Thermochromic liquid crystalline medium

Details Thermochromic liquid crystalline medium Thermochromic liquid crystalline medium

2002-09-19

2003-12-09

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C428S001300, C252S299010, C252S299610, C252S299620, C252S299640, C252S299670, C252S299700, C106S031030, C106S031130, C106S031720, C106S031960

Reexamination Certificate

active

06660345

ABSTRACT:

The invention relates to an improved thermochromic liquid crystalline medium. The invention also relates to a thermochromic ink comprising a thermochromic liquid crystalline medium encapsulated in a light transmissive polymeric material. The invention further relates to the use of a thermochromic liquid crystalline medium or ink in decorative applications like pigments, inks and paints, cosmetics, thermodiagnostic applications like medical thermography, thermometry, optical and electrooptical applications, and security applications and devices. The invention also relates to a security marking or device comprising a thermochromic liquid crystalline medium or thermochromic ink, in particular a security marking or device comprising two thermochromic liquid crystalline media or inks that differ in their chirality to produce a hidden effect, and to a document of value comprising such a security marking. Further aspects of the invention are discussed below.
BACKGROUND OF THE INVENTION
Thermochromic liquid crystal materials, such as thermochromic compounds or compositions are characterized in that they show a visible colour change upon temperature variation. One well known class of thermochromic materials are specific types of cholesteric liquid crystals (CLCs). CLCs exhibit a helically twisted molecular structure wherein the pitch p of the molecular helix is related to the reflected wavelength &lgr; and the average refractive index n of the liquid crystal by equation (1)
&lgr;=
n·p
  (1)
In thermochromic CLCs the reflection wavelength &lgr; shows a significant temperature dependence. If the reflected wavelength is inside the visible range, the thermochromic CLC material undergoes a visible colour change upon variation of the temperature. As the cholesteric liquid crystals are optically birefringent, they typically do not reflect a single wavelength, but a narrow band of wavelengths wherein the bandwidth &Dgr;&lgr; is defined according to equation (2)
&Dgr;&lgr;=&Dgr;
n·p
  (2)
with &Dgr;n being the birefringence of the liquid crystal material.
The above described thermochromic effect is exploited in a wide range of applications, such as decorative applications like pigments, inks or paints, cosmetics, thermodiagnostic applications like medical thermography, thermometry, optical and electrooptical applications, and security markings and devices.
For the above mentioned applications, typically cholesteric liquid crystalline compositions are used comprising achiral and chiral liquid crystalline or mesogenic compounds, which are encapsulated as small droplets in light transmissive polymer microcapsules, for example of gelatine or gum arabic, or in a continuous matrix of a transparent binder polymer.
In order to exhibit thermochromic behaviour, the cholesteric liquid crystalline composition should exhibit a cholesteric phase and an underlying smectic phase, i.e. a smectic phase below the temperature range of the cholesteric phase.
Thermochromic compositions are disclosed for example in EP 0 386 198, EP 0 423 293, EP 0 564 959 and GB 2 280 681.
However, the thermochromic compositions of prior art have several drawbacks. Thus, the thermochromic materials known in the prior art do often exhibit thermochromic colour play only in a narrow temperature range.
Therefore, to achieve a satisfying thermochromic effect in prior art thermochromic mixtures often a large amount of chiral components is needed, which in some cases make up more than 50% by weight of the total mixture.
On the other hand, it is generally desired to reduce the amount of chiral components in a liquid crystal mixture for several reasons. Thus, chiral compounds do often reduce the mesophase range and clearing point of a liquid crystal mixture. Furthermore, the preparation of chiral compounds with high enantiomeric purity is often difficult and expensive. Also, chiral compounds for liquid crystalline mixtures are usually not available in the same broad variety like e.g. nematic compounds. Thus, when the chiral compounds make up a large part of the mixture it is difficult to vary the composition of the mixture in order to adapt its properties to specific requirements. Finally, due to the presence of high amounts of chiral compounds it is often difficult to achieve mixtures with a high birefringence.
Another feature of CLC's is their ability to reflect circularly polarised light. The state of polarisation of light reflected depends upon the “handedness” of the helical structure formed within the thermochromic liquid crystalline medium, which is itself determined by the structure of the chiral compound(s) used in the mixture.
SUMMARY OF THE INVENTION
The above mentioned effect can be exploited in security markings and devices by providing a “hidden” effect. By using two thermochromic liquid crystalline media that differ only by their chirality, i.e. by the total twist sense of their chiral component that can be made up by one or more chiral compounds, a hidden design could be produced when the device is viewed through a suitable circular polariser.
Thus, it was an aim of the invention to provide thermochromic media which show a colour change over a wide temperature range, including room temperature or temperatures close to room temperature, and which do not contain high amounts of chiral compounds.
The inventors have found that the above aim can be achieved by providing thermochromic media according to the present invention. The inventive media are characterized in that they comprise a liquid crystalline host mixture with a nematic phase and an underlying smectic phase, to which a small amount of an optically active component is added, wherein said optically active component comprises one or more chiral dopants with a high helical twisting power (HTP).
The HTP of a chiral dopant describes its ability to induce or enhance a helical twist in a medium consisting of a liquid crystalline host mixture and the dopant. The HTP is defined by equation (3)
HTP
=(
p·c
)
−1
  (3)
wherein p is the helical pitch p (in &mgr;m) and c the concentration (in % by weight) of the chiral dopant in the medium.
The inventors have found that, when using chiral compounds with a high HTP of at least 20 &mgr;m
−1
already in small amounts in a liquid crystal host mixture, it is possible to obtain a thermochromic medium with a wide temperature range of the thermochromic colour play. Furthermore it was found that by using these dopants, it is possible to provide a thermochromic medium wherein the width and position of the temperature range of the thermochromic colour play can easily be controlled by careful selection of the amount and HTP of the chiral compounds and of the smectic-nematic phase transition temperature (T
S-N
) of the host mixture.
One object of the invention is a thermochromic liquid crystalline medium comprising a liquid crystalline host mixture and an optically active component, characterized in that the liquid crystalline host mixture has a smectic—nematic phase transition temperature in the range from about 0 to 60° C. and the optically active component comprises at least one chiral dopant having a helical twisting power (HTP) of at least 20 &mgr;m
−1
.
Another object of the invention is a thermochromic ink comprising a thermochromic liquid crystalline medium as described above encapsulated in a light transmissive polymeric material, such as a continuous polymer matrix or polymer microcapsules.
Another object of the invention is the use of a thermochromic medium or thermochromic ink in decorative applications like pigments, inks and paints, cosmetics, thermodiagnostic applications like medical thermography, thermometry, optical and electrooptical applications, and security applications and devices.
Another object of the invention is a security marking or device comprising a thermochromic liquid crystalline medium or thermochromic ink according to the present invention.
Another object of the invention is a document of value comprising a security marking or device com

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