U.S. patent application number 10/365488 was filed with the patent office on 2003-10-09 for method of preparing an anisotropic polymer film on a substrate with a structured surface.
This patent application is currently assigned to Merck Patent GmbH. Invention is credited to Bleasdale, Tony, Gregory, Guy, Hammond-Smith, Robert, Howarth, Peter, Kuntz, Matthias, Marshall, Allan, Patrick, John, Riddle, Rodney.
Application Number | 20030189684 10/365488 |
Document ID | / |
Family ID | 28459444 |
Filed Date | 2003-10-09 |
United States Patent
Application |
20030189684 |
Kind Code |
A1 |
Kuntz, Matthias ; et
al. |
October 9, 2003 |
Method of preparing an anisotropic polymer film on a substrate with
a structured surface
Abstract
An anisotropic polymer film with improved alignmentis prepared
by coating onto a substrate with a structured surface. The polymer
films thereby obtained are useful in optical and electrooptical
devices for decorative and security applications.
Inventors: |
Kuntz, Matthias;
(Seeheim-Jugenheim, DE) ; Patrick, John; (Poole,
GB) ; Gregory, Guy; (Livingston, GB) ;
Bleasdale, Tony; (Chorley, GB) ; Riddle, Rodney;
(Poole, GB) ; Hammond-Smith, Robert; (Dammerham,
GB) ; Howarth, Peter; (Molton, GB) ; Marshall,
Allan; (Hawkhurst, GB) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Assignee: |
Merck Patent GmbH
Darmstadt
DE
|
Family ID: |
28459444 |
Appl. No.: |
10/365488 |
Filed: |
February 13, 2003 |
Current U.S.
Class: |
349/123 |
Current CPC
Class: |
B42D 2033/26 20130101;
Y10T 428/1041 20150115; Y10T 428/1036 20150115; C09K 2323/03
20200801; C09K 2323/00 20200801; Y10S 428/916 20130101; C09K
2323/031 20200801; B42D 25/29 20141001; C09K 2323/02 20200801; G02F
1/13363 20130101; B44F 1/10 20130101; Y10T 428/1005 20150115; B42D
25/364 20141001; G02B 5/3016 20130101; B42D 25/425 20141001; Y10T
428/10 20150115; B42D 2035/24 20130101 |
Class at
Publication: |
349/123 |
International
Class: |
G02F 001/1337 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 13, 2002 |
EP |
02003070.6 |
Claims
1. A method of preparing an anisotropic polymer film comprising a
polymerized liquid crystal material with planar orientation, said
process comprising: coating a polymerizable liquid crystalline or
mesogenic material onto a substrate with a structured surface,
aligning the polymerizable liquid crystalline or mesogenic
material, polymerizing the polymerizable liquid crystalline or
mesogenic material, and optionally removing the polymerized film
from the substrate.
2. A method according to claim 1, wherein the substrate surface
comprises an area of fine grooves or grating, by which the
molecules of said polymerizable liquid crystal material are
homogeneously aligned with their molecular long axes substantially
parallel to the substrate surface and substantially parallel to the
direction of said fine grooves or grating.
3. A method according to claim 2, wherein the direction of the fine
grooves or grating is substantially the same over the entire
substrate.
4. A method according to claim 2, wherein the substrate comprises
at least two areas having different direction of the fine grooves
or grating.
5. A method according to claim 2, wherein said grooves or grating
have a width of 0.2 to 2.0 .mu.m and a depth of 0.05 to 0.6
.mu.m.
6. A method according to claim 3, wherein said grooves or grating
have a width of 0.2 to 2.0 .mu.m and a depth of 0.05 to 0.6
.mu.m.
7. A method according to claim 4, wherein said grooves or grating
have a width of 0.2 to 2.0 .mu.m and a depth of 0.05 to 0.6
.mu.m.
8. A method according to claim 2, wherein said grooves or grating
have a symmetrical profile.
9. A method according to claim 2, wherein said grooves or grating
have an asymmetrical profile.
10. A method according to claim 2, wherein said grooves or grating
have a profile that is rectangular, square, saw tooth, triangular,
trapezoidal, sine wave or an approximation to these profiles.
11. A method according to claim 1, wherein the substrate comprises
a carrier layer.
12. A method according to claim 11, wherein the substrate comprises
one or more additional layers on the carrier layer.
13. A method according to claim 1, wherein the structured surface
of said substrate is formed by applying heat and pressure in
combination with micro-engraved shims to the substrate.
14. A method according to claim 1, wherein the structured surface
of said substrate is formed by interferography, photolithography,
embossing, ion beam etching, electron beam etching, ruling or cast
curing.
15. An anisotropic polymer film with planar alignment obtained by a
method according to caim 1.
16. In an optical or electrooptical devices, for decorative or
security applications, comprising an anisotropic polymer film, the
improvement wherein said film is according to claim 15.
17. In an optical retardation or compensation film, quarter wave
foil, or polariser comprising an anisotropic polymer film, the
improvement wherein said film is according to claim 15.
18. In a liquid crystal display comprising an anisotropic polymer
film, the improvement wherein said film is according to claim
15.
19. In a liquid crystal display comprising a retardation or
compensation film, quarter wave foil or polariser, the improvement
wherein said retardation or compensation film, quarter wave foil or
polariser is according to claim 17.
20. In a security marking or device comprising an anisotropic
polymer film, the improvement wherein said film is according to
claim 15.
21. In a hot stamping foil, laminate, label, data carrier, document
of value, ID or credit card, banknote, security thread, cheque or
CD comprising a security marking or device, the improvement wherein
said security marking or device is according to claim 20.
22. A method of aligning a liquid crystalline or mesogenic material
in a continuous coating process on a conveying substrate comprising
orienting the molecular long axes of the molecules of the liquid
crystalline or mesogenic material to be substantially parallel to
the plane of the substrate and at any desired angle to the
substrate moving direction by using a substrate with a structured
surface.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method of preparing an
anisotropic polymer film with improved alignment on a substrate
with a structured surface, to polymer films thereby obtained and
their use in optical and electrooptical devices, and for decorative
and security applications.
BACKGROUND AND PRIOR ART
[0002] Anisotropic polymer films comprising a polymerized liquid
crystal material with uniform orientation are known in prior art.
They are usually prepared by coating a thin layer of a
polymerizable liquid crystal mixture onto a substrate, aligning the
mixture into uniform orientation and polymerizing the mixture.
[0003] For specific applications it is required to induce planar
alignment in the liquid crystal layer, i.e., where the liquid
crystal molecules are oriented substantially parallel to the layer.
The alignment is then frozen in by polymerizing the liquid crystal
mixture in situ. For example, oriented films or layers of
polymerized nematic liquid crystal material with planar alignment
are useful as A-plate compensators or polarizers. Another important
application is oriented films or layers of polymerized cholesteric
liquid crystal material having twisted molecular structure. If the
cholesteric material has planar alignment, these films show
selective reflection of light where the reflection colour is
dependent on the viewing angle. They can be used, for example, as
circular polarizers, colour filters or for the preparation of
effect pigments for decorative or security applications.
Furthermore, patterned films are known comprising regions of
different orientation direction. These can be used in optical
elements as mentioned above for decorative purposes or in security
devices.
[0004] Planar alignment can be achieved, for example, by treatment
of the substrate onto which the liquid crystal material is coated.
The most usual method of surface treatment is to rub the substrate
surface prior to application of the liquid crystal material. In
case of rod-shaped liquid crystal molecules, these will align
themselves with their long axes parallel to the rubbing direction.
Alternatively, it is possible to apply an alignment layer, for
example of polyimide, to the substrate, which can then subsequently
be rubbed or which will induce the desired alignment. Other methods
are the application of shear forces or the addition of surface
active compounds to the liquid crystal material.
[0005] Reviews of conventional alignment techniques are given, for
example, by I. Sage in "Thermotropic Liquid Crystals", edited by G.
W. Gray, John Wiley & Sons, 1987, pages 75-77, and by T. Uchida
and H. Seki in "Liquid Crystals--Applications and Uses Vol. 3",
edited by B. Bahadur, World Scientific Publishing, Singapore 1992,
pages 1-63. A review of alignment materials and techniques is given
by J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981),
pages 1-77.
[0006] However, the methods of the prior art have several
drawbacks. The use of alignment layers or additives in the liquid
crystal material leads to increased costs. Rubbing of the substrate
or the application of shear forces are usually carried out in one
direction, so that the liquid crystal molecules will adopt planar
alignment into one preferred direction over the entire film. The
formation of patterned films comprising regions of different
orientation is difficult with these methods.
[0007] Other methods to prepare patterned films known in prior art
imply photoorientation or photoisomerisation of the liquid crystal
material. However, this requires the additional use of
photoisomerisable or photoorientating materials and of special
techniques like photomasking or photopolymerisation under linearly
polarised light.
[0008] Rubbing of a substrate also implies several drawbacks when
used in mass production of liquid crystal polymer films. Therein,
usually a flexible long film substrate is continuously unwound from
a roll and rubbed directly before being coated with a polymerizable
liquid crystal material,. which is then polymerized and may be
removed from the substrate. Whereas rubbing and thus alignment of
the liquid crystal molecules can easily be achieved by conveying
the substrate over a rubbing roller or between two rollers,
alignment at any desired angle to the substrate moving direction
requires more complicated rubbing stages, where the rollers can be
positioned at an angle to the conveying direction of the substrate.
Also, rubbing in a direction at right angles to the substrate
conveying direction is not possible with this method.
[0009] An aim of the present invention is to provide a method of
uniform alignment of liquid crystal material in the production of
polymer films, wherein this method does not have the drawbacks of
the prior art, allows alignment in any desired direction within the
film plane and also the formation of patterned films, and is
suitable for mass production and coating on a continuously
conveying substrate. Other aims are directly evident to the expert
from the following description.
[0010] The inventors have found that these aims can be achieved and
the above drawbacks can be overcome by using a substrate with a
structured surface, for example with a surface profile in the form
of gratings or fine grooves. A polymerizable liquid crystal
material coated onto the substrate will spontaneously align in the
direction of the grating, and the alignment can then be fixed by
polymerization. The surface profiles or gratings can be formed by
known techniques, like for example embossing, photolithography, or
interferography. The direction of the gratings can be freely chosen
at any desired angle, and patterned structures may also be
formed.
[0011] The use of substrates with a profiled or structured surface
has been described in prior art for the alignment of low molar mass
liquid crystals in switchable or thermochromic display devices. For
example, U.S. Pat. No. 4,834,500 discloses a method of aligning a
thermochromic cholesteric liquid crystal material between flexible
walls having a surface that is profiled with a series of fine
grooves and ridges, whereby the molecules of the cholesteric
material are aligned substantially parallel to the wall. U.S. Pat.
No. 5,724,113 describes a method to induce tilted alignment in a
nematic, smectic or cholesteric liquid crystal cell by providing an
alignment layer with an asymmetric surface grating onto the cell
walls. U.S. Pat. No. 5,754,264 discloses a method of surface
treatment to achieve a pretilt in a ferroelectric liquid crystal
cell by providing symmetrical or asymmetrical monogratings to the
surface of the cell walls. U.S. Pat. No. 5,764,325 discloses a
method to achieve surface alignment and surface tilt in a twisted
nematic liquid crystal cell by providing a grating of grooves with
an asymmetric profile to the surface of the cell walls. WO 97/14990
and WO 99/34251 describe a bistable liquid crystal cell wherein the
cell wall is provided with a surface alignment grating that permits
the liquid crystal molecules to adopt two different pretilt
angles.
[0012] However, these documents do not mention polymerizable liquid
crystal materials or the formation of polymer films with uniform or
patterned orientation in large scale production.
SUMMARY OF THE INVENTION
[0013] One object of the invention is to provide a method of
preparing an anisotropic polymer film comprising a polymerized
liquid crystal material with planar orientation, comprising the
steps of coating a polymerizable liquid crystalline or mesogenic
material onto a substrate with a structured surface, aligning the
material, polymerizing the material and optionally removing the
polymerized film from the substrate.
[0014] Another object of the invention is an anisotropic polymer
film obtained by a method according to the present invention.
[0015] Another object of the invention is the use of an anisotropic
polymer film according to the present invention in optical or
electrooptical devices, for decorative or security
applications.
[0016] Another object of the invention is a method of aligning a
liquid crystalline or mesogenic material in a continuous coating
process on a conveying substrate such that the molecules of the
liquid crystalline or mesogenic material are oriented with their
molecular long axes substantially parallel to the plane of the
substrate and at any desired angle to the substrate moving
direction, by using a substrate with a structured surface.
[0017] Another object of the invention is a security marking or
device comprising a polymer film according to the present
invention, and its application in hot stamping foils, laminates,
labels, data carriers or documents of value like ID or credit
cards, banknotes, security threads, cheques or CDs.
[0018] Upon further study of the specification and appended claims,
further objects and advantages of this invention will become
apparent to those skilled in the art.
DEFINITION OF TERMS
[0019] The term `substrate` as used in this application refers to
any underlying layer or substrate.
[0020] The term `film` as used in this application includes
self-supporting, i.e., free-standing, films that show more or less
pronounced mechanical stability and flexibility, as well as
coatings or layers on a supporting substrate or between two
substrates.
[0021] The term `liquid crystal or mesogenic material` or `liquid
crystal or 'mesogenic compound` should denote materials or
compounds comprising one or more rod-shaped, board-shaped or
disk-shaped mesogenic groups, i.e., groups with the ability to
induce liquid crystal phase behaviour. The compounds or materials
comprising mesogenic groups do not necessarily have to exhibit a
liquid crystal phase themselves. It is also possible that they show
liquid crystal phase behaviour only in mixtures with other
compounds, or when the mesogenic compounds or materials, or the
mixtures thereof, are polymerized.
[0022] For the sake of simplicity, the term `liquid crystal
material` is used hereinafter for both liquid crystal materials and
mesogenic materials, and the term `mesogen` is used for the
mesogenic groups of the material.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Preferably, the substrate surface onto which the
polymerizable liquid crystal material is coated comprises an area
of fine grooves or grating, by which the molecules of said
polymerizable liquid crystal material are homogeneously aligned
with their molecular long axes substantially parallel to the
substrate surface and substantially parallel to the direction of
said fine grooves or grating. In terms of substantially parallel,
the difference in the direction of the molecular axis and the plane
of the substrate surface and the direction of the axis of the
grooves is preferably 0-10 degrees, very prepferably 0-2 degrees
and especially 0-1 degrees.
[0024] The direction of the grooves or grating can be the same over
the entire substrate. It is also possible that the substrate
comprises different areas, or a specific pattern or image
comprising different areas, having different directions of the
grooves.
[0025] The grooves can have a symmetrical or asymmetrical profile.
They can have, for example, a profile that is rectangular, square,
saw tooth, triangular, trapezoidal, sine wave or an approximation
to these profiles.
[0026] The grating or grooves have typical dimensions of a width of
0.2 to 2.0, preferably 0.8 to 1.2 .mu.m, and a depth of 0.05 to
0.6, preferably 0.25 to 0.4 .mu.m.
[0027] The substrate used for preparing the inventive film may
comprise a carrier layer made of paper or polymers like for
example, polypropylene, polyethylene terephthalate, triacetyl
cellulose, or co-polymers thereof.
[0028] The substrate may be isotropic or anisotropic. For example,
it is possible to use a birefringent substrate comprising a
uniaxially or biaxially stretched or compressed film of the above
mentioned materials.
[0029] The substrate may be removed from the polymerised liquid
crystal material after polymerisation for example by using a
release layer consisting of a mineral wax, natural wax, or other
materials known in the art. Other release transfer methods may also
be used.
[0030] The substrate may comprise of one or more additional layers
on the surface of the carrier layer. These layers may be
thermoplastic, thermosetting, or actinic radiation-curable
materials, like for example, but not limited to, polyacrylate,
vinyl polymer, polystyrene, polyamide, epoxy, or any copolymers
thereof.
[0031] The polymerizable liquid crystal material coated on the
substrate can be covered with a second substrate. The second
substrate may have a structured surface or not. In case the second
substrate has a structured surface with a fine grating, the
dimensions, profile or direction of this may be different from or
identical to the first substrate in selected areas or over the
entire second substrate.
[0032] The grating can be provided to the substrate by any method
known to those skilled in the art for this purpose, as described,
for example, in U.S. Pat. No. 5,754,264 or in the references cited
therein. Suitable methods are, for example, embossing, cast curing,
or application of a profiled layer of a photopolymer formed by an
interferographic or photolithographic process.
[0033] In a preferred embodiment of the present invention, a fine
grating is prepared by embossing the substrate surface. This can be
achieved by applying heat and pressure to the substrate in
combination with a metal and preferably nickel shim micro-engraved
with the desired image or pattern. These shims can also be made out
of other materials.
[0034] Preferred embodiments of the invention relate to a method
wherein
[0035] the direction of the fine grooves or grating is
substantially the same over the entire substrate.
[0036] the substrate comprises at least two areas having different
direction of the fine grooves or grating.
[0037] the grooves or grating have a width of 0.2 to 2.0 .mu.m and
a depth of 0.05 to0.6 .mu.m.
[0038] the grooves or grating have a symmetrical profile.
[0039] the grooves or grating have an asymmetrical profile.
[0040] the grooves or grating have a profile that is rectangular,
square, saw tooth, triangular, trapezoidal, sine wave or an
approximation to these profiles.
[0041] the substrate comprises a carrier layer of a polymer or
paper.
[0042] the substrate comprises a carrier layer in combination with
one or more other layers containing thermoplastic, thermosetting,
or actinic radiation-curable materials.
[0043] the structured surface of said substrate is formed by
applying heat and pressure in combination with shims to the
substrate.
[0044] the structured surface of said substrate is formed by
interferography, photolithography, embossing, ion beam etching,
electron beam etching, ruling or cast curing.
[0045] In another preferred embodiment the embossed film is
prepared on a carrier film of, for example, polyester,
polypropylene or polyethylene. Preferably, the carrier film is a
polyester film, like, e.g., the commercially available
Hostaphan.RTM. ( Mitsubishi Polyester Film) or Melinex.RTM. (from
Du Pont.). The carrier film thickness is typically in the range
from 8 to 175 .mu.m, depending on the use of the final anisotropic
film.
[0046] In another preferred embodiment the embossed film is a hot
stamping foil or comprises a part of a hot stamping foil.
[0047] In another preferred embodiment a reflective and opacifying
layer is applied, for example, by vacuum deposition of a metal,
preferably aluminium, onto the upper surface of the polymerizable
liquid crystal layer, in a thickness of typically 100 to 500 .ANG.,
preferably 125 to 250 .ANG..
[0048] In a further preferred embodiment the reflective layer is
added by laminating the polymerizable liquid crystal layer to a
reflective layer, like for example the metal surface of a
metallised polyester film, by means of, e.g., a pressure sensitive
adhesive.
[0049] In a further preferred embodiment the reflective layer
comprises one or more reflective pigments as a continuous layer or
a printed design on another substrate.
[0050] In a further preferred embodiment of the present invention
the anisotropic polymer film comprising the polymerized liquid
crystal material is prepared by a continuous fabrication or
manufacturing process. The continuous manufacturing process
comprises at least one of the steps of embossing the substrate,
coating, aligning and polymerizing the polymerizable liquid crystal
material, applying additional layers like the embossable layer,
release layer, reflective or opacifying layer or adhesive layer,
and optionally removing one or both of the substrates. Preferably,
at least the steps of coating, aligning and polymerizing the liquid
crystal material, and optionally the steps of embossing the
substrate and applying additional layers, are carried out in a
continuous manufacturing process.
[0051] In a further preferred embodiment the layers containing the
aligned polymerizable liquid crystal material are printed by a hot
stamping process, and the carrier layer is removed. The hot
stamping process is known in the art. The stamped films are
especially useful for decorative or security purposes, like product
or identification labels or security markings on documents of value
like bank notes or ID cards.
[0052] The polymerizable liquid crystal material is preferably
coated onto the structured surface of the substrate as a thin layer
with a thickness of preferably 0.2 to 50 .mu.m. The liquid crystal
material can be applied by any suitable surface coating or printing
technique that is known to the skilled in the art, like, for
example, spin coating, gravure coating or printing, flexographic
coating or printing, offset coating or printing or Meyer bar
coating. The polymerizable liquid crystal material can also be
dissolved in a solvent, preferably in an organic solvent. The
solution is then coated onto the substrate, for example, by
spin-coating or other known techniques, and the solvent is
evaporated off. In most cases it is desirable to heat the mixture
in order to facilitate the evaporation of the solvent.
[0053] The polymerizable liquid crystal material is preferably a
nematic, smectic, chiral nematic (cholesteric) or chiral smectic
material. Nematic materials are especially preferred. In case of a
cholesteric material, preferably a substrate or surface comprising
a light absorbing material, like a dark or black substrate, is
used.
[0054] In another preferred embodiment the liquid crystal material
is a nematic liquid crystal material, preferably with a pattern of
at least two areas having different alignment directions. Such a
film is transparent when viewed at normal conditions, but shows a
pattern of interference colours when viewed through a linear
polariser against a reflective background or when viewed between
two polarisers.
[0055] The above embodiments are particularly suitable for use as
false-proof security markings on banknotes or documents of value,
by which the document is easy to authenticate when viewed through
one polariser against a dark or reflective background or when
viewed between two polarisers.
[0056] The liquid crystal material of the anisotropic polymer film
is preferably a polymerizable or crosslinkable material that is
polymerised or crosslinked during or after evaporation of the
solvent. It preferably comprises at least one polymerisable
mesogenic compound having one polymerisable functional group and at
least one polymerisable mesogenic compound having two or more
polymerisable functional groups.
[0057] If the polymerisable LC material comprises polymerisable
mesogenic compounds having two or more polymerisable functional
groups (di-or multireactive or di-or multifunctional compounds),
upon polymerisation a three-dimensional polymer network is formed,
which is self-supporting and shows a high mechanical and thermal
stability and a low temperature dependence of its physical and
optical properties. By varying the concentration of the
multifunctional mesogenic or non mesogenic compounds the crosslink
density of the polymer film and thereby its physical and chemical
properties such as the glass transition temperature, which is also
important for the temperature dependence of the optical properties
of the polymerised film, the thermal and mechanical stability or
the solvent resistance can be tuned easily.
[0058] The polymerisable mesogenic mono-, di- or multireactive
compounds can be prepared by methods which are known per se and
which are described, for example, in standard works of organic
chemistry such as, for example, Houben-Weyl, Methoden der
organischen Chemie, Thieme-Verlag, Stuttgart. Typical examples are
described, for example, in WO 93/22397; EP 0 261 712; DE 19504224;
DE 4408171 and DE 4405316. The compounds disclosed in these
documents, however, are to be regarded merely as examples that do
not limit the scope of this invention.
[0059] Examples representing especially useful monoreactive
polymerisable mesogenic compounds are shown in the following list
of compounds, which should, however, be taken only as illustrative
and is in no way intended to restrict, but instead to explain the
present invention: 1
[0060] Examples of useful direactive polymerisable mesogenic
compounds ate shown in the following list of compounds, which
should, however, be taken only as illustrative and is in no way
intended to restrict, but instead to explain the present invention
2
[0061] In the above formulae, P is a polymerisable group,
preferably an acryl, methacryl, vinyl, vinyloxy, propenyl ether,
epoxy or styryl group, x and y are each independently 1 to 12, A is
1,4-phenylene that is optionally mono- di or trisubstituted by
L.sup.1 or is 1,4-cyclohexylene, v is 0 or 1, Z.sup.0 is --COO--,
--OCO--, --CH.sub.2CH.sub.2-- or a single bond, Y is a polar group,
Ter is a terpenoid radical like, e.g., menthyl, Chol is a
cholesteryl group, R.sup.0 is a nonpolar alkyl, or alkoxy group,
and L.sup.1 and L.sup.2 are each independently H, F, Cl, CN or an
optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl
or alkoxycarbonyloxy group with 1 to 7 C atoms.
[0062] The term `polar group` in this connection means a group
selected from F, Cl, CN, NO.sub.2, OH, OCH.sub.3, OCN, SCN, an
optionally fluorinated carbonyl or carboxyl group with up to 4 C
atoms or a mono- oligo- or polyfluorinated alkyl or alkoxy group
with 1 to 4 C atoms. The term `nonpolar group` means an alkyl group
with 1 or more, preferably 1 to 12 C atoms or an alkoxy group with
2 or more, preferably 2 to 12 C atoms.
[0063] In case CLC materials are used, these preferably comprise a
nematic or smectic host material and one or more chiral dopants
that induce a helical twist in the host material. The chiral
dopants can be polymerisable or not. They can be mesogenic or
liquid crystal compounds, but do not necessarily have to be liquid
crystalline.
[0064] Especially preferred are chiral dopants with a high helical
twisting power (HTP), in particular as disclosed in WO 98/00428.
Further typically used chiral dopants are, e.g., the commercially
available S 1011, R 811 or CB 15 (from Merck KGaA, Darmstadt,
Germany).
[0065] Very preferred are chiral dopants selected from the
following formulae 3
[0066] including the (R,S), (S,R), (R,R) and (S,S) enantiomers not
shown,
[0067] wherein E and F have each independently one of the meanings
of A given above, v is 0 or 1, Z.sup.0 is --COO--, --OCO--,
--CH.sub.2CH.sub.2-- or a single bond, and R is alkyl, alkoxy,
carbonyl or carbonyloxy with 1 to 12 C atoms.
[0068] The compounds of formula II are described in WO 98/00428,
the compounds of formula III synthesis are described in GB
2,328,207, the entire disclosure of which is incorporated into this
application by reference.
[0069] Polymerisable chiral compounds are preferably selected from
the above formulae Ik to Ip, and IIc to IIe. It is also possible to
use compounds of formula Ia to Ii wherein R.sup.0 or Y comprise a
chiral C atom.
[0070] The amount of chiral dopants in the liquid crystal material
is preferably less than 15%, in particular less than 10%, very
preferably less than 5% by weight of the total LC material (without
the solvent).
[0071] Polymerisation of the polymerisable liquid crystal material
takes place by exposing it to heat or actinic radiation. Actinic
radiation means irradiation with light, like UV light, IR light or
visible light, irradiation with X-rays or gamma rays or irradiation
with high energy particles, such as ions or electrons. Preferably,
polymerisation is carried out by UV irradiation. As a source for
actinic radiation for example a single UV lamp or a set of UV lamps
can be used. When using a high lamp power the curing time can be
reduced. Another possible source for actinic radiation is a laser,
like, e.g., a UV laser, an IR laser or a visible laser.
[0072] The polymerisation is carried out in the presence of an
initiator absorbing at the wavelength of the actinic radiation. For
example, when polymerising by means of UV light, a photoinitiator
can be used that decomposes under UV irradiation to produce free
radicals or ions that start the polymerisation reaction. When
curing polymerisable mesogens with acrylate or methacrylate groups,
preferably a radical photoinitiator is used, when curing
polymerisable mesogens vinyl and epoxide groups, preferably a
cationic photoinitiator is used. It is also possible to use a
polymerisation initiator that decomposes when heated to produce
free radicals or ions that start the polymerisation. As a
photoinitiator for radical polymerisation for example the
commercially available Irgacure 651.RTM., Irgacure 184.RTM.,
Darocur 1173.RTM. or Darocur 4205.RTM. (all from Ciba Geigy AG) can
be used, whereas in case of cationic photopolymerisation the
commercially available UVI 6974.RTM. (Union Carbide) can be used.
The polymerisable LC material preferably comprises 0.01 to 10%,
very preferably 0.05 to 5%, in particular 0.1 to 3% of a
polymerisation initiator. UV photoinitiators are preferred, in
particular radicalic UV photoinitiators.
[0073] The curing time is dependant, inter alia, on the reactivity
of the polymerisable mesogenic material, the thickness of the
coated layer, the type of polymerisation initiator and the power of
the UV lamp. The curing time according to the invention is
preferably not longer than 10 minutes, particularly preferably not
longer than 5 minutes and very particularly preferably shorter than
2 minutes. For continuous production short curing times of 3
minutes or less, very preferably of 1 minute or less, in particular
of 30 seconds or less, are preferred.
[0074] The polymerisable liquid crystal material can additionally
comprise one or more other suitable components such as, for
example, catalysts, sensitisers, stabilisers, inhibitors,
co-reacting monomers, surface-active compounds, lubricating agents,
wetting agents, dispersing agents, hydrophobing agents, adhesive
agents, flow improvers, defoaming agents, deaerators, diluents,
reactive diluents, auxiliaries, colourants, dyes or pigments.
[0075] In particular, the addition of stabilisers is preferred in
order to prevent undesired spontaneous polymerisation of the
polymerisable material for example during storage. As stabilisers
in principal all compounds can be used that are known to the
skilled in the art for this purpose. These compounds are
commercially available in a broad variety. Typical examples for
stabilisers are 4-ethoxyphenol or butylated hydroxytoluene
(BHT).
[0076] Other additives, like, e.g., chain transfer agents, can also
be added to the polymerisable material in order to modify the
physical properties of the resulting polymer film. When adding a
chain transfer agent, such as monofunctional thiol compounds like,
e.g., dodecane thiol or multifunctional thiol compounds like, e.g.,
trimethylolpropane tri(3-mercaptopropionate), to the polymerisable
material, the length of the free polymer chains and/or the length
of the polymer chains between two crosslinks in the inventive
polymer film can be controlled. When the amount of the chain
transfer agent is increased, the polymer chain length in the
obtained polymer film is decreased.
[0077] It is also possible, in order to increase crosslinking of
the polymers, to add up to 20% of a non mesogenic compound with two
or more polymerisable functional groups to the polymerisable
material alternatively or in addition to the di- or multifunctional
polymerisable mesogenic compounds to increase crosslinking of the
polymer. Typical examples for difunctional non-mesogenic monomers
are alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1
to 20 C atoms. Typical examples for non-mesogenic monomers with
more than two polymerisable groups are trimethylolpropane
trimethacrylate or pentaerythritol tetraacrylate.
[0078] In another preferred embodiment the mixture of polymerisable
material comprises up to 70%, preferably 3 to 50% of a
non-mesogenic compound with one polymerisable functional group.
Typical examples for monofunctional non-mesogenic monomers are
alkylacrylates or alkylmethacrylates.
[0079] It is also possible to add, for example, a quantity of up to
20% by weight of a non-polymerisable liquid-crystalline compound to
adapt the optical properties of the resulting polymer film.
[0080] The polymerisation is preferably carried out in the liquid
crystal phase of the polymerisable material. Therefore, preferably
polymerisable mesogenic compounds or mixtures with low melting
points and broad liquid crystal phase ranges are used. The use of
such materials allows a reduction of the polymerisation
temperature, which makes the polymerisation process easier and is a
considerable advantage especially for continuous production. The
selection of suitable polymerisation temperatures depends mainly on
the clearing point of the polymerisable material and, inter alia,
on the softening point of the substrate. Preferably, the
polymerisation temperature is at least 30 degrees below the
clearing temperature of the polymerisable mesogenic mixture.
Polymerisation temperatures below 120.degree. C. are preferred.
Especially preferred are temperatures below 90.degree. C., in
particular temperatures of 60.degree. C. or less.
[0081] The anisotropic layer obtained by the inventive process can
be used in optical elements, like reflective polarisers,
retardation films, compensators, colour filters or holographic
elements, especially in reflective films with patterned optical
properties, in adhesives, synthetic resins with anisotropic
mechanical properties, for the preparation of liquid crystal
pigments, in decorative and security applications, especially in
security markings that are applied to items or documents of value
for easy identification or prevention of falsification, in
nonlinear optics, optical recording or information storage.
[0082] The anisotropic polymer film according to the present
invention is especially useful as security marking for
identification and prevention of copying or counterfeiting of high
value documents like ID cards, bank notes, share certificates, etc.
The anisotropic polymer film can be either included in a laminate
or adhesively bound to the surface of the document or as a
transparent or watermark area.
[0083] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilise the
present invention to its fullest extent. The following examples
are, therefore, to be construed as merely illustrative and not
limited by the remainder of the disclosure in any way
whatsoever.
[0084] In the foregoing and in the following examples, unless
otherwise indicated, all temperatures are set forth uncorrected in
degrees Celsius and all parts and percentages are by weight.
[0085] The entire disclosure of all applications, patents and
publications, cited above and below, and of corresponding European
Application No. 02003070.6, filed Feb. 13, 2002 is hereby
incorporated by reference.
EXAMPLES
Example 1
[0086]
1 The following polymerisable mixture is formulated Toluene 70.0%
compound A 6.9% compound B 3.6% compound C 9.1% compound D 8.5%
Fluorad FC-171 .RTM. 0.1% Irgacure 907 .RTM. 1.8% (A) 4 (B) 5 (C) 6
(D) 7
[0087] Compound (A) and its preparation are described in GB
2,280,445. Compounds (B), (C) and (D) can be prepared according to
or in analogy to the methods described in D. J. Broer et al.,
Makromol. Chem. 190, 3201-3215 (1989). Irgacure 907 is a
commercially available photoinitiator (Ciba Geigy). Fluorad FC 171
is a commercially available surfactant (3M).
[0088] This solution is coated on to an embossed film, where the
embossing is in a single direction, using for example a wire wound
bar to give a 6 micron wet film. This coating is then dried at
60.degree. C. for one minute and cured by exposing to the light
from a medium pressure mercury lamp for one minute. When viewed
through a linear polariser against a reflective background a dark
blue colour is seen.
Example 2
[0089] The solution of Example 1 is coated on to an embossed film
where the embossing is in domains having differing directions using
a wire wound bar to give a 6 micron wet film. This coating is then
dried at 60.degree. C. for one minute and cured by exposure to the
light from a medium pressure mercury lamp for one minute. When
viewed through a rotating linear polariser against a reflective
background a dark blue colour is seen only in regions having the
appropriate alignment.
[0090] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0091] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various conditions and usages.
* * * * *