Near Ultraviolet Photostability of Liquid Crystal Mixtures

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Abstract
Liquid crystal material photostability was investigated for application in high intensity projectors operating with both near ultraviolet (UV) and visible light. Commercially available mixtures were studied in transparent test cells and their lifetimes were correlated with their chemical structures. Substantial improvements were observed in the photostability of hybrid field effect cells by using pure nematic mixtures whose major components had one or two cyclohexyl groups in place of phenyl groups. However. overriding the importance of these structural effects, polar impurities present in some of the nematic mixtures with cyclohexane groups caused a large decrease in photostability. Results were obtained by examining the photostability of eight different commercial liquid crystal mixtures, with highly positive dielectric anisotropy, whose components include a variety of structural types. Although the resistivity of each liquid crystal mixture decreased steadily with exposure, the reported end of life corresponded to the first observable change in the liquid crystal alignment toward a higher off-surface tilt. Near-UV/visible exposures (356-790 nm band) from a 900 W xenon lamp resulted in lifetimes of 3800 h for ZLI-1800 and 800 h for ZLI-1132 compared to only 230 h for a BDH-E7 cell.