched nanosilica sol and the spherical nanosilica sol used in Example 2.

[0055] (2) Application Examples

[0056] The melamine resin / nanosilica composite microspheres prepared in the examples and comparative examples were used as light dispersants in optical materials. The preparation method is as follows:

[0057] 0.1 g of the composite microspheres was added to 100 g of ethyl acetate and ultrasonically dispersed for 30 minutes. The dispersion was transferred to a three-necked flask, and 20 g of PMMA particles (Xi'an Qiyue Biological Technology Co., Ltd., MW: 5000) or polycarbonate particles (Puyang Shengtong Poly Source New Material Co., Ltd., JY-PC-910L type) were added (PMMA particles were used in Examples 1-2 and Comparative Examples 1-4, and polycarbonate particles were used in Example 3). The mixture was heated to 45°C in a water bath and stirred until completely dissolved to obtain a dispersion. The dispersion was poured into a silica gel disc mold with a diameter of 6 cm, the solvent was dried, and the light diffusion film was demolded. The light transmittance and haze were measured using a TH110 type haze meter (Cai Mu Technology (Zhejiang) Co., Ltd.). The test results are shown in Table 1.

[0058] Table 1 Light diffusion performance of the microspheres of the examples and comparative examples

[0059]

[0060] As can be seen from the above table, the samples of Examples 1-3 show excellent light diffusion performance, with high haze and high light transmittance.

[0061] ​The branched nano-silica and the spherical nano-silica are combined together, the branched nano-silica has high surface energy and rich surface active sites, preferentially adsorbs melamine-formaldehyde prepolymer in the prepolymerization stage, quickly nucleates through hydrogen bond and electrostatic interaction, the geometric anisotropy of the branched structure causes the difference of local reaction rate, promotes the non-uniform growth of the microsphere surface, and forms the initial rough structure; the spherical nano-silica gradually migrates to the microsphere surface in the temperature polycondensation stage (85 DEG C) due to the low Brownian motion rate, is combined with the melamine formaldehyde crosslinking network through physical adsorption, and forms a dense shell layer, the coating of the large particle size spherical particles inhibits the excessive smoothing of the microsphere surface, further strengthens the concave-convex structure of the microsphere surface, and then forms the unique surface structure similar to the lychee.

[0062] The above examples only illustrate the principles and effects of the present application, and are not used to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical thought disclosed by the present application should be covered by the claims of the present application.

Claims

1. A method for preparing melamine resin / nanosilica composite microspheres, characterized in that, The method comprises the following steps: a) mixing deionized water, melamine, aqueous formaldehyde solution, branched nanosilica sol and spherical nanosilica sol to obtain a mixture; b) adjusting the pH value of the mixture to 7.5-8.0 by using ammonia water; c) heating the mixture to 70-80 DEG C and stirring for 20-40 min to obtain a transparent reaction product; d) heating to 80-90 DEG C, adjusting the pH value to 5.0-6.0 by using nitric acid to initiate polymerization, and reacting for 2-8 h to obtain the composite microspheres after post-treatment; the branched nanosilica sol has an average length of 50-100 nm and a diameter of 10-20 nm; the spherical nanosilica sol has an average particle size of 80-120 nm; the concentration of the aqueous formaldehyde solution is 20-45 wt%; the mass ratio of melamine to the aqueous formaldehyde solution is 1:(1.5-2.5); the mass ratio of the total mass of silica in the branched nanosilica sol and the spherical nanosilica sol to the mass of melamine is (0.1-0.5):

1.

2. A melamine resin / nanosilica composite microsphere, characterized in that, The composite microspheres have a rough surface morphology like a lychee shell and are prepared by the method of claim 1.

3. An optical diffuser, characterized by, The composite microspheres are prepared by the method of claim 1.

4. An optical material, characterized by, The light diffusing agent comprises a transparent resin and the composite microspheres of claim 3.

5. The optical material of claim 4, wherein, The transparent resin is one of polymethyl methacrylate and polycarbonate.

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