Novel versatile pressure-sensitive adhesives for polarizing film of TFT-LCDs: Viscoelastic characteristics and light leakage performance

Abstract

One of the main problems in thin film transistor liquid crystal displays (TFT-LCD) is a phenomenon called “light leakage”, seriously affecting black–white contrast and color brightness. It occurs due to a thermal shrinkage of the polarizing film in TFT-LCDs, caused by a heat from the backlight unit. A pressure-sensitive adhesive (PSA) used for assembling of the polarizing film to the TFT-LCD panel can relieve the stress and minimize the light leakage. PSAs are designed specifically for a certain size LCDs, and cannot be used for another size LCD because of poorer light leakage. Obviously, there is a certain necessity to develop a universal PSA applicable simultaneously for LCDs with different sizes, such as mobile phones, digital cameras, navigation systems, computer monitors, HDTVs, etc. In this paper, we introduced N-vinyl pyrrolidone as a comonomer to a conventional copolymer of butyl acrylate, acrylic acid, and 2-hydroxy ethyl methacrylate. It resulted in a higher elasticity modulus and a higher shrinkage resistance of the PSA. A significant reduction of the light leakage to some extend was observed at increase of the hardener content for 2.5 and 7 in samples. However, it was accompanied by deterioration of the peel strength below spec requirements. Lowering molecular weight of the polymers from 626,000 to 205,000 Da resulted in excellent light leakage of both small and large specimens. Dynamic mechanical analysis confirmed that zero light leakage was achieved through a synergy of two mechanisms: high modulus, i.e. resistance to thermo-induced shrinkage, and high stress relaxation of lower molecular weight polymers. The results of this work are perspective for creation of a universal PSA for polarizing film in TFT-LCDs of different sizes.