Thermo-Mechanical and Hygro-Mechanical Stress Analyses of an Organic Multilayer Sheet

Abstract

A polarizing plate, which is an important part of a liquid crystal display panel, is made by sandwiching an organic polarizer between protecting films. An organic polarizer is both a hygroscopic and orthotropic material. The hygroscopic swelling and drying shrinkage of the organic polarizer can cause the polarizing plate to crack and the liquid crystal display panel to warp. The diffusion coefficient and Henry’s law coefficient were measured using a thermo-gravimetric analyzer (TGA) under controlled humidity, while the coefficient of moisture expansion (CME) was measured using a thermo-mechanical analyzer (TMA), also under controlled humidity. The thermo-mechanical and hygro-mechanical deformation of a polarizing plate was analyzed using the finite element method (FEM). This analysis was performed as follows. The distribution of the moisture concentration was analyzed according to Fick’s law. The equation of Fick’s law is similar to that of the transient heat conduction, and the FEM for the transient heat conduction was utilized for the transient diffusion analysis. The hygro-mechanical analysis was then carried out in a way similar to the thermal stress analysis. Thermal stress was analyzed separately using the FEM. Finally, the obtained hygro-mechanical strain and stress were added to the thermal strain and stress, respectively. The measured CME of a polarizing plate corresponds to the analyzed CME using the CMEs of a polarizer and protecting films. The warpage of a liquid crystal display panel sometimes causes light leakage along the frame of the display panel due to contact of the display panel with the bezel of the frame. The warpage was analyzed according to the thermo-mechanical strain and the hygro-mechanical strain. The contact between the display panel and the bezel, which causes the light leakage, was estimated.