GB2398077A

GB2398077A - Polymerised liquid crystal film with improved adhesion

Info

Publication number: GB2398077A
Application number: GB0402098A
Authority: GB
Inventors: Donald Gordon Graham; Owain Llyr Parri
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2003-01-31
Filing date: 2004-01-30
Publication date: 2004-08-11
Anticipated expiration: 2024-01-30
Also published as: TWI349028B; JP4837255B2; GB2398077B; TW200417596A; JP2004339476A; GB0402098D0; KR20040070076A

Abstract

The invention relates to a film comprising polymerised liquid crystal (LC) material with improved adhesion to a substrate, in particular to a plastic substrate, to methods for preparing such a film, to a polymerisable LC material used for the preparation of such a film, to a multilayer comprising such a film, and to the use of the film, material and multilayer for optical, electrooptical, decorative or security devices and applications. The film comprising the polymerised LC material is characterised in that it is obtainable from a polymerisable material comprising not more than 7% by weight of compounds having two or more polymerisable groups.

Description

- 1 - 2398077 Polymerised Liquid Crystal Film with Improved Adhesion

Field of Invention

The invention relates to a film comprising polymerized liquid crystal (LC) material with improved adhesion to a substrate, in particular to a plastic substrate, to methods for preparing such a film, to a polymerisable LC material used for the preparation of such a film, to a multilayer comprising such a film, and to the use of the film, material and multilayer for optical, electrooptical, decorative or security devices and applications.

Background and Prior Art

Polymerisable liquid crystal (LC) materials are commonly used for the preparation of optical films in liquid crystal displays. These materials usually contain a certain amount of compounds with two or more polymerisable groups (di- or multi-functional), which are crosslinked to give a hard film. However, these hard films often do not readily adhere to the surface of the plastic substrates commonly used in the manufacturing process, because the polymerization and crosslinking process causes the film to shrink. Films made from such LC materials are therefore usually delaminated and re-attached to another substrate, such as glass or the plastic of a polariser, for use in a liquid crystal display. However, the processes of delamination and application to an alternative substrate are costly in time and materials. It also gives two potential points at which loss of film may occur by damage during the delamination or relamination, leading to a lower overall yield of product.

In prior art it has also been reported to use adhesion and aligning layers to bond an LC polymer film to plastic substrates. For example, US 5,631,051 discloses a method of preparing an optical compensation sheet on a transparent substrate of triacetyl cellullose (TAC), by first providing an adhesion layer of gelatins on the TAC film. Then an aligning layer is formed by coating a solution of

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denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerizable groups, onto the gelatins layer, evaporating the solvent and rubbing the surface of the polymerised PVA layer unidirectionally, Finally an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerized.

US 5,747,121 discloses a method of preparing an optical compensation sheet by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerizable groups, onto a transparent substrate, evaporating the solvent and rubbing the surface of the PVA layer unidirectionally.

Then an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerized.

Afterwards the film is subjected to heat treatment whereby the PVA layer and the discotic LC layer are reported to be chemically bonded to each other via free, crosslinkable groups.

However, the above methods require many separate coating steps.

Furthermore, the use of several intermediate layers, such as adhesion or aligning layers, comprising isotropic materials like gelatins or PVA can negatively influence the optical performance of the optical film.

Therefore, there is a need for an advantageous method to provide a film of crosslinked LC material that does well adhere to a plastic substrate, whilst saving processing time and allowing to reduce losses due to processing damage of the LC film.

The inventors of the present invention have found that the above mentioned drawbacks of prior art methods can be overcome by using a polymerisable LC material comprising a low percentage of di- or multifunctional compounds to prepare an LC polymer film with low degree of crosslinking for use e.g. as optical component. Such a film does better adhere to plastic substrates as commonly used in the optical films industry. Since the low crosslinked LC film is soft and - 3 can be sensitive against mechanical stress, it is preferably covered by a hardcoat or by another polymerized LC film comprising a high degree of crosslinking. The highly crosslinked and hard LC film can then act as protective layer for the soft LC film, and the soft LC film can act as adhesive layer for the hard LC film when laminated to a plastic substrate or another film.

WO 96/10768 describes the preparation of a film comprising polymerised LC material with tilted or splayed orientation. The orientation of the LC material is achieved by the application of a pre tilt layer that also comprises polymerized LC material. However, WO 96/10768 does not disclose to use LC materials with different amount of di- or multifunctional compounds, and does not address the problem of poor adhesion of hard LC films and/or mechanical damage of soft LC films.

Definition of Terms

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The term 'film' includes self-supporting, i.e. free-standing, films or layers of material that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.

The term 'liquid crystal or mesogenic material' or 'liquid crystal or mesogenic compound' includes 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 (LC) compounds with rod-shaped or 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 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 for both liquid crystal materials and mesogenic materials, and the term mesogen' is used for the mesogenic groups of the material.

The term 'director' means the preferred orientation direction of the long molecular axes in case of calamitic compounds, or of the short molecular axis in case of discotic compounds, of the mesogens in a liquid crystal material.

The term 'planar structure' or 'planar orientation' refers to a layer of optically anisotropic material wherein the optical axis is substantially parallel to the plane of the layer.

The term 'homeotropic structure' or'homeotropic orientation' refers to a layer of optically anisotropic material wherein the optical axis is substantially perpendicular to the plane of the layer.

The terms 'tilted structure' or'tilted orientation' refers to a layer of optically anisotropic material wherein the optical axis is tilted at an angle between O and 90 degrees relative to the plane of the layer.

The term 'splayed structure' or 'sprayed orientation' means a tilted orientation as defined above, wherein the tilt angle within the layer varies monotonuously in the range from 0 to 90 , preferably from a minimum to a maximum value, in a direction perpendicular to the plane of the layer.

Unless stated otherwise, the tilt angle of a splayed layer is given as the average tilt angle (Dave which is defined as d 0'(d) _ d'=0 Wave d - 5 wherein 6'(d') is the local tilt angle at the thickness d' within the layer, and d is the total layer thickness.

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

Unless stated otherwise, in the following the percentages by weight (wt.%) of individual solid or liquid crystalline compounds in a polymerisable liquid crystal material relate to the total amount of solid or liquid crystalline compounds in said material.

Summary of the Invention

The invention relates to a film comprising polymerized liquid crystal (LC) material, characterized in that it is obtainable from a polymerisable material comprising not more than 7 % by weight of compounds having two or more polymerisable groups.

The invention further relates to a polymerisable LC material comprising not more than 7 % by weight of compounds having two or more polymerisable groups.

The invention further relates to a multilayer comprising a plastic substrate, at least one LC polymer film according to the present invention provided on the substrate, and at least one hardcoat film with improved resistance against mechanical stress provided on the - 6 surface of said at least one LC polymer film facing away from the substrate.

The invention further relates to a method of preparing a polymer film or a multilayer according to the present invention.

The invention further relates to the use of a film, material, multilayer or method according to the present invention in optical, electrooptical, information storage, decorative and security 1 0 applications.

The invention further relates to an optical component or device comprising a film, material or multilayer according to the present invention.

The invention further relates to a liquid crystal display comprising a film, material or multilayer according to the present invention.

The invention further relates to an authentification, verification or security marking or a coloured image comprising a film, material or multilayer according to the present invention.

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.

Detailed Descrintion of the Invention As mentioned above, a low crosslinked LC film according to the present invention is soft and in some applications tends to be sensitive against mechanical stress. Therefore, in a preferred embodiment the low crosslinked LC film according to the present invention is protected by a hardcoat or protective film against damage. - 7

Suitable hardcoats or protective films are known to the expert and published in the literature. Especially suitable and preferred hardcoats are selected from crosslinked acrylate formulations containing mono-, dior multifunctional acrylates, like for example methyl acrylate, 1,6hexanediol-diacrylate (HDDA), tripropylene gylcol diacrylate (TPGDA) or pentaerythritol tetraacrylate, furthermore commercially available polymerisable formulations or hardcoats like Ebecryl@) 270, 5129 or 350 (from UCB Chemicals, Ltd.), Topcoat 604 (from Eques Coatings B.V., NL), or mixtures or O modifications of the above.

The invention further relates to a method of preparing a multilayer by providing a layer of a polymerisable LC material comprising not more than than 7 wt. % of compounds having two or more polymerisable groups onto a substrate onto a plastic substrate, - optionally aligning the LC material into uniform orientation, - polymerizing the LC material,and - providing a hardcoat onto the free surface of the polymerised LC material.

The low crosslinked LC film according to the present invention has good adhesion in particular to plastic substrates, and therefore can be used as adhesive or base coating for subsequent LC layers which otherwise would not well adhere to the substrates.

Furthermore, the low crosslinked LC film according to the present invention can impart alignment to subsequent LC layers that are coated on top of the low crosslinked LC film. It can therefore be used as alignment layer for subsequent LC layers. By variations in the thickness of the LC film according to the invention, it is possible to affect the orientation, in particular the tilt angle, of the subsequent LC layer. For example, a thinner LC film according to the invention with planar orientation can be used to induce planar orientation in a subsequent LC layer. A thicker LC film according to the invention - 8 with titled or splayed orientation can be used to induce tilted or splayed orientation in a subsequent LC layer.

Thus, another preferred embodiment of the present invention relates to a multilayer comprising a base layer of a low crosslinked or soft LC film, and further comprising at least one further polymerized LC film which has a higher degree of crosslinking than the base layer and is therefore harder. The hard LC film serves as protective layer for the soft LC film, while the soft LC film serves as alignment layer and/or as adhesive layer for the hard LC film when coated or laminated onto a plastic substrate or another optical film component in a display or optical device.

In a multilayer as described above, either the soft LC film or the hard LC film or both films can act as optical layer. Compared to conventional hardcoats or protective, adhesive or alignment layers comprising isotropic or stretched polymers, the combination of a hard and a soft LC film has the advantage that LC materials with similar optical properties can be used, which allows significant improvement of the optical performance of the final optical product.

There is a requirement in the optical films industry to keep the films to a minimum thickness. This results in a saving of the relatively expensive LC material. However, in a multilayer as described above it is not possible to reduce the thickness of the LC film that acts as optical layer, as the thickness of this layer is determined by the optical properties required. Therefore, if in this multilayer only one of the soft and the hard LC film acts as optical layer, it is preferred to reduce the thickness of the other layer, which then mainly functions to act as adhesive, aligning or protective layer for the optical layer.

The preferred thickness of a high or low crosslinked LC film acting as optical layer is determined by the optical properties desired from the film or the final product. If the low crosslinked film does not mainly act as optical layer, but e.g. as adhesive or aligning layer, or if the high crosslinked LC film does not mainly act as optical layer, but e.g. - 9 as protection or hardcoat layer, its thickness is preferably not greater than 1 Urn, in particular not greater than 0.5 m, very preferably not greater than 0.2 m.

The invention further relates to a method of preparing a multilayer by providing a layer of a first polymerisable LC material, which comprises not more than 7 wt. % of compounds having two or more polymerisable groups, onto a plastic substrate, - optionally aligning said first LC material into uniform orientation, - polymerizing said first LC material, - providing a layer of a second polymerisable LC material, which comprises more than 20 wt. % of compounds having two or more polymerisable groups, onto the free surface of said first polymerised LC material, - optionally aligning said second LC material into uniform orientation, and polymerising said second LC material.

The low crosslinked LC film preferably has a planar orientation.

The high crosslinked LC film preferably has a planar or tilted or splayed orientation.

Suitable plastic substrates are known to the expert and described in the literature, like for example conventional substrates used in the optical films industry. Especially suitable and preferred 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 LC films are preferably prepared from the polymerisable LC material by in-situ polymerization. In a preferred method of preparation the polymerisable LC material is coated onto a substrate, - 1 0 oriented into the desired orientation and subsequently polymerized for example by exposure to heat or actinic radiation as described for example in WO 01/20394, GB 2,315,072 or WO 98/04651.

Thus, the invention further relates to a method of preparing a polymerized LC film with improved adhesion to a substrate, by - providing a layer of a polymerisable LC material comprising not more than 7 wt. % of compounds having two or more polymerisable groups onto a substrate, - optionally aligning the LC material into uniform orientation, - polymerizing the LC material, and - optionally removing the polymerised film from the substrate.

Especially preferably the polymerisable LC material used for the preparation of the low crosslinked film comprises (in % by weight) - from 70 to 99 %, preferably from 85 to 99 %, very preferably from 93 to 99 % of one or more compounds having one polymerisable group, - from more than 0 to 7 %, preferably from 1 to 7 %, very preferably from 2 to 7 %, most preferably from 4 to 7 % of one or more compounds having two or more polymerisable groups, - from O to 15 %, preferably from 0 to 10 %, very preferably from 0 to 5 % of non-polymerisable compounds, - from O to 10 %, preferably from 0.1 to 6 % of one or more polymerization initiators.

The polymerisable compounds used in the polymerisable LC material are preferably mesogenic or liquid crystalline compounds. Thus, the polymerisable LC material typically comprises one or more polymerisable chiral or achiral mesogenic or liquid crystalline compounds. It is preferably a mixture comprising one or more polymerisable compounds having one polymerisable group (monoreactive) and one or more polymerisable compound having two or more polymerisable groups (di- or multireactive). 11

In another preferred embodiment the polymerisable LC material comprises up to 20 by weight % of a monoreactive non-mesogenic compound with one polymerisable functional group. Typical examples are alkyl acrylates or alkyl methacrylates with alkyl groups of 1 to 20 C atoms.

In another preferred embodiment the polymerisable LC material used for the preparation of the low crosslinked film does not contain compounds having more than two polymerisable groups.

In another preferred embodiment the polymerisable LC material used for the preparation of the low crosslinked film is an achiral material, i.e. it does not contain chiral compounds.

The non-polymerisable compounds include for example additives like surfactants, catalysts, sensitizers, stabilizers, chain-transfer agents, inhibitors, lubricating agents, wetting agents, dispersing agents, hydrophobing agents, adhesive agents, flow improvers, defoaming agents, deaerators, diluents, reactive diluents, colourants, dyes or other auxiliaries.

The polymerisable LC material used for the preparation of the high crosslinked film preferably comprises more than 20 %, in particular more than 40 %, very preferably more than 60 % by weight of compounds having two or more polymerisable groups.

In case of high crosslinked LC films it is also possible to add up to 20 % by weight of a non-mesogenic compound with two or more polymerisable functional groups to the polymerisable LC material alternatively or in addition to the di- or multireactive polymerisable mesogenic compounds to increase the degree of crosslinking. Typical examples for direactive nonmesogenic monomers are alkyl diacrylates or alkyl dimethacrylates with alkyl groups of 1 to 20 C atoms. Typical examples for multireactive nonmesogenic monomers are trimethylpropane trimethacrylate or pentaerythritol tetraacrylate. - 12

The polymerisable LO materials as described above and below are another object of the invention.

Pol ymerisable mesogen ic mono-, d i- and mu ltireactive compounds used for the present 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, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

Examples of suitable polymerizable mesogenic compounds that can be used as monomers or comonomers together with the compounds according to the present invention in a polymerizable LC mixture, are disclosed for example in WO 93/22397, EP 0 261 712, 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 shall not limit the scope of this invention.

Examples of especially useful chiral and achiral polymerizable mesogenic compounds (reactive mesogens) are shown in the following lists which should, however, be taken only as illustrative and is in no way intended to restrict, but instead to explain the present invention: P-(CH2)XO COO 43 R (R1) P-(CH2)XO COO R (R2) P-(CH2)XO COO <} R (R3) - 1 3 (R4) P(CH2)xo 3 coo OCO <3 R P-(CH2)XO COO l R (R5) P-(CH2)xo i3Z}Z R (R6) P(CH2)X- = J vR (R7) P-(CH2)X CH-CH-coo R (R8) P(CH2)x {Z RO (R9) P(CH2)xO (R10) P-(CH2)XO if R (R11) P-(CH2)XO (COO)U 3t CH2cH(CH3) C2H5 (R12) c' * (R13) P-(CH2)XO coo coo <3 CH2CH(cH3)c2Hs - 14 P-(CH2)x 3 COO-Ter (R14) P-(CH2)X0 <3 COO-Chol (R15) P-(CH2)x COO - (R16) P(CH2) xO O-CO R (R17) P(CH2)x COO OCO o(cH2)yp (R18) L' L2 P(CH2)X0 <3 CH2CH2 CH2CH2 <3 o(CH2)yP (R19) p '03Co26LO2Có p (R20) P(CH2)XO4Z óCH=CHCO (R21) oooCCH=CHó[Z iVO(CH2)yP P(CH2)XO}Z 4 COO H(R22) H {)Vo(CH2)yP - 15 P(CH2) 0 4Z 0 Z0: O(CH2\P In the above formulae, P is a polymerisable group, preferably an acryl, methacryl, vinyl, vinyloxy, propenyl ether, epoxy, oxetane or styryl group, x and y are identical or different integers from 1 to 12, A is 1,4 phenylene that is optionally mono-, di- or trisubstituted by L', or 1,4 cyclohexylene, u and v are independently of each other 0 or 1, Z is COO-, -OCO-, -CH2CH2-, -CH=CH-, -C_C- or a single bond, R is a polar group or an unpolar group, Ter is a terpenoid radical like e.g. menthyl, Chol is a cholesterol group, L, L' and L2 are independently of each other H. F. Cl, ON or an optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 C atoms, and r is 0, 1, 2, 3 or 4. The phenyl rings in the above formulae are optionally substituted by 1, 2, 3 or 4 groups L. The term 'polar group' in this connection means a group selected from F. Cl, CN, NO2, OH, OCH3, OCN, SCN, an optionally fluorinated alkycarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy; 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 optionally halogenated alkyl, alkoxy, alkycarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy group with 1 or more, preferably 1 to 12 C atoms which is not covered by the above definition of 'polar group'.

For the preparation of cholesteric LC films with helically twisted structure, the polymerisable LC material preferably comprises one or more achiral polymerisable mesogenic compounds and at least one chiral compound. The chiral compound can be selected from non polymerisable chiral compounds, like e.g. conventional chiral dopants, or polymerisable chiral compounds, all of which can be mesogenic or non-mesogenic. - 16

Suitable polymerisable chiral compounds are for example those shown in the above list. Further suitable chiral polymerisable compounds are e.g. the commercially available Paliocolour O materials (from BASF AG, Germany) .

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, R or S3011, R- or S-4011, R- or S-5011, or CB 15 (from Merck KGaA, Darmstadt, Germany). Very preferred are chiral compounds with a high helical twisting 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 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 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 LC 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 96/02597. Especially preferred, however, are LC materials not containing a binder or dispersion auxiliary.

In another preferred embodiment the polymerisable LC material comprises an additive that induces or enhances planar alignment of - 17 the liquid crystal material on the substrate. Preferably the additive 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-1710 (from 3M Co.), or Zonyl FSN @) (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 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 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 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 available Irgacure@) or Darocure@) series (all from Ciba Geigy AG) can be used, whereas in case of cationic photopolymerisation the commercially available UVI 6974 (Union Carbide) can be used.

The polymerisable LC material can additionally comprise one or more other suitable components such as, for example, catalysts, sensitizers, stabilizers, chain-transfer agents, inhibitors, co-reacting monomers, surface-active compounds, lubricating agents, wetting agents, dispersing agents, hydrophobing agents, adhesive agents, flow improvers, - 18 defoaming agents, deaerators, diluents, reactive diluents, auxiliaries, colourants, dyes or pigments.

It is also possible to add one or more chain transfer agents to the 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 thiol compounds. When adding a chain transfer agent, the length of the free polymer chains and/or thelength 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.

The LC films or multilayers according to the present invention are useful as optical elements like polarisers, compensators, circular polarisers or colour filters in liquid crystal displays or projection systems, as decorative image, for the preparation of liquid crystal or 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 The LC films or multilayers according to the present invention can be used in displays of the transmissive or reflective type. They can be used in conventional LCDs, in particular 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 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 (rr-cell) displays, furthermore in displays of the TN (twisted nematic), HTN (highly twisted nematic) or - 19 STN (super twisted nematic) mode, in AMD-TN tactive 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.

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.

Example 1 - Preparation of a multilaver comprising a low crosslinked LC film and a high crosslinked LC film Polymerisable LC mixture M1 with a low amount of direactive compounds was formulated as follows M1: (1)41.6% monoreactive (2)32.0% monoreactive (3)16.2% monoreactive (4)5.7% direactive Irgacure 9074.0% Fluorad FC1710.5% :0-0 IN (1) :0 oóO- CH (2) 20 O TO SIX C3H7 (3) to-(cH2)3_oooo- (CH2)3-of 1 0 (a) Irgacure 907 is a commercially available photoinitiator (from Ciba AG). Fluorad FC 171 is a commercially available non-ionic fluorocarbon surfactant (from 3M Corp.).

Polymerisable LC mixture M2 with a high amount of direactive compounds was formulated as follows M2: (1) 7.0% monoreactive (2) 21.0% monoreactive (4) 66.5% direactive Fluorad FC171 0.5% Irgacure 907 5.0% Both M1 and M2 were prepared as a 30% w/w solution iri toluene/cyclohexanone (7: 3).

The substrate used in all cases was triacetyl cellulose (TAC).

A TAC substrate was rubbed 3 x 1000 mm and coated with the solution of M1 using a No 1 wire wound bar. The solvent was allowed to evaporate at room temperature and the resulting film was polymerized by 4 passes through the Minicure UV lamp system at 20m/min. - 21

The resulting film was, a clear, planar aligned film having 100% adhesion when tested by the cross hatch method.

Samples of this film were then coated with the solution of M2, again using a No 1 wire wound bar, evaporating the solvent at room temperature, and curing the film as described above. Adhesion was tested in a similar manner.

In the case of M2 being applied to the cured M1 without rubbing, a clear, highly crosslinked polymer film of M2 with tilted structure was produced on top of the M1 layer. The adhesion of the M2 layer to the M1 layer was tested using the cross hatch method and found to be 100%. The low crosslinked film of M1 thus acts as an adhesive and alignment layer for the high crosslinked film of M2.

Rubbing the layer of M1 before being coated with M2 caused a loss of alignment, a scattering film, and no adhesion.

ExamnIe 2 - Reduction of film thickness Using a No 1 bar with a 30% w/w solution as described in example 1 yielded a calculated film thickness of approximately 1.8 Em for each single layer. Coating both M1 and M2 with a No 1 bar created a film of approximately 3.0 1lm total thickness.

In applications where M1 serves only as alignment and adhesive layer, its thickness should preferably be reduced to save the relatively expensive LC material. By using solutions with higher content of solvents it is possible to reduce the thickness of the polymerisable LC layer aver evaporation of the solvent.

Films were prepared using a No 1 bar but diluting the M1 solution to 15%, 10%, 5% and 1%, and the adhesion and alignment of the films was measured. The results are shown in table 1. - 22

Table 1

% w/w RMM M1 Film Adhesion of M2 Film solution Thickness M1 to M2 alignment am Film % 1.3 100 Tilted 100 Tilted 0.3 _ 100 Tilted 0.2 100 Planar 1 0 Planar Thicknesses of the films M1 were measured using an optical fringe measurement system. The 30% solution gave a measured thickness of 1.3 m. The 10% solution gave a measured thickness of 0.34 m.

From the above results it can be seen that there is a minimum thickness of approximately 0.2 Em below which adhesion is not achieved. It can also seen that thicker films of M1 give rise to a tilted film of M2 rather than a planar film which would normally be expected from the mixture formulation of M2.

Exa mole 3 - Pre pa ration of a m u lti layer comurisi no a low crossli nked LC film and a cholesteric LC (CLC) film Polymerisable cholesteric LC mixture M3 was formulated as follows (1) 7.1% monoreactive (2) 11.2% monoreactive (3) 9.7% monoreactive (4) 47. 1% direactive (5) 10.2% direactive Paliocolour LC756 10.2% direactive Fluorad FC171 0. 5% Irgacure 907 4.0% - 23 o (CH2)6-oooo-(CH2)6-0): (5) Paliocolour LC756 is a commercially available chiral polymerisable material (from BASE AG, Ludwigshafen, Germany).

A piece of TAC substrate was rubbed 3 x 1000 mm and coated with a 5% w/w solution of M1 (example 1) using a No 2 wire wound bar.

The theoretical thickness of the resulting coating would be 0.6 m.

The solvent was allowed to evaporate to leave a liquid crystal film which was then cured under UV light.

The film produced was then rubbed 2 x 1000 mm and coated with the cholesteric LC mixture M3 using a No 1 wire wound bar. After evaporation of the solvent the liquid crystal film was cured under UV light.

A clear, aligned CLC film reflecting in the UV range was produced. This was tested for adhesion using the standard cross hatch test. Adhesion was 100%.

Example 4 - Influence of surfactant Mixtures M1 and M2 of example 1 contain 0.5 % of the surface active component Fluorad FC171.

Polymer films of M1 and M2 were prepared as described in example 1, but wherein the surfactant FC171 was omitted. The same effects were observed with these mixtures and films as described in example 1.

Example 5 - Preparation of a multilayer comprising a low crosslinked LC film and a crosslinked isotropic hardcoat A polymer film of M1 ("M1 film") was prepared on a TAC substrate as defined in example 1. Several hardcoat formulations T1-T5 as shown - 24 below were prepared, coated on top of the M1 film and cured by UV exposure.

Irgacure() photoinitiators are commercially available from Ciba Geigy AG. Ebecryl 270, 5129 and 350 are commercially available from UCB Chemicals, Ltd.. Topcoat 604 C) is commercially available from Eques Coatings B.V.

5a) T1 Ebecryl 27060 % 1,6-hexanediol-diacrylate (HDDA)24 % tripropylene gylcol diacrylate (TPGDA)10 % Irgacure 9076 % The formulation was used as 25 % w/w solution in toluene. The hardcoat exhibited greater scratch resistance than the unprotected M1 film. This was demonstrated by the fingernail test.

Durability of this system was further checked by laminating the multilayer at the free surface of the TAC substrate via a pressure sensitive adhesive (PSA) to a glass slide. The entire stack was then placed in a chamber at 60 C and 90 % relative humidity. After 40 days (960 hours) no obvious deterioration of the slide had occurred.

The hardcoat surface was then laminated to a second piece of TAC via a PSA. On peeling the two pieces of TAC apart, the hardcoat layer and the M1 film came apart undamaged.

5b) T2 pentaerythritol tetraacrylate 80 % HDDA 15 % Irgacure 907 5 % r 25 The formulation was used as 50 % w/w solution in toluene. The scratch resistance of the coating was found to be very good, the fingernail test barely marked the coating.

5c) T3 methyl acrylate 47.5 % HDDA 26.4 % pentaerythritol tetraacr, vlate 21.1 % Irgacure 907 5.0 % The formulation was used without solvent. The scratch resistance of the coating was found to be excellent. On lamination to a second piece of TAC with PSA, and subsequent delamination the hardcoat layer was removed from the M1 film without damage to the M1 film.

5d) T4 Topcoat 604 (g) (from Eques Coatings B.V) was applied to the M1 film and cured. This material gave a hard coating which was resistant to vigorous abrasion.

be) T5 Ebecryl 5129 74.8 % HDDA 18.7 % Ebecryl 350 0.9 % Irgacure 500 5.6 % The formulation was used without solvent. A clear, scratch resistant hardcoat was obtained, which separated from the M1 film upon delamination. - 26

Claims

Claims 1. A film comprising polymerised liquid crystal (LC) material,

characterized m that it Is obtainable from a polymerisable material comprising not more than 7% by weight of compounds having two or more polymerisable groups.
2. A film as claimed in claim 1, wherem the polymerisable LC material does not contain compounds having two or more polymerisable groups.
3. A film as claimed in claim 1 or 2, wherein the polymerisable LC material comprises: - from 70 to 99% by weight of one or more compounds having one polymerisable group, - from O to 15% by weight of non-polymerisable compounds, - from 0 to 10% by weight of one or more polymerization initiators.
4. Polymerisable LC material as defined in any of claims 1 to 3.
5. Polymerisable LC material substantially as hereinbefore described with reference to mixture M1 in Example 1 or 4.
6. A film comprising polymerized LC matenal, character1sed in that it is obtainable from polymerisable LC material as claimed in claim 5.
7. A multilayer comptlsing a plastic substrate, at least one LC polymer film as claimed in any of claims 1 to 3 or claim 6 provided on the substrate, and at least one hardcoat film having improved resistance against mechanical stress provided on the surface of said at least one LC polymer film facing away from the substrate.
8. A multilayer as claimed in claim 7, wherein said hardcoat film is selected from crosslinked acrylate formulations containing mono-, di- or multifunctional acrylates. - 2
9. A multlayer as claimed In claim 7, wherem said hardcoat film comprises polymeriscd LC material comprising more than 20% by weight of compounds having two or more polymensable groups.

s
10. A multilayer substantially as hereinbefore described with reference to: a) Example 1, obtained by the application of M2 to the cured M1 without rubbing; or b) any of Examples 3 - 5.
11. A method of preparing a polymerised LC film, comprising: - providing a layer of polymerisable LC material as claimed in claim 4 or 5 on a substrate, - optionally aligning the LC material into uniform orientation, - polymerising the LC material, and - optionally removing the polymerised film from the substrate.
12. A method of preparing a multilayer as claimed in claim 7 or 8, comprising: - providing a layer of polymerisable LC material as claimed in claim 4 or 5 on a plastic substrate, - optionally aligning the LC material into uniform orientation, - polymerising the LC material, and - providing a hardcoat film on the free surface of the polymerised LC material.
13. A method of preparing a multllayer substantially as hereinbefore described with reference to any of Examples 1-4.
14. A method of preparing a multllayer as claimed in claim 9, comprising: - providing a layer of a first polymerisable LC material, as claimed in claim 4 or 5, on a plastic substrate, - optionally aligning said first LC material into uniform orientation, - polymerising said first LC material, - 28 provdng a layer of a second polymerisable LC material on the free surface of said first polymerised LC material, wherein said second polymerisable LC material comprises more than 20% by weight of compounds having two or more polymerisable groups, s - optionally aligning said second LC material into uniform orientation, and polymerising said second LC material.
15. A method of preparing a multlayer substantially as herembefore described with reference to Example 5.
16. Use of a film, material, multilayer or method as claimed in any of claims 1 to in optical, electrooptical, information storage, decorative or security applications.
17. An optical component or device comprising at least one film, material or multlayer as claimed in any of claims 1 to 10.
18. A liquid crystal display comprising at least one film, material or multilayer as claimed in any of claims 1 to 10, or a component as claimed in claim 17.
19. An authentlficatton, verification or security marking or coloured image comprising at least one film, material or multilayer as claimed in any of claims 1 to or a component as claimed in claim 17.
20. An object or document of value comprising an authentification, verification or security marking or image as claimed in claim 19.