How to Choose the Main Raw Materials of Water-soluble LED Light Diffusion Coatings?

The glass tube LED lamp has high requirements for uniformity, transmittance, haze, safety, adhesion, stability, non-pollution and price grid of the light diffusion coating. To meet these requirements, it is necessary to study the selection of resin and light diffuser and the manufacturing process. The application of waterborne LED light diffusion coating avoids the environmental pollution caused by low organic volatile matter (VOC) emission, is safe and convenient to use, and develops rapidly under the promotion of large-scale development of straight glass LED lamps.

1 selection of water-soluble film-forming resin

The film resin which meets the technological requirements of the equipment is the key to the harmless waterborne photodiffusion coating technology. At present, the film-forming resins that can be used as LED light diffusion coating are waterborne alkyd resin, waterborne acrylic resin, waterborne polyurethane resin and so on. These three kinds of resins can be used separately or in combination according to a certain proportion to avoid the performance defects when using a certain kind of resin alone. The performance differences of these three kinds of water-soluble film-forming resins are described below.

1.1 Waterborne alkyd resin-the first film-forming agent developed

Waterborne alkyd resin is an early developed waterborne coating, and its film-forming mechanism is similar to that of traditional solvent-based alkyd resin, which is cured by oxidation and cross-linking of unsaturated fatty acids without adding cosolvent (film-forming agent) and free of volatile organic compounds. In addition, waterborne alkyd resin has good wettability, strong bearing capacity, good permeability, leveling and fullness, easy coating and good coating effect. However, the polymer chain is easy to be hydrolyzed and the durability of the film is poor. If the ignition time is too long, it will change color slightly. However, when the resin is modified by acrylic acid or polyurethane after self-emulsification, the durability can be improved and can be used in the light diffusion layer.

1.2 Waterborne polyacrylic resin-the most ideal film-forming agent at present

Waterborne acrylic resin includes acrylic resin-emulsion, acrylic resin aqueous dispersion and acrylic resin aqueous solution. The waterborne polyacrylic resin used as water diffusion coating is one of the emulsions. according to the monomer composition, it is usually divided into pure acrylic emulsion, styrene-acrylic emulsion, acetate-acrylic emulsion, silicone-acrylic emulsion, tertiary vinegar (tert-carbonate-vinyl acetate) emulsion, tert-acrylic (tert-carbonate-acrylate) emulsion, fluorocarbon emulsion, fluoro-acrylic emulsion and so on. Acrylic resin emulsion has the advantages of fast drying speed, high hardness, low cost, good weather resistance (not easy to change color at ignition point), and avoids the disadvantages of poor film-forming, low gloss, solvent resistance, hot adhesion and cold brittleness when used in LED light diffusion coating. Waterborne polyacrylic resin is the most ideal film-forming agent for light diffusion coating in terms of comprehensive performance and performance-to-price ratio.

1.3 Water-soluble polyurethane resin-the first choice for high-end coatings

Waterborne polyurethane coating uses waterborne polyurethane resin and water as medium, which has the advantages of low toxicity, not easy to burn, not polluting the environment, energy-saving, safety and so on. The coating has high hardness, strong adhesion and good flexibility. The one-component polyurethane coating polymer has a large number of relative components, and there is no cross-linking reaction in the film-forming process, so it is convenient to use. Two-component waterborne polyurethane coatings must be mixed before use, cross-linking reaction occurs in the film formation process, and the film performance is better. However, most of this resin is produced from abroad, the cost is high, the performance-to-price ratio is not ideal, and it is only used in some high-end products.

2. Comparative analysis of light diffusion agent

2.1 Technical parameters of light diffusion coating

(1) Light transmittance-the ratio of the light flux through the light diffusion layer lamp to the light flux through the light diffusion coated glass tube is expressed as a percentage. However, there is a problem of the difference between the upper and lower thickness of the straight tube lamp in the process of manufacturing. Now many manufacturers use the comparison of the luminous flux before and after the glass tube is coated with the light diffusion layer measured in the integrating sphere under the same conditions.

2) haze-the ratio of the scattered light flux through the glass tube (deviating from the incident light direction) to the transmitted light flux is expressed as a percentage (in this method, the scattered light flux deviating from the incident light direction of more than 2.5 °is used to calculate haze), which is often measured by haze meters.

(3) Anti-aging time-the rate of light decay caused by coating at a specific burning point at the working temperature. The light decay rate of 1 000 h or 10 000 h is commonly used to express it.
Light transmittance and haze are important indicators to measure the transparency of LED lamps. The challenge of light diffuser is to obtain high haze while ensuring high light transmittance and uniform and soft lighting effect. The key criterion for the excellence of light diffuser is to reduce the light decay caused by light diffusion as much as possible in the process of efficiently exerting the effect of light diffusion.

2.2 Inorganic light diffuser.

There are many kinds of materials that can be used as inorganic light diffuser, such as calcium carbonate, talcum powder, zinc oxide, titanium dioxide, silicon oxide and so on. The optical diffuser can increase the haze of the coating, adjust the rheological energy, improve the mechanical strength and improve the durability of the film.

2.2.1 calcium Carbonate (CaCO3).

Calcium carbonate includes heavy calcium carbonate and light calcium carbonate. When light calcium carbonate is used as raw material of suspension, attention should be paid to the amount. The dissociation of Ca2+, from free calcium oxide in water, affects the storage stability of suspension, so the content of free calcium oxide in light calcium carbonate is an important index for the preparation of light diffusion coatings. Heavy calcium carbonate, including white powder and calcite powder, is made from calcite powder with high purity. Heavy calcium carbonate is relatively dense and easy to precipitate, so it is necessary to pay attention to the prevention of sedimentation when it is used in suspension.

2.2.2 talc (3MgO ·4SiO2 ·H2O).

The chemical composition of talc powder is magnesium silicate hydrate, which can not only improve the flexibility of the film, but also eliminate the internal stress during curing, and has a good leveling property.

2.2.3 Zinc oxide (ZnO).

Zinc oxide, also known as zinc white, has anti-mildew, whitening effect, and can improve the light resistance and pulverization of the film. Among them, Zn2+ can cause some coatings to thicken and coagulate, and will not be used alone, and attention should be paid to the dosage and compatibility with the corresponding coatings, and the thermal stability test of the formula should be carried out.

2.2.4 Titanium dioxide (TiO2).

Titanium dioxide (titanium dioxide) is divided into three different crystalline states: rutile type, anatase type and plate titanium type. Rutile titanium dioxide has high reflectivity, strong hiding power, high refractive index, light resistance, heat resistance and durability, and is not easy to yellowing, pulverization and degradation. it can be thickened, thixotropic and flow-proof in the coating. Anatase type is not suitable to be used as light diffuser because it is easy to be pulverized under light, while plate titanium type is unstable and has no application value.

2.2.5 Silicon dioxide (SiO2).

Natural silica is a neutral white powder with chemical properties consistent with silicon dioxide and high stability, but there is a great difference in physical state. Some of the powders have large particles, impure color and certain light absorption, so the light efficiency is low when directly used as a light diffuser. Natural silica can be divided into three types: natural amorphous silica, natural crystalline silica and natural diatomite. Among them, the particle size of natural amorphous silica is mostly less than 40 μ m, which deviates from the best range of application and is not an ideal optical diffuser. Natural diatomite is silica containing crystalline water, with a particle size of 4 ~ 12 μ m. Its absorbance varies with different manufacturing methods, and its quality fluctuates greatly and is difficult to control. The particle size of natural crystalline silica is 1.5 ~ 9.0 μ m, and the particle size is suitable. The product can be selected after purification, which can improve the mechanical energy of the film.

2.3 Application of organic light diffuser.

Organic light diffusers are transparent or translucent organic resin particles, and the most commonly used ones include micron particles such as polymethyl methacrylate (PMMA), polystyrene, silicone resin and acrylic resin. Most of the light emitted from the LED can pass through these particles, which is different from the inorganic light diffuser in two ways.

The main results are as follows: (1) there is a difference in the refractive index between the organic light diffuser and the substrate, in which the light is refracted many times to obtain an excellent haze, rather than the diffuse effect caused by multiple reflection. In this way, the light transmittance is good, the light loss is less, and the self-absorption of light caused by multiple reflections is avoided, thus the light efficiency is improved and the problem of light leveling is solved.

(2) the organic resin particles have strong triboelectrification characteristics, which can be rapidly dispersed in other optical diffusers and uniformly attached to the surface of other powder particles during dry mixing. This orderly mixing improves the mixing characteristics, fluidity and formability of the powder. compared with inorganic powders, these granular resins have better compatibility with organic resins as film-forming agents and are easier to disperse in waterborne resins.

As far as the current application effect is concerned, the silicone resin particles with a refractive index of 1.41 to 1.43 have excellent light transmittance and high haze. Compared with inorganic microparticles, silicone resin microparticles have lower specific weight and better heat resistance; light transmittance and stability are higher than those of other organic materials, and the addition amount is less; compared with acrylic resin light diffuser, it has better heat resistance and high temperature resistance, low addition and high performance-to-price ratio; compared with PMMA, it is more resistant to high temperature and does not change color. In the ideal mass fraction test, the effective light diffusion coefficient of silicone resin particles can reach 76.7%, which is the highest among the known organic light diffusers.

3 nano-particle materials.

The particle size of the optical diffuser discussed above is all in the micron level. for the optical diffuser, if it is too fine, the haze is not good, and if it is too coarse, the light transmittance is not good. From the comprehensive consideration of light transmittance and haze, the best particle size range of light diffuser is 2 ~ 20 μ m. However, the effect of nano-sized particles on the coating is also worthy of attention

3.1 Physicochemical analysis of nano-particle materials in coatings.

When the particles enter the nanometer scale, the increase of surface active centers enhances the reaction ability of chemical catalysis and photocatalysis, and gives the coating self-cleaning ability under the action of ultraviolet and oxygen. The secondary chemical bond can occur between the surface active center and the functional group of the film-forming material, which greatly increases the rigidity and strength of the coating and makes it not easy to scratch. The surface energy of nano-material is very high, and it can be hydrophobic and oleophobic at the same time after modification. when used in light diffusion coating, it can significantly improve the stain resistance and aging resistance of the coating. When the nano-particle material is used in the light diffusion coating, it can increase the adhesion between the coating and the substrate glass, improve the mechanical strength, and the strong force and filling effect between the nano-particles and the coating, which is helpful to the interface bonding between the coating and the glass.
The wavelength of visible light (400~750nm) is much larger than the particle size of the nanoparticles, which can pass through the particles directly, thus ensuring the high transparency of the nanocomposite coating.

3.2 nanometer silica.

Artificial precipitated silica is a white amorphous powder with an average particle size of 20~110nm, which belongs to nanometer scale. It will be attached to the polymer surface in the aqueous system, while a small amount of negative charge on the particle surface makes it mutually exclusive and difficult to flocculate, thus improving the stability of the system. After adding nano-meter silica, the coating is not easy to be delaminated, can prevent flow hanging, and has anti-aging and thermal stability. However, when the pH value of the system is lower than 8.5, the surface charge of the nano-silica dispersion will decrease, and the stability of the system will also decrease, so the nano-silica dispersion should be mixed with the resin emulsion before adding other components.

3.3 nanometer titanium dioxide.

Nano-titanium dioxide is a good nano-coating material, and it can be self-clean and antibacterial at the same time. Nano-titanium dioxide powder can be used in coatings to make it have bactericidal function. The irradiation of light can cause the surface of titanium dioxide to form a wonderful super amphiphilic (hydrophilic and lipophilic two-phase coexistence). Under the light whose wavelength is less than 400nm, the particles can absorb short-wave light radiation higher than the width of its forbidden band, produce electron transition, and the valence band electrons are excited to the conduction band and form electron-hole pairs, which transfer energy to the surrounding medium and induce photochemical reaction, so they have photocatalytic performance.
The addition of titanium dioxide and other nanoparticles to the light diffusion coating can not only improve the aging resistance, but also significantly increase the hardness and adhesion of the coating.

4 concluding remarks.

At present, the waterborne light diffusion coating technology is the focus of the straight tube LED lamp manufacturers. The main formula raw materials are analyzed and studied, hoping to promote the research, development and application of environment-friendly LED light diffusion coatings.