A study on the thermal stability of water-soluble dyes and their application on ink-jet printed color filters

Abstract

While color filters produced by pigment-dispersed method have good thermal and photo-chemical stability, they have low chromatic properties due to the aggregation behavior of pigment particles used as colorants. Dyes can be attractive alternatives to overcome this limitation due to the reduced light scattering resulting from the fact that they are dissolved in the media and exist in molecular form. Although dyes have superior spectral characteristics, they are not widely used as coloring materials for liquid crystal display color filters owing to their low thermal stability. In this study, several perylene and phthalocyanine derivatives having high thermal stability were synthesized and then used to fabricate color filters by a spin-coating technique. The spectral and thermal stabilities of the color filters were investigated in comparison with those of pigment-dispersed color filters. The transmittance and color properties of the prepared color filters were higher and the thermal stability was similar to those of the pigment-dispersed color filters. In order to manufacture a simple process of LCD color filters, ink-jet printed color filters were introduced using the synthesized RGB water-soluble dyes. Even though its chromaticity was unsatisfied due to their lower pattern thickness, the prepared dyes could be successfully applied to ink-jet printed LCD color filters since the thermal stability of ink-jet printed color filters was similar to that of spin-coated color filters. Experimental studies on thermal stability of the water-soluble dyes were also conducted. For the investigation of structure–thermal stability relationships and thermal decomposition mechanism of water-soluble dyes, a series of structurally isomeric water-soluble azo naphthalene dyes were synthesized, and dye-based red color filters were fabricated using the synthesized azo dyes. Superior transmittance (98.9%) at 650 nm and a wide color gamut (62.8%) were achieved using one of the color filters. Thermal properties of the prepared isomeric dyes were examined by thermogravimetric analysis and differential scanning calorimetry. All the prepared isomeric azo dyes had a degradation temperature above 280 oC, varied with the dye structure. The degradation temperatures of the dyes were presumed to be related to the intra- and inter-molecular interactions of the structural isomers. It is confirmed that their intra-molecular interactions were similar from quantum mechanics calculations. However, the degradation temperatures generally increased as the proportion of their density. According to the molecular mechanics calculations, the structural differences gave rise to change of electrostatic attraction, steric hindrance and linearity of the dyes, which in turn caused changes in their molecular packing geometry, density and degradation temperature. Thermal decomposition mechanism of the water-soluble dyes was also discussed. In case of the prepared isomeric azo dyes with strong electrostatic inter-molecular interactions decomposed gradually without melting and their degradation temperatures increased as the proportion of their inter-molecular interaction. Thus, when the prepared water-soluble dyes had higher density – stronger inter-molecular interaction, they exhibited higher degradation temperatures since the dyes with stronger inter-molecular interactions could have stabilized intra-molecular interactions of the dye molecules. And the aggregates with larger diameters and higher aggregation numbers, would have stronger inter-molecular interaction and greater stabilized intra-molecular interactions than those with smaller particle sizes. This result could explain that, for the prepared water-soluble dyes, dye aggregates with smaller particle size degraded first and those with larger particle size degraded later.