The goal of the present project has been to develop light diffusing polymer beads, tailor-made for products to be used in optical films, LED lighting covers/panels, cell phones, architectural lighting fixtures, signs and light guide applications in the automotive industry. Small, monodisperse polymer particles, Ugelstad beads, has proven to be ideal for ensuring uniform light diffusion. The beads are perfectly spherical polymer particles with very narrow particle size distribution and identical chemical composition and properties. Many of the light diffusing products used in the industry are based on PC or PMMA due to their high transparency and relatively low cost. The beads that have been developed by Microbeads have therefore been mixed with PC or PMMA and injection molded into discs and test bars which have been used for further investigation (Figure 2).

Development of a method for measuring the refractive index of the beads

When dealing with optical performance products, it is important to know the refractive index of the beads and of the matrix material which the beads are compounded into. It is especially challenging to measure the refractive index (RI) of the small beads. Norner Research has developed a fast and simple method for determining the RI of small beads. The principle of the method is very elegantly illustrated in Figure 3 by mixing the beads (powder) into a solution consisting of two solvents with different refractive indices. Usually, when the powder is mixed into a transparent liquid, the solution will appear opaque unless the solution and the powder have exactly the same refractive index. The refractive index of the solution is determined by the refractive index and volume fraction of each solvent component. By gradually adding a solvent with a higher refractive index to a mixture of beads and solvent with a lower refractive index, the mixture of beads and solution will gradually change from opaque to transparent and to opaque again. By keeping control of the volume fractions of solutions, it is then possible to calculate the refractive index of the beads when the solution appears transparent.

Figure 1 Monodisperse beads developed by Microbeads

Project highlights

The project has gained substantial knowledge about how polymer beads refract light waves through a polymer by carrying out systematic experiments with the most commonly used polymers in LED displays.

Over 100 polymer samples with varying amounts, sizes and compositions of polymer beads have been produced. Different optical properties have been measured and compared with polymer without beads. We have also succeeded in systematically changing the beads’ refractive index by changing the chemical composition of the beads.

A method for measuring the refractive index of particles has been developed, and all particles in the project have been measured.

Optical measurements of polymer samples with these new beads confirm that a larger difference in the refractive index between the base polymer and the polymer beads increases light scattering but reduces the light transmittance.

Systematic investigations of how the addition of particles affects various mechanical properties have also been carried out.

The main project result is that due to the perfectly spherical shape and the identical size and composition of the beads from Microbeads, it is possible to predict and tailor optical properties based on the size and addition level of beads.

Figure 2 Injection moulded discs, ready for optical investigation

Figure 3 Small beads in a solution for determination of the refractive index of the beads