Effect of Light diffusion agent on Properties of Light Diffusion Plastics

As a solid-state semiconductor device that directly converts electrical energy into light energy, light-emitting diode (LED) not only has a solid structure, shock resistance, fast light response, long life, but also low energy consumption. LED is a point light source. In practical applications, it is usually necessary to convert it into a line light source or a surface light source to make the light bright and soft. To achieve this transformation, light diffusion materials are needed. Photodiffusive materials are generally composed of light diffusion agents and polymers.

1. Light diffusion agent

The light diffusion agent is made by special processing and surface treatment. The particle size is generally between 1 μ m and 10 μ m, and the average particle size is about 2 μ m. The beads have the function of astigmatism, good fluidity and good compatibility with optical resin substrate.

At present, optical diffuser is mainly divided into inorganic type and organic type. Inorganic light diffusion agent mainly include inorganic particles such as silicon dioxide, titanium dioxide, calcium carbonate, aluminum hydroxide and glass beads, while organic light diffusion agent mainly include cross-linked polystyrene, polymethacrylate and other organic polymer microspheres.

Inorganic Light Diffusion Agent

When adding inorganic particles such as silicon dioxide, titanium dioxide, and calcium carbonate, light will be diffused through the surface of tiny particles for countless times to achieve the effect of uniform light. Although inorganic particles have good heat resistance, they have different shapes, large particle size deviation, and lack of uniform diffusivity of light; inorganic light diffusion agent is a solid microsphere from a microscopic point of view, and light can not pass through this solid sphere. It will affect the transmission of a lot of light, and only part of the light is refracted through, thus affecting the brightness or transmission of light. Those who are required to have a light transmittance above 50% for lampshades cannot choose inorganic light diffusion agent.

Organic light diffusion agent
Cross-linked polystyrene and polymethacrylate microspheres have good shape uniformity, controllable particle size deviation, high light transmittance, but low heat resistance. During light diffusion polymer extrusion granulation and high-temperature injection molding or extrusion molding of light guide devices, polymer particles are prone to deformation, thus affecting the uniform diffusion of light. Moreover, the light resistance of the polystyrene light diffuser is poor, and it is easy to yellowing, which affects the service quality and service life of LED.

Organic-inorganic hybrid organosilicon microsphere light diffuser has been developed in recent years. Silicone microsphere light diffuser not only has the characteristics of high heat resistance, light resistance and aging resistance of inorganic particles, but also has the characteristics of homogeneity of organic particle shape, high light transmittance and good light diffusion uniformity, and its refractive index can be changed with the change of synthetic monomers, and the surface of the particles can be modified in situ by light diffusion. It has a broad application prospect to improve the compatibility and adaptability of light diffusion particles with polycarbonate (PC), polymethyl methacrylate (PMMA), polystyrene (PS) and other light guide plate matrix resins. Therefore, silicone microspheres are expected to become a high-performance optical diffuser for LED. Silicone light diffusion agent is usually a microsphere product formed from methyltrimethoxysilane and phenyl trimethoxysilane by hydrolysis, condensation and cross-linking reaction. the particle size distribution is between 1 μ m and 8 μ m, and the average particle size is 2 μ 4 μ m.

2 light diffusion plastics
When the light diffuser is added to the PC resin, the light diffuser is spherical and uniformly dispersed in the PC resin to form an island structure. Due to the different refractive index of the PC resin and the light diffuser, the light is similar to specular reflection on the surface of the light diffuser, and the light diffusion effect is achieved after multiple reflections.

There are generally two methods for the preparation of photodiffusion materials, one is polymerization, the other is blending modification, each of which has its own characteristics. According to the principle of light refraction, the polymerization method is to select the co-polymerization of two kinds of polymer monomers with different refraction and poor compatibility or piecewise polymerization to further prepare light scattering materials. The commonly used polymerization method is to prepare two kinds of monomers with different reaction activity, because the reaction activity of the scatterer monomer is different from that of the matrix monomer, and the scatterer monomer produces self-polymerization or block copolymerization with the matrix monomer. in this way, the optical properties of the condensed nuclei formed in the respective polymerization chains are uniform, and the light is reflected and refracted at the boundary of the condensed nucleus, thus forming scattering. Polymerization is widely used in the preparation of poly (methyl methacrylate)-based (PMMA)-based photodiffusion materials, but it is rarely used in the preparation of PC-based photodiffusion materials, and the related reports and studies are also very rare.

The blending modification method is to add the light diffuser to the PC resin. The light diffuser is spherical and uniformly dispersed in the PC resin to form an island structure. Due to the different refractive index of the PC resin and the light diffuser, the light is similar to specular reflection on the surface of the light diffuser, and the light diffusion effect is achieved after many times of reflection. At the same time, the amount of optical diffuser, particle size and distribution, refractive index determine the optical properties of the material. At present, many new types of light diffusion materials are produced by using methods, because this method is similar to polymer doping, and the process is simple, especially in light diffusion plates with very large consumption, this method can be produced continuously and has high production efficiency. .

The light diffusion agent is added to PC, PMMA, PS and other transparent plastics to obtain light diffusion plastics, which not only cover the light source and dazzling light, but also make the whole transparent resin emit softer, beautiful and elegant light, so as to achieve the comfortable effect of light transmission and opacity.
In order to solve the problems of glare and blue light harm of LED lighting, starting from organosilicon light diffusion materials, titanium dioxide (TiO2) and ceria (CeO2) were used to modify and functionalize the surface of light diffusion agent, so as to obtain light diffusion materials with good optical properties and blue light shielding function.

What’s the effect of Light diffusion agent on Properties of Light Diffusion Plastics?

The types of light diffusion agent can be divided into three categories: organic particles, inorganic particles and composite materials. Scatterer particles need to meet the following three points:
The main results are as follows:

(1) there are some differences between the optical properties and the matrix materials.
(2) there should be no or less absorption of the transmitted light.
(3) the size of the particles must meet certain requirements.

At the beginning of the research, the inorganic scatterer particles are widely used, but these particles are hard and irregular, so they are easy to damage the equipment during processing, and the dispersed particles are not uniform enough. If the size of the particle itself is too large, the surface of the polymer material will not be smooth. Therefore, inorganic particles are gradually replaced in production practice. The compatibility between organic scattering particles and substrate is better than that of inorganic particles, so it gradually takes the place of inorganic particles. It is found that the scattering particles with shell-core structure have greater advantages, because the particles with this structure are composed of core and shell, and the outermost core-shell materials can be well compatible, thus improving the dispersion characteristics of scattering particles. At the same time, due to the close bonding between particles, the mechanical properties of the composites have also been improved. If a tougher material is used to make the core, it will improve the impact performance of the material.

Refractive index of optical diffusion agent
According to the light scattering theory, the light scattering effect of different light diffusion polymers with the same volume and the same diameter light diffusion agent is directly related to the refractive index. In PC light diffusion materials, the difference of refractive index between light diffusion agent particles and matrix resin directly determines the diffusion effect and light transmission effect of light diffusion materials.

The particle diameter of the light diffusion agent
The particles of the light diffusion agent are dispersed in the matrix resin materials, and the diameter of these particles also affects the properties of the composites. It is found that at a certain doping concentration, the light transmittance increases gradually with the increase of the particle diameter of the light diffuser, while the diffusivity increases rapidly and begins to decline after reaching the peak. With the increase of the particle diameter of the light diffusion agent, the reverse scattering effect is weakened, while the forward scattering effect is enhanced, and the light transmittance is enhanced.

If small particles such as nanoparticles are mixed in PC, the diffusivity will mainly depend on the scattering ability of the light diffusion agent particles, in which case the scattering coefficient is small and the diffusivity is also very low. When the diameter of the particles increases gradually, the scattering ability of the particles increases, which leads to the increase of diffusivity. If the diameter of the particles continues to increase, the scattering ability will no longer be greatly affected, and the scattered light is mostly concentrated in the front, so the diffusivity shows a downward trend.

Doping amount of light diffusion agent particles
The doping amount of light diffusion agent particles is also an important factor to determine the scattering effect of materials. In the numerical simulation study, it is found that the doping amount of scattering particles can determine the uniformity of the output light surface. If the doping amount of the scatterer particles in the medium exceeds the critical point, the strongest part of the output light will appear far away from the light source, and when the concentration of the particles is just at the critical point, the output light intensity distribution is very uniform. With the increase of the amount of scatterer particles, the maximum intensity of the output light moves from far to near. Therefore, as long as we effectively control the number of doping particles, we can obtain uniformly distributed light.