Nematic liquid crystal mixtures for bistable display device

A nematic liquid crystal medium with weak zenithal anchoring energy for bistable display devices, comprising:

• a) at least 40% by weight of compounds with strong molecular polarisability (Δε>10) and strong dipole moment (μ>1D), however with weak zenithal anchoring, the compounds including:
• AI compounds selected from the group consisting of 4-pentyl-4′-cyanobiphenyl, 4-heptyl-4′-cyanobiphenyl, 3-fluoro-4-cyanophenyl-4′-ethylbenzoate, 3-fluoro-4-cyanophenyl-4′-propylbenzoate, 3-fluoro-4′-cyanophenyl-4′-butylbenzoate and 3-fluoro-4-cyanophenyl-4′-pentylbenzoate;
• and
• AII or AIV compounds selected from the group consisting of 4-cyanophenyl-4′-trans-butyl-cyclohexylbenzoate, 3-fluoro-4-cyanophenyl-4′-trans-pentyl-cyclohexylbenzoate and 4-pentyl-trans-cyclohexyl-benzoate-4′-phenyl-carboxylate of 4-cyano-3-fluorobenzene
• b) 0 to 50% by weight of compounds with weak anchoring selected from the group consisting of 4-pentylphenyl-4′-pentylcyclohexyl-carboxylate, 4-pentylphenyl-4′-propylcyclohexylcarboxylate, 4,4′-bis(4-propyl-cyclohexyl)-3-fluorobiphenyl, 4,4′-bis(4-propyl-cyclohexyl)-biphenyl, 4-pentylphenyl-4′-propylcyclohexylcarboxylate, 4-pentylphenyl-4′-propylphenylcarboxylate, 4-pentylphenyl-4′-pentylphenylcarboxylate, 4-propylphenylethynyl-4′-butylphenyl and 4-hexylcarboxyphenylethynyl-4′-hexylcarboxylphenyl;
• wherein the relative proportions of the above-mentioned compounds used in the composition of the nematic liquid crystal mixture are determined to simultaneously obtain the following physical characteristics for the said mixture:
• a nematic-isotropic liquid transition temperature (TN-I) of the liquid crystal mixture greater than or equal to +50° C.,
• a nematic range ΔTTN of the liquid crystal mixture greater than or equal to 50° C.,
• a positive dielectric anisotropy larger than or equal to 8×10−11 F/m (value measured at a temperature of +20° C.).
• a weak zenithal anchoring on at least one of the plates of the bistable display, characterised by a breaking voltage Uλ/2>25 volts for a cell with thickness d such that Δn d =295 nm (value measured at a temperature of +20° C.).