New liquid-crystal formulations keep automobile touch displays working in extreme cold and heat
Three new LC mixtures operate from below -40° C to higher than 100° C.
Liquid-crystal (LC) displays have recently proliferated in automobiles -- in head-up displays, instrument cluster displays, navigation systems, and entertainment displays. However, today's LC display technology has trouble in cold, as well as very hot, weather -- the images blur and the displays are slow to respond in extreme temperatures.
Shin-Tson Wu, professor at the University of Central Florida--College of Optics and Photonics (CREOL; Orlando, FL), and his colleagues from the University of Central Florida, Xi'an Modern Chemistry Research Institute (Xi'an, China), and DIC Corporation (Tokyo, Japan) have developed three new LC mixtures that overcome previous physical limitations on upper and lower operation temperatures.1
According to Wu, the LCs should have a clearing point higher than 100° C and a melting point below -40° C. Beyond this range, the LC would be nonfunctional because it will be either frozen or isotropic.
Low viscosity at low temperatures
To keep their LCs operating over such a wide temperature range, the researchers mixed a dozen three-ring and four-ring compounds together with low-molecular-weight compounds. This type of mixture is considered a eutectic system. In addition to boosting the clearing point, the mixtures exhibit low viscoelastic coefficients and activation energies. These properties play a key role in maintaining low viscosity of the LCs at low temperatures, as the response time of a LC display device is mainly governed by the viscoelastic coefficient and the LC layer thickness.
Current European automotive standards require a response time for pixels to change from one brightness to another of 200 ms at -20° C and 300 ms at -30° C, which are insufficiently rapid to avoid an image blur. The response time reported by Wu and his colleagues is about 10 ms, or roughly 20 times faster than the European requirements. Additionally, these mixtures enable field-sequential color display at an elevated temperature, which results in a tripling in resolution density and display brightness. This approach improves the ambient contrast ratio of head-up displays in the daytime.
Future work for Wu and his colleagues includes developing extremely thin LC displays for integration with rear-view mirrors to eliminate blind spots for drivers, as well as improving the readability of all types of automotive displays in harsh sunlight.
1. F. Peng et al., Optical Materials Express 6, 717-726 (2016); doi: 10.1364/OME.6.000717