Cholesteric flakes

Details Cholesteric flakes Cholesteric flakes

1998-08-14

2001-03-27

Michl, Paul R. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymerizing in presence of inert solid masses so as to...

C106S493000, C252S299010

Reexamination Certificate

active

06207770

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to cholesteric polymer flakes obtainable from a chiral polymerizable mesogenic material. The invention also relates to methods of manufacturing such cholesteric flakes.
The invention further relates to the use of certain chiral and achiral polymerizable compounds with one or more terminal polymerizable groups for the manufacturing of such flakes.
The invention also relates to the use of such cholesteric flakes as effect pigments in spraying or printing inks or paints or colored plastics for different applications, especially for automotive use, cosmetic products and security applications.
BACKGROUND OF THE INVENTION
Cholesteric liquid crystals exhibit a helically twisted molecular orientation resulting in special optical properties. When a cholesteric LC is irradiated with unpolarized light, interaction of the helix structure with incident light of a selected wavelength will result in reflection of 50% of its intensity as circularly polarized light of a given handedness (left-handed or right-handed according to the handedness of the helix) while the other 50% are transmitted as circularly polarized light of the opposite handedness. The wavelength &lgr; of the reflection maximum depends on the pitch p of the helix and the average refractive index n of the cholesteric LC material according to the following formula:
&lgr;=
n.p
Since the color effect of cholesteric optical materials is based on selective light reflection and not on absorption like in conventional dyes or pigments, extraordinary color-properties can be obtained like for example higher color saturation, wider color range and iridescent appearance. These materials exhibit a unique reflection pattern, because the reflected wavelength will change if the incident light propagates through the cholesteric LC at an angle to the direction of the helix axis.
However, to achieve good color properties when applied in inks or paints, a uniform alignment of the cholesteric LC with the orientation of the helix axis parallel to the viewing direction is required. Furthermore, low molar mass cholesteric LC's are best used in most applications in the liquid state if they are confined to small capsules or droplets. In this case the temperature dependence of the reflected wavelength is another problem. On the other hand polymeric cholesteric LC's which are used in the solid state have to be aligned above their glass transition or melting temperature respectively which requires high temperatures. Both embodiments are therefore inconvenient for production and limited in their applications.
By making flakes or platelets of prealigned cholesteric polymer material several problems of the prior art can be circumvented. To produce such flakes a cholesteric polymer material is coated onto a substrate and aligned to achieve uniform orientation of the helical axis normal to the surface. The film is then cured and ground to yield small flat flakes which can be dispersed e.g. in a transparent binder for the use as inks or paints. These inks can be used at room temperature without the need of further alignment.
PRIOR ART
Such polymer flakes have been described earlier. In U.S. Pat. No. 5,364,557 flakes based on cholesteric LC polysiloxanes are disclosed. However, the alignment of an LC polymer as described there is difficult to achieve and has to be carried out above the glass transition, which requires high temperatures (120 to 150°C.) and optionally auxiliary alignment means such as electric or magnetic fields.
Patent application WO 94/22976 describes flakes made by coating two separate films of cholesteric LC polysiloxanes which are aligned at high temperatures, optionally crosslinked and subsequently laminated together or are coated on different sides of a base polymer plate. As an alternative low molar mass cholesteric LC's with high melting points are described which have to be cooled down rapidly after alignment to obtain an oriented glass. However, these methods require high temperature changes and, as the application is confined to laminae of cholesteric materials, imply a complicated production process with many subsequent steps.
Furthermore, both documents describe the preferred use of prefabricated cholesteric LC side chain polysiloxanes. As it is known to the skilled in the art such polymers are usually synthesized by attaching mesogenic side chains in a polymer analogous addition reaction to a polysiloxane backbone which has been polymerized in advance. Since the optical properties and thus the color appearance of the polymer flakes are depending mainly on the ratio and the chemical structure of the mesogenic side chains, they are already determined in the polymer prior to the flake preparation. On the other hand the mechanical properties of the flakes are heavily influenced by the chain length and the crosslink density of the polymer, which are fixed during synthesis of the polysiloxane backbone and/or during flake production. It is therefore difficult to control all physical and material properties of the pigment flakes obtained in this way.
The German Application DE 4,419,239 describes cholesteric pigment flakes made of three dimensional chiral polymer networks and containing polysiloxanes with cholesterol side chains and methacrylate groups as crosslinking agent However, besides cholesterol no other chiral groups are disclosed. Furthermore, the polymer material also has to be prepared in two subsequent polymerization steps as described above.
SUMMARY OF THE INVENTION
Thus the aim of this invention is to provide cholesteric flakes for use as pigments that can be made in a very simple manner which also enables easy and direct control of the optical and mechanical properties of the product.
It was now found that this can be achieved by using certain polymerizable mesogenic materials and a process according to the present invention.
The term “flakes” as it is used throughout the claims and the description of this invention comprises small size particles with dimensions of 1 &mgr;m to 2 mm. For example, these particles can be granules of a symmetric or unsymmetric shape, or platelets having average lateral dimensions several times larger than the thickness, or mixtures of both platelets and granules.
One of the objects of the invention are cholesteric flakes obtainable from a chiral polymerizable mesogenic material by a process including the following steps:
(a) coating said material onto a substrate which is then optionally covered by a second substrate,
(b) aligning the coated material into a planar orientation,
(c) curing the aligned material into a polymer film,
(d) removing the polymer film from the substrate, and
(e) grinding it, optionally under cooling.
In a preferred embodiment of the present invention the chiral polymerizable mesogenic material comprises at least two polymerizable mesogenic compounds exhibiting at least one terminal poymerizable group that is linked, optionally via a spacer group, to a mesogenic core and is selected from the following formulae:
CH
2
═CW—COO—  I1
WCH═CH—O—  I2
CH
2
═CH—Ph—(O)
n
—I3
in which W denotes H, CH
3
or Cl and n is 0 or 1.
In another preferred embodiment of the invention the chiral polymerizable mesogenic material comprises at least two polymerizable mesogenic compounds, wherein each of said compounds exhibits a polymerizable group of the formulae I1 to I4 that is different from at least one other compound.
In another preferred embodiment of the present invention the chiral polymerizable mesogenic material is comprising at least one achiral polymerizable mesogenic compound and at least one chiral polymerizable mesogenic compound, wherein at least one of these compounds exhibits two or more polymerizable groups.
In another preferred embodiment of the invention at least one of the polymerizable mesogenic compounds is a fumarate.
In another preferred embodiment the achiral polymerizable mesogenic compound exhibits two or more polymerizable groups.
In another preferred embodiment the chi

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