This invention relates to optical material comprising particles of a liquid crystal material in a light transmissive medium. Such optical material may be used, for example, in ink for security marking documents.

Cholesteric liquid crystals (CLCs) and cholesteric liquid crystal polymers (CLCPs) exhibit circular dichroic properties in their aligned state. This means they have the ability to separate incident unpolarized white light into a narrow wavelength band of circularly polarized light which is reflected, whilst transmitting the rest of the light (i.e. white light minus one circular polarization state in the narrow wavelength band). A typical transmission characteristic of such an aligned CLC is shown in figure 1 of the accompanying drawings, which shows a graph of percentage transmission versus wavelength for a red cholesteric liquid crystal. Different materials are used to produce different senses of circular polarization.

The wavelength at which maximum reflection (or minimum transmission) occurs can be chosen to lie anywhere in the near ultra-violet (UV), visible or infrared (IR) parts of the electromagnetic spectrum by suitable choice of CLC or CLCP pitch. For a specific

CLC or CLCP, the wavelength (λ) of peak reflection and the width (Δλ) of the narrow wavelength band are determined by the formulae:-

λ = n.p Δλ = Δn.p

where n is the average refractive index of the CLC or CLCP, Δn is the birefringence of the CLC or CLCP, and p is the helical pitch of the layer of CLC or CLCP. Cholesteric liquid crystals are also known as chiral nematics. Similar optical properties will be observed for chiral smectic liquid crystals and chiral smectic liquid crystal polymers.

In a known optical material disclosed in Australian patent AU-488662, the particles are small capsules of a CLC in its liquid state, together with a light transmissive medium. The thermochromic properties of this material are used to make inks for documents of value. Such an optical material has a number of disadvantages including poor colour purity and brightness, restricted viewing angles if the polarization properties

of the reflected light are to be preserved, susceptibility to damage during high pressure printing, and the inability of the capsules to be given particular shapes.

An alternative way to make a security element for a banknote is disclosed in

Canadian patent CA-2032587, in which a solid CLCP in the form of a thread or a 2 dimensional film laminated to the banknote is used. This has disadvantages associated with thread manufacture and film lamination. Also such an element may not survive frequent folding.

An object of the present invention is to enable these disadvantages to be mitigated. According to the invention there is provided an optical material comprising particles of a liquid crystal material in a light transmissive medium, characterized in that the particles are laminae of a chiral liquid crystal which has an oriented form and which is a solid at room temperature. This material has the advantage of a small temperature dependence of the colour, ability to withstand high pressure printing, and difficulty in reproducing the colours photographically for security applications.

The liquid crystal may be a high melting point cholesteric or chiral smectic liquid crystal, or a cholesteric or chiral smectic liquid crystal polymer. The medium may be, for example, a liquid, gel or soft solid.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:-

Figure 1 shows the transmission characteristic versus wavelength for a red cholesteric liquid crystal material discussed hereinbefore,

Figure 2 shows a first embodiment in the form of a mixture of CLCP laminae and a light transmissive medium provided on a surface, Figure 3 shows a second embodiment in the form of sequential layers each of a mixture of CLCP flakes and a substantially transparent medium provided on a surface,

Figure 4 shows in cross-section an optical material including polymer particles coated with a layer of CLCP and a light transmissive medium.

In the first embodiment shown in figure 2 an optical material including laminae 1 of a sohd cholesteric liquid crystal polymer and a light transmissive medium 2 is provided as a cosmetic on the surface of a person's fingernail 3 to give an iridescent pattern. In this embodiment the medium 2 is transparent nail varnish, and the laminae

are in the form of small flakes. Two types of CLCP flake are present, specifically flakes of Wacker LC-Silicone CC3767 (red) and flakes of CC3939 (blue) cross-linked polymer respectively, these polymers being obtainable from "Consortium fur Electrochemische Industrie" of Munich, Germany. The major dimensions of the flakes are approximately 200 microns by 100 microns and they are approximately 10 microns thick; the flakes make up 20% of the mixture by weight.

The flakes 1 of CLCP are made in the following way. Two clean glass plates with optically flat surfaces are coated with a thin layer of nylon by spin coating. These layers are rubbed with a velvet or silk cloth to provide a layer to align the mesogenic groups (this technique is well known for Liquid Crystal Display manufacture). Two self adhesive tape spacers 50 microns thick are provided at opposite edges of a major surface of each plate, and a solution of 33 parts of the un-crosslinked CLCP in 66 parts toluene with 1 part photo-initiator is applied and spread evenly across the plates using a doctor blade or squeegee supported by the spacers. The tape spacers are removed from the glass plates and the plates are then placed side by side in a vacuum oven to drive off the solvent. The plates are then placed with the CLCP coatings facing one another with small pillar spacers at each corner between the plates. These spacers are such that they melt or deform at a predetermined temperature (in this case 85°C), and are sized so as to leave a clear air gap between the two CLCP coated surfaces before the spacers deform. This assembly is put in a bag sealer and encapsulated in a plastic bag from which the air is removed so that the CLCP coated surfaces face one another with no air between. This assembly is then put into a furnace at a temperature of 85°C. This melts the pillars so that the CLCP plates move towards one another. The CLCP also softens so that a film with flat surfaces aligned by the nylon coating is formed between the plates. The assembly is then exposed to UV radiation at this elevated temperature to cross-link the polymer (other temperatures such as for example 70°C, or even room temperature for blue material, may be used as an alternative). The assembly is then allowed to cool, after which it is removed from the bag and prised apart. It is usual for portions of the area of the film (having the full layer thickness of approximately 10 microns) to adhere to each plate at this stage. Flakes of the polymer are then scraped off the plates using a hard instrument, and further ground with an agate mortar and pestle if required. Other grinding techniques such as ball milling or triple roll milling or ultrasonic agitation may

be used as an alternative.

Optical materials with different properties may be formed by using different ratios of CLCP weight to medium weight. Mixtures with CLCP content as low as 0.5% may have a speckled appearance, whereas mixtures with CLCP content as high as 80% can be used to give a glittering metallic appearance. In use, when such optical material is applied to a surface, the flakes tend to lie in the plane of the surface and show strong iridescent colours. The colour effects produced by CLCPs are durable as they are not due to dyes but rather due to a structured matrix of molecules. Thus a surface coated with such a material may be sanded down and itself coated with a transparent lacquer to improve the colour effects or surface smoothness.

In the second embodiment shown in figure 3, two inks (a) and (b) are made up using:-

(a) particles of CLCP consisting of flakes 4 of Wacker LC-Silicone CC3767 (red) crosslinked and aligned, made as described above, together with an epoxy binder acting as a substantially transparent medium 7,

(b) particles of CLCP consisting of flakes 5 of Wacker LC-Silicone CC3939 (blue) crosslinked and aligned, made as described above, together with an epoxy binder acting as a substantially transparent medium 11 , which may be identical to the medium 7.

These inks are printed and cured sequentially as successive layers on the surface 6 of a document. The flakes constitute 15% by weight of the inks prior to printing. In applications such as inks for banknotes and documents of value an elastomeric light transmissive medium which acts as a binder or contains an additional binder is preferably used so that the material can survive folding and/or abrasion of the document. One such material is RTV 615 A silicone resin and curing agent supplied by GE Silicones, 4500 AC Bergen op Zoom, the Netherlands.

Although in the embodiments described above CLCPs are used for the laminae, flakes of a high melting point cholesteric liquid crystal which has an oriented form and which is a solid at room temperature may be used as an alternative. Such laminae may be formed by quenching the aligned CLC rapidly in liquid nitrogen to form a glass, and

then grinding this glass in an agate mortar and pestle whilst it is still cold. Chiral smectic liquid crystals may also be used as an alternative.

The third embodiment shown in figure 4 takes the form of flakes 8 of a black coloured base polysiloxane polymer coated on each side with a layer 9 of an aligned polysiloxane CLCP in an epoxy resin 10 which acts as a light transmissive medium and binder. In this figure the flakes are shown in cross-section. Forming the flakes in this way enables colour effects to be obtained which may be stronger and less dependent on the colour of any background or the colour of the light transmissive medium or other component of the mixture. More than one layer of CLCP may be applied to the base polymer, and each layer need not be applied to d e whole surface of the polymer particle. Base polymers other than polysiloxane may be used, for example dyed mylar or PVA. The particles need not be coloured black. Stronger colour effects may be produced if a particle is given a specific colour, and a layer of CLCP which reflects light of the same colour and one sense of circular polarization is provided thereon. The effect may be enhanced if the particle is provided with a second layer of a CLCP which reflects light of substantially the same colour with the opposite sense of circular polarization to that of the first layer and which at least partially overlaps the first layer.

The particles need not be of a polymer, and may be provided with a reflective surface under the CLCP layer. For example, particles of aluminium or mica may be used with one or more CLCP layers thereon. The particles or laminae do not have to be planar. Concave or convex particles or laminae may be employed to produce iridescence with an apparent depth different from the thickness of the laminae.

Other light transmissive media such as petroleum jelly, amyl acetate, cellulose acetate butyrate + methyl ethyl ketone, polyvinyl alcohol + water, or polyurethane lacquer may be used as an alternative in any of the above embodiments. Coloured substantially transparent media may be used as another alternative. For the coating to retain the polarization properties of the flakes the medium or binders chosen should be free from birefringence. However, decorative effects may still be obtained even if a birefringent medium is used.

Although specific UV crosslinkable cholesteric liquid crystal polysiloxane polymers have been described in the above embodiments, many other types of CLCP

such as, for example, polyacrylates may be used as an alternative. Specifically the polymers need not be UV crosslinkable. Chiral smectic liquid crystal polymers may be used as an alternative. Similarly the method of manufacture described is not the only way to make such flakes - polymer films with such good surface quality are not always necessary, and for some applications coated drums or flexible plastic sheeting or aluminium foil may be used in the manufacture in place of the glass plates. Flake dimensions from a few microns across to hundreds of microns may be used for different effects.

Optical materials may be formed using laminae with a single reflection characteristic. Optical materials may also be formed using mixtures of laminae with different reflection characteristics. Laminae may also be made each with a plurality of narrow wavelength reflection bands and one or more senses of circular polarization by using multiple layers. Materials may be made using mixtures of laminae reflecting in the same narrow wavelength band but different senses of circular polarization. Such laminae will reflect approximately 50% of unpolarized incident light in a specified narrow wavelength band, but almost 100% in that band where they overlap.

The reflection pattern of the coated surface is in general random and unique, as in general the reflective particles are randomly dispersed within the medium and of random size. Each pattern may be optically read in the manner of a fingerprint and may be difficult to falsify. The surfaces of the particles themselves may be arranged to have an identifiable structure, such as an interference layer (for example a 1/4 wave plate) or a diffraction grating or hologram which can diffract incident light to give additional iridescent colours. Such a grating or hologram may be coated with a further transparent layer (for example of indium tin oxide) to enhance the diffraction efficiency by increasing the change in refractive index between the surface of the lamina and the medium. The reflection characteristics from a multiplicity of such lamina disposed randomly may give a pattern similar to an X-ray powder diffraction pattern with characteristic rings which can be used to identify the optical material present Such a pattern may also be obtained by rotating an object such as a CD marked with the optical material. The reflection characteristics may be used to code information. The codes may be read by inspecting through a circular or linear polarizer. Many combinations of flakes with different colours and polarization characteristics are possible. The colour effects are particularly suitable

for document and banknote printing as the reflection characteristics cannot be reproduced photographically. UV or IR reflecting laminae may be used for security printing applications as such patterns need not be visible to the naked eye. "Hidden" patterns may be made by printing an area with a material with a specific colour and regions within that area with different senses of polarization of the reflected light. The pattern will then only be seen by viewing through a polarizer. A similar effect may be obtained by using regions with reflection characteristics which reflect different amounts at a specific wavelength but appear the same colour to the eye. The pattern may then be viewed using light of the specific wavelength to produce a pattern which would not be visible using a broad band light source.

The optical materials described above have many uses. For example they may be used to protect credit cards or CDs or documents of value such as banknotes from counterfeiting. Iridescent decorative effects may be produced for car paints, cosmetics including nail varnish, eye shadow, lipstick etc. UV reflecting flakes may be used to make sun screen products. Such optical materials together with abrasives may be used in facial scrubs. Lastly, printing the materials onto textiles enables interesting decorative effects to be obtained.