The requirements of LED Technology for Optical Diffusion Materials and the Development conditions of Optical Diffusion Materials

1.1 LED Introduction

In recent years, as the global energy crisis, the rapid development of LED. it has been widely used in lighting, backlight, display and other industries. Because of its advantages of environmental protection, energy saving and long service life, LED has a broad application prospect. With the development of LED lighting industry, LED light diffusion board is also increasing rapidly. Nowadays, LED light diffusion materials are monopolized by several large foreign companies (such as Teijin, Asahi Huacheng, South Korea LG, etc.). which cost is quite high.

1.1.1 LED Technology and requirements for Optical Diffusion Materials.

LED-TV backlight technology is the most important application field of LED lighting. LCD TV production technology in LED-TV mainly adopts direct white LED backlight technology. The direct backlight is a point light source array composed of hundreds of LED. In order to provide a uniform backlight to the liquid crystal panel, it is necessary to add a light diffusion plate in front of the LED array. The high diffusivity of the diffusion plate can effectively prevent the LED point light source from imaging on the liquid crystal panel. Indoor and outdoor lighting is the most promising application field of LED. When it is used in indoor and outdoor lighting, glare is caused by the high color temperature and high pointing performance of LED light sources, which is easy to cause light loss to human eyes. Therefore, it is necessary to use the light diffusion plate to turn the high brightness LED point light source or line light source into a uniform and soft plane light source to improve the uniformity of light intensity and the utilization efficiency of light energy and prevent glare.

1.1.2 Application and Market Prospect of LED

The shell material used in LED lighting, both light diffusion material, as shown in figure 1.1, refers to the material that can not only make light pass through but also effectively scatter light, which can transform point and line light sources into line and surface light sources. Evaluate the material through which light can effectively scatter light, which can convert point and line light sources into line and surface light sources. The two most important indexes to evaluate the optical properties of light diffusion materials are light transmittance and haze. In order to make the light soft and uniform, it is generally required that the light transmittance of the light diffusion material is more than 50% and the haze is more than 90%.

Due to the contradiction between light transmittance and non-toxic, the haze often increases, and the light transmittance decreases. How to balance these two indicators, the choice of optical diffuser is particularly important. With the rapid development of the LED lighting industry, it is necessary to improve the production efficiency of light diffusion to achieve continuous production. Most of the light diffusion materials are prepared by blending transparent polymer matrix materials with light-scattering particles. Light scattering particles include inorganic microsilica, glass beads, and organic polymer particles, such as polymethacrylate, polystyrene PS, poly (ethylene terephthalate), etc. We use PC, which has excellent mechanical properties and heat resistance, as the matrix material, and study the effects of different kinds of optical diffusers with different micro-morphologies on the optical properties of LED lighting light diffusion materials, so as to provide powerful reference data for industrial production.

1.2 Development status of Optical Diffusion Materials at Home and abroad

Light diffusion material refers to the optical material with certain light transmittance and haze, which can convert point light source and line light source into line light source and surface light source, and achieve the effect of uniform distribution of incident beam light intensity. it is widely used in flat panel display, lighting engineering, laser, projection imaging, and other technical fields. Nowadays, with the maturity of high-power light-emitting diode LED chip manufacturing technology, and its wide application in automobile lighting, signal indicators, outdoor display screen, indoor and outdoor lighting and other fields, the demand for LED light diffusion particles is increasing rapidly.

At present, the optical diffusion materials with superior performance in the domestic market mainly come from foreign companies such as Teijin and Asahi Kasei, and the processing is more expensive. Independent research and development of domestic brands is the key way to break its monopoly. In addition, because haze and transmittance are important indexes to evaluate the optical properties of light diffusion materials, they contradict each other. Many factors affect the optical properties of light diffusion materials, and the interaction is more complex, so the development of light diffusion materials with high transmittance and high haze has become the focus of the current research.

The research of polymer light diffusion materials first came from the United States, and then quickly triggered an upsurge of research and development of polymer light diffusion materials in various countries. In 1944, Yoshio Ohtsuka et al used PC as the matrix and doped CaCO3, PMMA particles to prepare light diffusion materials to achieve the effect of light scattering. In 2000, Mcneil LE et al prepared light diffusion films by doping TiO2 particles into transparent acrylic resin and analyzed the scattering coefficient and multiple scattering function in detail in theory, which provided a reference for the following research work. In 2004, Kim GH et al used PMMA as the matrix, in which glass fiber was added to prepare the light diffusion film and applied in the LCD backlight module so that the light emitted by LCD was uniform and had good physical properties. From 1998 to 2004, Keio University in Japan began to apply high-performance scattering polymer to liquid crystal backlight lighting guide plate, and put forward the concept of lattice-free scattering light guide plate for the first time, while keeping the electric power of the light source unchanged. The brightness of the liquid crystal backlight has been doubled, which has attracted global attention. In 2005, this technology was acquired by SONY of Japan and industrialized, which was used in the production of ultra-thin notebook computers. In 2007, Beijing University of Chemical Technology prepared light diffusion materials by adding Pr-MMA and SBR (styrene-butadiene rubber) to PS by in-situ polymerization, with a light transmittance of 79.9% and haze of 83.11%. In 2009, Meng Qinghua et al synthesized nano-PMMA/PS light scattering agent and added it to PMMA matrix materials to prepare light diffusion composites with haze and light transmittance of 70-80%. In 2009, Wang et al prepared light diffusion films with water droplets dispersed in silicone polymers, with a light transmittance of about 88%. The scattering effect is shown in figure 1.2. With the increasing demand for LED and other optical materials, the research on optical diffusion materials is still of positive significance.

1.2.1 How to chose the optical diffuser?

Commonly used light diffusers include inorganic particles such as TiO2, BaSO4, SiO2, CaCO3, Al2O3, and glass beads, as well as organic polymer particles such as PS and silicone resin.

What’s requirements should be met when choosing the scattering materials?
(1 ) the scatterer material and the matrix material should have specific different optical properties (such as refractive index).
(2) the scatterer material should absorb little or no light.
(3) the size of the scatterer particles must meet certain requirements, and the particles should not be too small or too large, otherwise the scattering effect is not obvious, the particle size is very weak, and the light scattering increases with the increase of the particle size, but when it reaches a certain size, this linear relationship is no longer valid.

1.2.2 How to chose the Light Diffusion Materials?

At present, the light diffusion materials produced at home and abroad are mainly divided into surface scattering materials and bulk scattering materials. Traditional light scattering materials are surface scattering materials, and their scattered light mainly depends on surface structures, such as microlenses, pyramids, rough surfaces and other microstructure. The methods of preparing surface scattering light scattering materials include surface texturing, spraying, laser candle engraving, hot pressing, and ultrasonic embossing. The advantages of spraying and surface texturing methods are simple operation and low cost, and the disadvantage is that it is difficult to achieve ideal light transmittance. The surface scattering materials prepared by laser candle carving and ink chrysanthemum molding have good performance, and can even accurately control the beam shape, but the requirements for the instruments and molds used are very high and the cost is very high. The cost of hot pressing method is relatively low, but because the heating and cooling process is time-consuming and the efficiency is low, it can not be used in large-scale production.

Most of the bulk scattering light scattering materials are transparent polymer materials with scattered light scatterers, which achieve the purpose of light diffusion by adding light diffusers which are different from the refraction of the matrix in the transparent collective materials. This material uses the global scattering mechanism, that is, the interior and surface of the material play a scattering role, and the sample has high light scattering, high transparency and good comprehensive properties.
At present, volume scattering light scattering materials, as a new type of light scattering materials, gradually replace the traditional light scattering materials in many application fields and begin to be widely used in LED lighting, liquid crystal display and other fields.

The materials we use are generally volume scattering materials. PC has a light transmittance of 89%, good mechanical properties, low hygroscopicity and good flame retardancy, but ultraviolet or skin line irradiation can easily make it yellowing. PS has a light transmittance of 90%, low hygroscopicity and poor heat resistance. Because of its low surface hardness and brittleness, it is easy to produce cracks and crazes, and it is easy to color deterioration under long-term ultraviolet radiation. PMMA is extremely transparent, with a light transmittance of 92% in the visible range, good processability and strong anti-ultraviolet aging properties, but it has strong moisture absorption, poor toughness and flammability. ABS has two properties of impact resistance, heat resistance and low temperature resistance, and it is easy to process, the scale of the product is stable, and the surface gloss is good, but because ABS is a blend, its light transmittance is poor. Considering the optical properties, mechanical properties, processability and other factors of the resin, PC and PMMA become the first choice of light scattering matrix materials.