GB2395201A

GB2395201A - Broadband reflective film

Info

Publication number: GB2395201A
Application number: GB0321689A
Authority: GB
Inventors: Owain Llyr Parri; Kim Slaney; Shirley Ann Marden; Tara Perrett
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2002-09-24
Filing date: 2003-09-16
Publication date: 2004-05-19
Anticipated expiration: 2023-09-16
Also published as: GB2395201B; KR101114641B1; GB0321689D0; KR20040026632A

Abstract

The invention relates to a process for preparing a broadband reflective film comprising polymerised chiral liquid crystal material with high birefringence, to a reflective film obtainable by said process, to the use of the reflective film in optical, electrooptical, information storage, decorative and security applications, and to a liquid crystal display comprising the reflective film. The reflective film is characterised by the polymerisable liquid crystal (LC) material having a birefringence W n greater than or equal to 0.2. The polymerisable LC material may comprise mono-, di- or multireactive chiral and/or achiral mesogenic compounds.

Description

239520 1

- 1 Broadband Reflective Film Field of the Invention

5 The invention relates to a process for preparing a broadband reflective film comprising polymerised chiral liquid crystal material with high birefringence. The invention further relates to a reflective film obtainable by such a process. The invention further relates to the use of the reflective film in optical, electrooptical, information storage, 10 decorative and security applications, and to a liquid crystal display comprising the reflective film.

Background and Prior Art

15 Reflective films comprising polymerized liquid crystal (LC) material have been proposed in prior art for a variety of uses, inter alla for use

as broadband or notch polarisers, as colour filters in displays or projection systems, and for decorative purposes like e.g. for the preparation of coloured image films or cholesteric pigment flakes.

20 They typically comprise one or more layers of polymerised cholesteric LC (CLC) material having a helically twisted orientation wherein the helix axis is perpendicular to the film plane, and show selective reflection of circular polarised light.

25 For application e.g. as broadband reflective polariser in LC displays, it is desirable that the bandwidth of the reflective film comprises a substantial portion of the visible wavelength range, whereas for an application as notch polariser or as coloured reflective film e.g. for decorative or security applications, often films having a specific 30 reflection colour are desired.

In particular broadband reflective polarisers, also known as circular polarisers, which are transmitting circularly polarised light of a broad wavelength band covering a large part of the visible spectrum, are 35 suitable as polarisers for backlit liquid crystal displays. If unpolarised light is incident on such a reflective polariser, 50% of the light

- 2 intensity are reflected as circularly polarised light with the same twist sense as that of the molecular helix, whereas the other 50% are transmitted. The reflected light is depolarized (or its sense of polarization is reversed) in the backlight of the display, and is 5 redirected onto the polariser. In this manner theoretically 100% of a given waveband of the unpolarised light incident on the reflective polariser can be converted into circularly polarised light. The circularly polarised light can be converted into linear polarised light by means of a quarter wave film (QWF) optical retarder and 10 optionally also a compensation film.

The bandwidth AX of the waveband reflected by a broadband reflective film as described above is given by the birefringence An of the CLC material and the pitch p of the cholesteric helix according to 15 the following equation = An x p A simple, but neither very effective nor economic way to provide a 20 broadband reflective polariser is to stack several reflective films with different reflection wavebands on top of each other.

There are also methods described in prior art for the direct

preparation of broadband reflective polarisers from a CLC precursor, 25 including methods to create a gradient of the pitch p within the film and methods utilizing LC materials with high birefringence An, both of which leads to a broadening of the waveband according to the abvove equation. However, the methods disclosed in prior art so far

do have various drawbacks.

EP 0 606 940 and Broer et al. Nature, Vol. 378, pp. 467 (1995), disclose a circular reflective polariser with a bandwidth of up to 400 nm and methods of its manufacture. These methods exploit the diffusion of polymerisable LC compounds, also known reactive 35 mesogens (RM), with different reactivity and chirality during polymerization of a LC film, and the use of a dye to give a gradient of

curing power. As a result a polymerised CLC film is obtained wherein the cholesteric pitch p varies continuously from one edge to the other. However, this process is rather slow and in some cases even takes several minutes to complete. This is incompatible with most 5 methods to fabricate polarisers on continuously moving substrates such as plastic films.

A process for the production of broadband reflective films with varying pitch on plastic substrates is described in the WO 97/35219, 10 US 6,099, 758 and EP 0 982 605. Though this process is completed in the order of 15 to 30 seconds, and is thus faster than that used by Broer et al., it is nevertheless still relatively difficult with respect to the control of the resultant reflection wavelength and bandwidth of the reflective polariser. For example, the process described in US 15 6,099,758 requires different substrates or oxygen barrier layers, and the process described in EP 0 982 605 uses a material showing an LO phase transition during polymerization.

Furthermore, the broadband reflective films described in prior art

20 comprise CLC materials with a low birefringence that is typically below 0.18. This means that the films need to have a large thickness, typically of more than 10 microns, in order to show efficient reflection over the whole visible spectrum. Films of this thickness are typically very difficult to align from a single surface. On the other hand, 25 aligning a film between two substrates with shearing can lead to problems with uniformity of thickness.

JP 2000-281629 describes the use of direactive ditolanes with a high birefringence in a polymerisable CLC mixture to produce a 30 broadband reflective film. In this case the broad waveband is formed as a consequence of the high birefringence An of the CLC material used, and obviously no further band broadening occurs during polymerization. However, the achieved bandwidth of 250 nm as described in the examples of JP 2000-281629 is still not large 35 enough for many applications, as it does not cover the entire visible spectrum. Furthermore, the use of a high amount of direactive

-4 compounds with high birefringence, like the ditolanes described in JP 2000-281629, can lead to inhomogeneous alignment in the film.

The aim of the present invention is to provide a method of preparing 5 broadband reflective films from a polymerisable LC material that allows better and more easy control of the reflection wavelength and the bandwidth of the film, good alignment of the LC molecules and gives thin films with uniform alignment, high birefringence and a broad reflection waveband, and allows easy and economic 10 fabrication even at large scales.

Another aim of the present invention is to provide a broadband reflective film that does not have the drawbacks of films known from prior art, in particular a film that has low thickness, high

15 birefringence, uniform alignment and a broad reflection waveband that ideally covers the entire visible spectrum.

Another aim of the invention relates to the advantageous use of such a broadband reflective film in optical, electrooptical, information 20 storage, decorative and security applications.

Another aim of the present invention is to provide a liquid crystal displays comprising a broadband reflective film as described above and below.

Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.

The above aims can be achieved by providing a broadband reflective 30 film and methods for its preparation as described in the present invention. Summar,v of the Invention 35 The invention relates to a process of preparing a reflective film with varying pitch from a chiral polymerisable liquid crystal (LC) material,

- 5 characterized in that the polymerisable LC material has a birefringence An greater than or equal to 0.2.

The invention further relates to a reflective film obtained by the 5 process as described above and below.

The invention further relates to a reflective film with varying pitch obtained from a chiral polymerisable liquid crystal (LC) material having a birefringence An greater than or equal to 0.2.

The invention further relates to a reflective film with varying pitch comprising a polymerized chiral liquid crystal (LC) material having a birefringence An greater than or equal to 0.2.

15 The invention further relates to the use of reflective film as described above and below as reflective broadband or notch polariser or as a multicoloured film or image.

The invention further relates to the use of reflective film as described 20 above and below in liquid crystal displays, as colour filter, in effect pigments, for decorative or security applications.

The invention further relates to a liquid crystal display comprising a reflective film as described above and below.

The invention further relates to an authentification, verification or security marking or a multicoloured image comprising a reflective film as described above and below.

30 The invention further relates to an object or document of value comprising an authentification, verification or security marking or an image as described above and below.

Definition of Terms

- 6 In connection with films as described in the present application, the following definitions of terms as used throughout this application are given. 5 The term 'firm' es used in this application includes selfsupporting, 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.

10 The term 'liquid crystal or mesogenic materiel' 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. Liquid crystal compounds with rod-shaped or 15 board-shaped groups are also known in the art as 'calamitic' liquid crystals. Liquid crystal compounds with a disk-shaped group are also known in the art as'discotic' liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they 20 show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.

For the sake of simplicity, the term 'liquid crystal material' is used 25 hereinafter for both liquid crystal materials and mesogenic materials, and the term 'mesogen' is used for the mesogenic groups of the material.

The term 'director' is known in prior art and means the preferred

orientation direction of the long molecular axes (in case of calamitic 30 compounds) or short molecular axis (in case of discotic compounds) of the mesogens in a liquid crystal material.

The term 'helically twisted structure' relates to a film comprising one or more layers of liquid crystal material wherein the mesogens are 35 oriented with their main molecular axis in a preferred direction within molecular sublayers, said preferred orientation direction in different

- 7 sublayers being twisted at an angle around a helix axis. The term helically twisted structure with planar orientation' means a film with helically twisted structure as described above, wherein the helix axis is substantially perpendicular to the film plane, i.e. substantially 5 parallel to the film normal. This definition includes orientations of the helix axis from 75 to 90 , preferably 80 to 90 , very preferably 85 to 90 and most preferably 88 to 90 relative to the film plane.

10 Detailed Descrintion of the Invention The reflective film is prepared from a polymerisable CLC material following general methods as described in WO 97/35219, US 6,099,758 and EP 0 982 605. In the method according to the present 15 invention, the CLC material is preferably coated on a plastic substrate, like e.g. PET or TAC, aligned into planar orientation and polymerized, preferably under an inert gas like e.g. nitrogen, to give a broad band film, preferably without the use of a top laminate and preferably without the use of an oxygen barrier film like e.g. PVA on 20 the substrate. The removal of the need to coat on a barrier layer, as well as the removal of the lamination step significantly improves the process of manufacturing the reflective film.

The CLC material is then polymerised e.g. by irradiation with a UV 25 lamp. The broad waveband is formed by a helix unwinding mechanism. This results in a helix distribution in the film with the side furthest away from the radiation source having a short pitch and reflecting e.g. blue light, and the side closest to the radiation source having a long pitch and reflecting e.g. red light. The advantage of 30 having a helix distribution is that the polymerised CLC film can reflect the necessary part of the visible spectrum without the need for using polymerisable mesogenic compounds with extremely high birefringence. Such compounds tend to be coloured and/or unstable to light and/or heat.

- 8 The polymerisable LC material is preferably a cholesteric or chiral smectic LC material, in particular a cholesteric LC material, and preferably comprises at least one monoreactive polymerisable mesogenic compound and at least one di- or multireactive 5 polymerisable mesogenic compound.

Especially preferably the LC material comprises at least one mono-, di- or multireactive achiral polymerisable mesogenic compound and at least one chiral compounds that is mono-, di- or multireactive or 1 0 non- reactive.

Further preferably the LC material comprises at least one monoreactive polymerisable mesogenic compound with a birefringence of at least 0.2. Preferably 60% or more of the 15 polymerisable mesogenic compounds used in the CLC material are monoreactive mesogenic compounds, like e.g. monoacrylates, that have a birefringence of at least 0.2, preferably in the range from 0.2 to 0.27, and are selected to give good planar alignment.

20 The following embodiments are especially preferred: - An of the CLC material is > 0.22, preferably > 0.25, - An of the CLC material is preferably from 0.2 to 0.4, very preferably from 0.2 to 0.3, 25 - the CLC material comprises 5-50% di- or multireactive mesogenic compounds, preferably diacrylates, - the CLC material comprises 30-90% monoreactive mesogenic compounds, preferably monoacrylates, 30 - the CLC material comprises at least 60 % monoreactive mesogenic compounds, preferably monoacrylates, with a birefringence of at least 0.2, preferably from 0.2 to 0.4, very preferably from 0.2 to 0.3, - the CLC material comprises one or more chiral compounds with a 35 HTP >30, which may be non-reactive or contain one or more reactive groups, and may be mesogenic or not,

- 9 - - the CLC material comprises 0.2-6% of one or more photoinitiators, - the CLC material optionally comprises 0.2-5% of a dye which absorbs at the same wavelength as the radiation used for curing, 5 preferably a UV dye, - the CLC material optionally comprises 0.05-1% of a material used to promote alignment, preferably one or more surface active agents, 10 - the CLC material is polymerised in its cholesteric phase, - the CLC material has a cholesteric phase in the range from 40 C or less to 80 C or more, preferably from 30 C or less to 95 C or more, very preferably from 20 C or less to 11 0 C or more, 15 - the CLC material is polymerized under an inert gas atmosphere, preferably under nitrogen, - the CLC material is polymerised on a substrate, preferably a polyester film, at an open surface, without the use of a top laminate, 20 - the substrate is a PET or TAC film, - during polymerization the waveband of the CLC material is broadened by a helix unwinding mechanism, such that the helical pitch in different regions of the CLC material increases at a 25 different extent, - the thickness of the reflective film is from 1 to 10 microns, preferably from 2 to 8 microns, very preferably from 3 to 7 microns, - the thickness of the reflective film is less than 6 microns.

- the polymerized cholesteric material in the reflective film has an asymmetrical distribution of the helical pitch, wherein preferably the pitch varies from a minimum value at one surface of the film to a maximum value at the opposite surface of the film,

- 10 Polymerisable mesogenic mono-, di- and multireactive compounds used for the instant invention 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, 5 Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

Typical examples are described for example in WO 93/22397, EP O 261 712, DE 195 04 224, WO 95/22586, WO 97/00600 and GB 2 351 734. The compounds disclosed in these documents, however, are to be regarded merely as examples that do not limit the scope of 10 this invention.

Examples representing especially useful and preferred mono- and direactive polymerisable mesogenic compounds are shown in the following list of compounds: P-(CH2)xO COO /3 R (la) 20 P-(CH2)xO COO R (lb) 25 P(cH2)xO coo OCO R (IC) P-(CH2)XO COO <} R (Id) P-(CH2)xO COO R (le) 35 P-(CH2)xO z zOq;< R (If)

- 11 P(CH2)x (COO)V: R (Ig) P(CH2)X-O R

v (Ih) 10 P(CH2)XO {}Z R (1i) {L)r lL)r 15 P(CH2)xo{}Z =: RO (Ik) P-(CH2) xO CH=CH-COO R (I m) P-(CH2)XO COO CH2CH(CH3)C2H5

(In) P-(CH2)XO COO COO CH2CH(CH3)C2H5

P-(CH2)x COO-Ter 30 (IP)

P-(CH2)xo 3 COO-Chol (Iq) 35 P-(CH2)Xo Coo ' (Ir)

- 12 P(CH2)x O̳ cod) r (Is) L1 L2 P(CH2)x coo oco o(cH2)yP (I la) L1 L2 P(CH2)xO CH2CH2 CH2CH2 O(CH2)yP 15 L1 2

POCO26O2CO̳ (IIC) 20 P-(CH2)XO CH=CHCO(:

0 ooccH=cH (CH2)y-p (I Id) P-(CH20O:H (CH2)yP In the above formulae, P is a polymerisable group, preferably an acr,vl, methacryl, vinyl, vinyloxy, propenyl ether, epoxy or styrene group, x and y are each independently 1 to 12, A and D are 1,4 phenylene that is optionally monodi or trisubstituted by L' or 1,4 35 cyclohexylene, u and v are 0 or 1, Z is -COO-, -OCO-, -CH2CH2-, CH=CH-, -C-C- or a single bond, R is a polar group or an unpolar

- 13 group, Ter is a terpenoid radical like e.g. menthyl, Chol is a cholesterol group, L, L, and L2 are each independently H. F. Cl, ON or an optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 C atoms, and 5 r is 0, 1, 2, 3 or 4. The phenyl rings are optionally substitute by 1, 2, 3 or 4 groups L. The term 'polar group' in this connection means a group selected from F. Cl, Br, I, ON, NO2, OH, OCH3, OCN, SON, an optionally 10 fluorinated alkoxy, alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with up to 3 C atoms or a mono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms.

The term 'unpolar group' means an alkyl group with 1 or more, 15 preferably 1 to 12 C atoms, an alkenyl group with 2 or more, preferably 2 to 8 C atoms or an alkoxy, alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 4 or more, preferably 4 to 12 C atoms.

A preferred polymerisable liquid crystal mixture comprises 20 - 5 - 50 %, preferably 5 - 35 %, very preferably 5 - 25 % of one or more direactive achiral mesogenic compounds, - 30 - 95 % preferably 50 - 85 % of one or more monoreactive achiral mesogenic compounds, wherein at least 60 %, based on 25 the total mixture, are compounds with a birefringence of at least 0.2, preferably from 0.2 to 0.3, - 0.1 to 10 %, preferably 0.5 to 5 % of one one or more chiral compounds which may be mono-, di- or multireactive or non reactive,and may be mesogenic or not.

30 and optionally further comprises one or more of the following components - 0.2-6% of one or more photoinitiators, - 0.2-5% of a dye which absorbs at the same wavelength as the 35 radiation used for curing, preferably a UV dye, - 0.05-1% of a material used to promote alignment, preferably one

- 14 or more surface active agents.

The monoreactive achiral compounds are preferably selected from above formulae la-li, in particular la, If, lg. Ih, li and Ik.

The monoreactive compounds with high birefringence are preferably selected from above formulae Ih, li and Ik.

Further preferred are monoreactive compounds with a birefringence 10 of at least 0.2 and one or more terminal nonreactive groups selected from H. F. Cl, Br, l, ON, optionally fluorinated C,3 alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy, C2 alkenyl, C alkinyl and the polar groups as defined above, preferably from F. Cl, ON, CH3, C2H5, n-C3H7, OCH3, OC2H5, CHO, COCH3, COOCH3, CF3, 15 OCF3, CH=CH2, and C-C-CH3, very preferably CH3, C2H5 and OCH3.

Particularly preferred compounds of this type are those of formula Ih, li and Ik, wherein R and L are terminal nonreactive groups as defined in the previous sentence and r is 0,1 or 2.

20 Highly birefringent compounds with long terminal alkyl groups (greater than three methylene units) or branched terminal alkyl groups are less preferred.

The direactive achiral compounds are preferably selected from 25 above formulae lla and lib, in particular lla.

Especially preferred are mixtures comprising one or more polymerisable compounds comprising an acetylene or tolane group with high birefringence, like compounds of formula Ih, li and Ik.

30 Further suitable and preferred polymerisable tolanes are described in GB 2,351,734, EP 02008229.3, EP 02008230.1 and EP 02008231.9.

For the preparation of planar films with helically twisted structure, the polymerisable LC material preferably comprises one or more achiral 35 polymerisable mesogenic compounds and at least one chiral compound. The chiral compound can be selected from non

- 15 polymerisable chiral compounds, like e.g. conventional chiral dopants, or polymerisable chiral compounds, all of which can be mesogenic or non-mesogenic.

5 Suitable polymerisable chiral compounds are for example those of above formulae Im-lr and llc-lle. Further suitable chiral polymerisable compounds are e.g. the commercially available Paliocolour 6) materials (from BASE AG, Germany).

10 Suitable nonreactive chiral dopants can be selected e.g. from the commercially available R or S 811, R or S 1011, R or S 2011 or CB 15 (from Merck KGaA, Darmstadt, Germany).

Very preferred are chiral compounds with a high helical twisting 15 power (HTP), in particular compounds comprising a sorbitol group as described in WO 98/00428, compounds comprising a hydrobenzoin group as described in GB 2,328,207, chiral binaphthyl derivatives as described in WO 02/94805, chiral binaphthol acetal derivatives as described in WO 02/34739, chiral TADDOL derivatives as described 20 in WO 02/06265, and chiral compounds having at least one fluorinated linkage group and a terminal or central chiral group as described in WO 02/06196 and WO 02/06195.

The polymerisable material is preferably dissolved or dispersed in a 25 solvent, preferably in an organic solvent. The solution or dispersion is then coated onto the substrate, for example by spin-coating or other known techniques, and the solvent is evaporated off before polymerization. In most cases it is suitable to heat the mixture in order to facilitate the evaporation of the solvent.

The polymerisable LO material may additionally comprise a polymeric binder or one or more monomers capable of forming a polymeric binder and/or one or more dispersion auxiliaries. Suitable binders and dispersion auxiliaries are disclosed for example in WO 35 96/02597. Especially preferred, however, are LO materials not containing a binder or dispersion auxiliary.

- 16 ln another preferred embodiment the polymerisable LC material comprises an additive that induces or enhances planar alignment of the liquid crystal material on the substrate. Preferably the additive 5 comprises one or more surfactants. Suitable surfactants are described for example in J. Cognard, Mol.Cryst.Liq.Cryst. 78, Supplement 1, 1-77 (1981). Particularly preferred are non-ionic surfactants, very fluorocarbon surfactants, like for example the commercially available fluorocarbon surfactants Fluorad FC-171<g) 10 (from 3M Co.), or Zonyl FSN hi) (from DuPont).

Polymerisation of the LC material is preferably achieved by exposing it to actinic radiation. Actinic radiation means irradiation with light, like UV light, IR light or visible light, irradiation with X-rays or gamma rays 15 or irradiation with high energy particles, such as ions or electrons.

Preferably polymerization is carried out by photoirradiation, in particular with UV light. 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 20 source for photoradiation is a laser, like e.g. a UV laser, an IR laser or a visible laser.

Polymerisation is carried out in the presence of an initiator absorbing at the wavelength of the actinic radiation. For example, when 25 polymerizing by means of UV light, a photoinitiator can be used that decomposes under UV irradiation to produce free radicals or ions that start the polymerization reaction. UV photoinitiators are preferred, in particular radicalic UV photoinitiators. As standard photoinitiator for radical polymerization for example the commercially 30 available Irgacureg) 907,1rgacure@) 651, Irgacure) 184, Darocure@) 1173 or Darocure@) 4205 (all from Ciba Geigy AG) can be used, whereas in case of cationic photopolymerisation the commercially available UVI 6974 (Union Carbide) can be used.

35 The polymerisable LC material can additionally comprise one or more other suitable components such as, for example, catalysts, sensitizers,

- 17 stabilizers, chain-transfer agents, 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, 5 colourants, dyes or pigments.

In another preferred embodiment the mixture of polymerisable material comprises up to 70%, preferably 1 to 50 % of a monoreactive non-mesogenic compound with one polymerisable 10 functional group. Typical examples are alkyl acrylates or alkyl methacrylates with alkyl groups of 1 to 20 C atoms.

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 15 polymerisable functional groups to the polymerisable LC material alternatively or in addition to the di- or multireactive polymerisable mesogenic compounds to increase crosslinking of the polymer.

Typical examples for direactive non-mesogenic monomers are alkyl diacrylates or alkyl dimethacrylates with alkyl groups of 1 to 20 C 20 atoms. Typical examples for multireactive non-mesogenic monomers are trimethylpropane trimethacrylate or pentaerythritol tetraacrylate. It is also possible to add one or more chain transfer agents to the 25 polymerisable material in order to modify the physical properties of the polymer film. Especially preferred are thiol compounds, such as monofunctional thiol compounds like e.g. dodecane thiol or multifunctional thiol compounds like e.g. trimethylpropane tri(3 mercaptopropionate), very preferably mesogenic or liquid crystalline 30 thiol compounds. When adding a chain transfer agent, 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 decreasing.

- 1 8 Suitable substrates for polymerization are for example polyester such as polyethyleneterephthalate (PET) or polyethylenenaphthalate (PEN), polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC), especially preferably PET or TAC.

The inventive reflective films are useful as broadband or notch polarisers, in particular as broadband reflective polarisers in liquid crystal displays, as optical or colour filters in displays or projectionsystems, as decorative image, for the preparation of liquid crystal or 10 effect pigments, and especially as reflective film with spatially varying reflection colours, e.g. as multicolour image for decorative, information storage or security uses, such as non-forgeable documents like identity or credit cards, banknotes etc 15 Preferably the broadband reflective film exhibits an asymmetrical structure of the helical pitch, with the pitch substantially increasing from a smaller, preferably a minimum, value at one edge of the film to a higher, preferably a maximum, value at the opposite edge of the film, in a direction perpendicular to the layer. Especiall preferably the 20 reflective film has a reflection wavelength in the blue visible region at one surface and a reflection wavelength in the red visible region at the opposite surface.

Compared e.g. to a film with a symmetrical variation of the pitch and 25 reflection wavelength, e.g. blue - red - blue, throughout the film thickness, a film with asymmetrical pitch variation is advantageous because it is easier to compensate, gives better off axis brightness and lower colour change. Furthermore, a reflective polariser with asymmetric pitch variation according to the present invention can be 30 made as thinner film compared to a reflective polariser film with the same bandwidth and symmetric pitch variation, because the former exhibits only one complete pitch gradient (e.g. blue - red) within the film thickness, whereas the latter exhibits two gradients (e.g. blue red - blue). For example, in case of an asymmetric film having a 35 thickness of about 10 microns, a symmetric film with approximately the same bandwidth exhibits a thickness of 15 to 20 microns.

- 1 9 However, thinner films are generally preferred for use in flat panel displays, and are also cheaper as they need less material.

In order to create linear polarised light, e.g. when used in a liquid 5 crystal display, a reflective polariser according to the present invention is preferably used in combination with an optical retardation film. The optical retardation film is comprising a layer of a birefringent material selected such that its optical retardation is approximately 0.25 times the wavelength of the centre of the bandwidth reflected by the broadband 10 reflective polarizer. As a result, this retarder serves as a quarter wave plate or foil (QWF) which converts circular polarised light into linear polarised light. As a QWF for example a stretched plastic film, such as stretched PET, PVA, PC or TAC can be used. It is also possible to use a film of an oriented polymerized liquid crystal material.

It is also possible to use a combination of two or more optical retardation layers, the retardation of these layers being selected in such a manner that due to the difference in retardation of the layers the nett retardation of the combination is approximately 0.25 times 20 the wavelength of the light reflected by the broadband reflective polariser over a substantial portion of the reflected bandwidth of the polariser. This combination of layers is then used as a QWF together with the inventive reflective polariser, in order to compensate the optical dispersion of the single QWFs.

It is also possible to use the broadband reflective polariser in combination with a compensation film in order to compensate the viewing angle dependence of the phase retardation of light transmitted by the reflective polariser. Suitable compensation films 30 are for example those having homeotropic orientation, e.g. as described in WO 98/00475, those having twisted orientation, those having planar orientation, e.g. as described in WO 98/04651, or those having tilted or splayed orientation, e.g. as described in US 5,619,352, WO 97/44409, WO 97/44702, WO 97/44703 and WO 35 98/12584, the entire disclosure of these documents being

incorporated into this application by way of reference.

- 20 It is also possible to use the broadband reflective polariser in combination with one or more compensation films having an extraordinary optical axis perpendicular to the film plane and 5 negative birefringence, also known as negative C plates, e.g. as described in WO 01/20393 the entire disclosure of which is

incorporated into this application by way of reference.

It is also possible to use the broadband reflective polariser in 10 combination with one or more linear polarisers.

The reflective film according to the present invention can be used as broadband reflective polariser in displays of the transmisive or reflective type. It can be used in conventional LCDs, in particular 15 those of the DAP (deformation of aligned phases) or VA (vertically aligned) mode, like e.g. ECB (electrically controlled birefringence), CSH (colour super homeotropic), VAN or VAC (vertically aligned nematic or cholesteric) displays, MVA (multi-domain vertically aligned) or PVA (patterned vertically aligned) displays, in displays of 20 the bend mode or hybrid type displays, like e.g. OCB (optically compensated bend cell or optically compensated birefringence), R OCB (reflective OCB), HAN (hybrid aligned nematic) or pi-cell (-cell) displays, furthermore in displays of the TN (twisted nematic), HTN (highly twisted nematic) or STN (super twisted nematic) mode, in 25 AMD-TN (active matrix driven TN) displays, or in displays of the IPS (in plane switching) mode which are also known as 'super TFT' displays. Especially preferred are VA, MVA, PVA, OCB and pi-cell displays. 30 The examples below serve to illustrate the invention without limiting it. In the foregoing and the following, all temperatures are given in degrees Celsius, and all percentages are by weight, unless stated otherwise. Unless stated otherwise, the bandwidth of the reflected waveband is given as the full width half maximum (FWHM), as 35 illustrated in Figure 2.

-21 Comparative ExamPIe The following polymerisable liquid crystalline mixture was formulated 5 Compound (1) 30.0 % compound (2) 26.0 % compound (3) 26.0 % compound (4) 10.5 % Paliocolour LC756 O 5.0 % 10 compound (5) 1.3% Irgacure 651 A) 0.7% FC171 6) 0.5%

go ': (1) o'\' COW IN (2) 25 F 0

0 0 (3) 30 0 Woo ook (4)

- 22 400b HI N NO (5) Compounds (1) and (2) can be prepared according to or in analogy to the methods described in D.J.Broer et al., Makromol.Chem. 190, 10 3201-3215 (1989). Compound (3) and its preparation are described in EP 0 972 818. The direactive compound (4) can be prepared as described in WO 93/22397. Paliocolour LC756 is a commercially available chiral polymerisable material (from BASE AG, Ludwigshafen, Germany). Irgacure 651 is a commercially available 15 photoinitiator (from Ciba AG, Basel, Switzerland). FC 171 is a commercially available non-ionic fluorocarbon surfactant (from 3M).

The birefringence An of the mixture was measured as 0.123.

20 The mixture was dissolved in a suitable organic solvent (e.g. xylene 50% w/w solids). The mixture was coated onto rubbed PET to give a 4 micron film when dried. The film was annealed for 1 minute at 85 C. The film was placed in a nitrogen atmosphere and irradiated at 0.3mWcm2 at a constant temperature of 80 C for 2 minutes.

25 The transmission spectrum measured with left and right handed circularly polarised light of the resulting film was recorded and is shown in Figure 1. The reflection band is narrow with multiple peaks.

Examole 1 The following polymerisable liquid crystalline mixture was formulated Comopund (6) 28.0 % compound (12) 27.3 % 35 compound (4) 10.5 % compound (10) 28.0 %

- 23 Paliocolour LC756 tE) 5.0 % Irgacure 651 O 0.7 % FC171 E) 05%

5 / 0

6om,0:,05 (1 o) o om-o','-o'b (1 2) The polymerisable mesogenic tolanes (10) and (12) can be prepared as described in GB 2 351 734.

The mixture has a birefringence An of 0.277.

The mixture was dissolved in a suitable organic solvent (eg xylene 50% w/w solids). This was coated onto rubbed PET to give a 4 micron film when dried. The film was annealed for 1 minute at 85 C.

The film was placed in a nitrogen atmosphere and irradiated at 25 0. 3mWcm2 at a constant temperature of 80 C for 2 minutes.

The transmission spectrum measured with left and right handed circularly polarised light of the resulting film was recorded and is shown in Figure 2. The bandwidth is approximately 340 nm.

30 Example 2

A liquid crystalline mixture was made as in example 1. The mixture was dissolved in an organic solvent and films made as described n example 1 with various thicknesses d as shown in the table below.

1 Film 1 2a 1 2b 1 2c 1 2d 1 2e 1 2f 129 1 2h

- 24 thickness (pm) 1 1.1 1 1.9 1 3.2 1 4 0 1 4.8 1 5.5 1 6.5 1 7.2 1 The transmission spectrum measured with circularly polarised light of the films were recorded and is shown in Figure 3. It can be seen that 5 reflection efficiency increases as the film thickness increases.

Example 3

The following polymerisable liquid crystalline mixture was formulated Compound (6) 38.8 % compound (7) 17.0 % compound (8) 17.0 % compound (4) 20.5 % 15 Paliocolour LC756 O 5.0 % compound (5) 0.5 % Irgacure 651 0.7 % FC171 (g) 0.5 % go-WoK o (6) 30 (7)

o /o, -- 'b 35 (8)

- 25 The polymerisable mesogenic tolanes (6), (7) and (8) can be prepared as described in GB 2 351 734.

The mixture was found to have a birefringence An of 0.255.

The mixture was dissolved in a suitable organic solvent (eg xylene 50% w/w solids). The mixture was coated onto rubbed PET to give a 4 micron film when dried. The film was annealed for 1 minute at 85 C. The film was placed in a nitrogen atmosphere and irradiated 10 at 0.5mWcm2 at a constant temperature of 90 C for 2 minutes.

The transmission spectrum measured with left and right handed circularly polarised light of the resulting film was recorded and is.

shown in Figure 4. The bandwidth is approximately 415 nm.

Example 4

The following polymerisable liquid crystalline mixture was formulated 20 Compound (6) 38.8 % compound (7) 17.5 % compound (4) 20.5 % compound (8) 18.0 % compound (9) 3.5 % 25 compound (5) 0.5 % Irgacure 651 @) 0.7 % FC171 @) 0.5 %

30 CH3O {a} COO {a co:? O - OOC {a} OOC {>OCH3 (9) The chiral dopant (9) and its preparation are described in WO 98/00428.

- 26 The mixture has a birefringence An of 0.261.

The mixture was dissolved in a suitable organic solvent (eg xylene 5 50% w/w solids). This was coated onto rubbed PET to give a 4 micron film when dried. The film was annealed for 1 minute at 85 C.

The film was placed in a nitrogen atmosphere and irradiated at 0.5mWcm2 at a constant temperature of 90 C for 2 minutes.

The transmission spectrum measured with left and right handed 10 circularly polarised light of the resulting film was recorded and is shown in Figure 5. The bandwidth is approximately 415 nm.

Example 5

15 The following polymerisable liquid crystalline mixture was formulated Compound (6) 34.0 % compound (7) 17.0 % compound (4) 20.5 % 20 compound (8) 17.0 % Paliocolour LC756 (g) 5.0 % Irgacure 907 O 6.0 % FC171 Hi) 0.5 % 25 The mixture has a birefringence An of 0.228.

The mixture was dissolved in a suitable organic solvent (eg xylene 50% w/w solids). The mixture was coated onto rubbed PET to give a 4 micron film when dried. The film was annealed for 1 minute at 30 85 C. The film was irradiated at 12mWcm2 at a constant temperature of 50 C for 1 minute. The transmission spectrum measured with left and right handed circularly polarised light of the resulting film was recorded and is shown in Figure 6. The bandwidth is approximately 235 nm.

Example 6

- 27 The following polymerisable liquid crystalline mixture was formulated Compound (6) 42.0 % 5 compound (7) 20.0 % compound (4) 10.5 % compound (10) 20.0 % Paliocolour LC756 O 5.0 % compound (5) 1.3 % 10 Irgacure 651 6) 0.7% FC171 6) 0.5%

The mixture has a birefringence An of 0.255.

15 The mixture was dissolved in a suitable organic solvent (eg xylene 50% w/w solids). A substrate was prepared by coating PET with a PVA solution to give a 1 micron film. This was rubbed. The mixture was coated onto the prepared substrate to give a 4 micron film when dried. The film was annealed for 1 minute at 85 C. The film was 20 placed in a nitrogen atmosphere and irradiated at 0.6mWcm-2 at a constant temperature of 80 C for 2 minutes. The transmission spectrum measured with left and right handed circularly polarised light of the resulting film was recorded and is shown in Figure 7. The bandwidth is approximately 265 nm.

Claims

1. A process of preparing a reflective film with varying pitch from a chiral polymerisable liquid crystal (LC) material, characterized in s that the polymerisable LC material has a birefringence An greater than or equal to 0.2.

2. A process as claimed in claim 1, wherein the polymerisable LC material comprises at least one mono-, di- or multireactive achiral to polymerisable mesogenic compound and at least one chiral compound that is mono-, di- or multireactive or non-reactive.

3. A process as claimed in claim 1 or 2, wherein the polymerisable LC material comprises at least one monoreactive polymerisable Is mesogenic compound with a birefringence of at least 0.2.

4. A process as claimed in any of the preceding claims, wherein An of the CLC material is greater than or equal to 0.2.

20

5. A process as claimed in any of the preceding claims, wherein the CLC material is polymerised in its cholesteric phase.

6. A process as claimed in any of the preceding claims, wherein the CLC material has a cholesteric phase in the range from 20 C or less 25 to 1 1 0 C or more.

7. A process as claimed in any of the preceding claims, wherein the CLC material comprises 5-50% di- or multireactive mesogenic compounds.

I

8. A process as claimed in any of the preceding claims, wherein the CLC material comprises 30-90% monoreactive mesogenic compounds. s

9. A process as claimed in any of the preceding claims, wherein the CLC material comprises one or more chiral compounds with a HIP of greater than 30, which may be non-reactive or contain one or more reactive groups, and which may be mesogenic or not.

to

10. A process as claimed in any of the preceding claims, wherein the CLC material optionally comprises - 5 to 50% of one or more direactive achiral mesogenic compounds, - 30 to 95% of one or more monoreactive achiral mesogenic I5 compounds, wherein at least 60%, based on the total mixture, are compounds with a birefringence of at least 0.2, - 0.1 to 10% of one or more chiral compounds which may be mono-, di- or multireactive or non-reactive, and may be mesogenic or not To and optionally comprises one or more of - 0.2 to 6% of one or more photoinitiators, - 0.2 to 5% of a dye which absorbs at the same wavelength as the radiation used for curing, preferably a UV dye, - 0.05 to 1% of a material used to promote alignment, as preferably one or more surface active agents.

11. A broadband reflective film obtainable by a process as claimed in any of the preceding claims.

30

12. A broadband reflective film as claimed in claim 11, which has a thickness of from 1 to 10 microns.

-3a

13 A broadband reflective film substantially as hereinbefore described with reference to Examples 1 to 6.

14. Use of a reflective film as claimed in any of claims 11 to 13 as a s reflective broadband or notch polariser or as a multicoloured film or image.

15. Use of a reflective film as claimed in any of claims 11 to 13 in a liquid crystal display, as a colour filter, in an effect pigment or for a to decorative or security application.

16. A liquid crystal display comprising a reflective film as claimed in any of claims 11 to 13.

Is

17. An authentification, verification or security marking or a multicoloured image comprising a reflective film as claimed in any of claims 11 to 13.

18. An object or document of value comprising an authentification, To verification or security marking or an image as claimed in claim 16.