LIGHT MODULATING LIQUID CRYSTAL DEVICES

Abstract: A method suitable for producing large scale light modulating liquid crystal devices having a very thin layer, say 15 mu m thick, of liquid crystal containing material (6), preferably a mixture capable of forming polymer stabilised liquid crystal ("PSCT") material, sandwiched between two substrates (2, 4), at least one of which is glass. The liquid crystal containing material (6) is applied to the first substrate (2) before the second substrate (4) is brought into contact, and the substrates (2, 4) are compressed, for instance between rollers (18, 22), to achieve the required thickness of liquid crystal material. Spacers keep the substrates (2, 4) the requisite distance apart. Liquid crystal containing material (6) capable of forming PSCT material also requires polymerising, say, by exposing the compressed sandwich to UV radiation.

Description: TITLE LIGHT MODULATING LIQUID CRYSTAL DEVICES DESCRIPTION Technical Field The invention relates generally to a method of making liquid crystal light modulating devices (sometimes known as optical shutters), and particularly to a method of making large area liquid crystal light modulating devices of the type used, for instance, as switchable privacy glazings. Background Art Liquid crystal light modulating devices for glazings are known which comprise a thin layer (of the order of 25μm) of liquid crystal material sandwiched between transparent plastics materials substrates, further laminated between sheets of glass. The substrates have an electrically conductive coating on the surface against the liquid crystal material, and the state of the liquid crystal material i.e. clear or scattering, is determined by a voltage applied across the material via the coatings. Liquid crystal light modulating devices are increasingly in demand for use in, for example, architectural glazings which offer selective privacy, for example, conference room partition walls or hospital ward door panels. At present, permanently private, translucent patterned glasses may be used in such glazings. Also, liquid crystal light modulating devices are seen as being of use on commercial aircraft as so-called "class dividers". The conventional way of dividing different classes of passengers on an aircraft is by means of curtains which are drawn back, presumably for safety reasons, during take-off and landing. One prior art form of liquid crystal light modulating device comprises droplets of a nematic liquid crystal material dispersed in a polymer. Such devices, often termed polymer dispersed liquid crystal ("PDLC") devices, either scatter or transmit light according to the orientation of the molecules in the liquid crystal material. When no voltage is applied across the liquid crystal/polymer mixture, the liquid crystal molecules have a random orientation and the refractive index of the droplets is an average of the normal and perpendicular refractive indices. This average value is not matched to the refractive index of the polymer and, as a result, the system scatters light. Under the influence of a voltage, the molecules in the liquid crystal material align themselves generally normal to the plane of the device and, if the parallel component of the refractive index matches that of the polymer, the material appears transparent. An inherent feature of PDLC devices, however, is high haze in the clear state when the device is viewed at anything other than the normal angle. As the viewing angle deviates from normal, mismatch with the polymer refractive index increases, resulting in haze. In fact, there is typically little or no change in the opacity of these devices from the on-state to the off-state when viewed at an angle of 70 s or more to the normal. In addition, the process for fabricating PDLC devices has proven problematical. The process may involve coating an aqueous solution of poly vinyl alcohol containing about 50% dispersed nematic liquid crystal material onto indium tin oxide ("ITO") coated polyester sheet, allowing the water to evaporate, and roll laminating a second sheet of ITO coated polyester on top of the liquid crystal material. For architectural applications, the polyester sheet sandwich cannot generally be used on its own, so the polyester sheets must then be laminated between sheets of glass, making the whole production process lengthy, in terms of the number of steps involved, and potentially costly. There is scope for an easily and cheaply producible, large area light modulating device which has low haze in the clear state, even at relatively large angles to the normal, and preferably operates at lower switching voltages than the 60-100N used for conventional devices. It has been suggested that an improved large area light modulating device could be produced using cholesteric texture liquid crystal material, rather than the nematic types used previously. Cholesteric liquid crystal material is described in detail, for example, in WO-A- 9219695 (Kent State University), which is incorporated by reference herein. Nematic liquid crystals align with their major axes parallel, but there is no lateral order or correlation between the ends of one molecule and those of its neighbours. Smectic liquid crystals are nematics in which the ends of the molecules do align, producing slabs or domains. Cholesteric liquid crystals have an additional degree of order in that molecules in adjacent nematic layers align with their directors at a slight angle to each other rather than parallel as in a true nematic. The result is that stacks of molecules are formed with a uniform twisted structure. The most common cholesteric systems are based on the cholesterol esters (hence the name), but nematic liquid crystal materials may also adopt a cholesteric structure in the presence of chiral liquid crystal dopants. Because of their periodic structure, cholesteric materials reflect light of a particular wavelength which is defined by the pitch of the helical arrangement of the liquid crystal molecules. With cholesteric liquid crystal material such as that described in WO-A-9219695, the helical pitch is tuned to reflect in the infra-red. Thus, in the clear state, the liquid crystals between two substrates adopt a planar structure, or Grandjean texture, and appear transparent with little or no haze in the visible region. When an electric field is applied, the liquid crystal molecules are turned to align the director along the field and the helical structure is now parallel to the plates. Ideally the system should adopt a so-called fingerprint texture. However, anchoring forces due to the surface of the substrates compete with the torque on the molecules due to the applied field and cause a large number of domains of fingerprint texture to be formed. This state is called the focal conic state and is scattering because of the abrupt changes in refractive index at the domain boundaries. If the applied field is increased beyond a threshold value, the liquid crystals are forced into the homeotropic state, where the helical structure is lost and the material again becomes transparent It is also possible to stabilise any of the cholesteric states with respect to the others by the addition of small amounts, typically up to 10%, of appropriate polymers. Both normal mode (off-scattering, on-clear) and reverse mode (off-clear, on-scattering) devices of 150mm x 150mm size have been demonstrated. These devices exhibit excellent transparency and low ha∞ in their respective clear states on visual inspection. The opacity in the scattering state can be increased by increasing the cell thickness, but at the expense of increasing operating voltage at a rate of approximately l-2V/μm. Previous nematic systems have typically operated in the range of 60-100V. In contrast, a system based on the polymer stabilised cholesteric texture materials can operate at a voltage less than 50V and probably less than 30V, with low clear- state haze and high scattering-state opacity. Light modulating devices using cholesteric liquid crystal material with stabilising polymers have come to be known as Polymer Stabilised Cholesteric Texture (PSCT) devices. In normal mode PSCT devices, the scattering focal conic texture is stabilised at zero field and the material is switched into the transparent homeotropic texture. The fabrication of normal mode devices involves applying an electric field during the polymerisation stage. Reverse mode devices are stabilised in the planar texture with the reflection peak in the infra-red at zero field by using a rubbed polyi ide alignment layer of the type used in twisted nematic liquid crystal displays. The scattering focal conic mode is stabilised in the on-state by the polymer. While PSCT devices have clear advantages over the original nematic technology, PSCT devices have proven rather difficult to manufacture and scale-up. Conventional PDLC devices are produced by pumping a relatively viscous 50/50 liquid crystal/polymer solution through a slot die onto a moving sheet of ITO coated polyester sheet. After allowing the solvent to evaporate, a second ITO coated polyester sheet is roll-laminated on top of the first, with the liquid crystal polymer mixture therebetween. The desired uniform spacing of, for example, 25±1 μm is achieved either by adding 25 μm polymer spheres to the coating solution or, more usually, the dry film is itself sufficiently uniform and hard that spacers are not required. PDLC liquid crystal mixtures .are usually 50% liquid crystal whereas PSCT mixtures are usually at least 90% (and more typically about 95%) liquid crystal, and therefore of low viscosity and complex rheology. Moreover, even when the monomer is polymerised, the resulting gel is mechanically weak and easily compressed. Thus, the PSCT systems currently require rigid substrates, such as glass, and may not be coated directly onto polyester sheets or the like without risking short circuits when compressed against a second sheet Small area PSCT devices, say of the order of 100mm x 100mm, have been made using two sheets of ITO coated glass as the substrates. For a reverse mode device, the substrates are typically coated with a polyimide precursor solution, heated at a temperature and for a time (for example at 275°C for sixty minutes) sufficient to convert the solution to polyimide, and rubbed unidirectionally using a velour cloth to complete preparation of the alignment layer. A dispersion of glass fibre spacers is then applied onto one substrate, the two glass sheets assembled to form a cell, and three of the four edges sealed using an adhesive, such as a UV- curable or epoxy adhesive. This cell is then suspended, with the open side down, over a small bath of the liquid crystal/monomer mixture, which is itself contained within a bell jar vacuum chamber. Evacuation of the chamber, lowering the open side of the cell into the liquid crystal mixture and re-pressurising the bell jar then results in the cell being uniformly filled. Sealing the fourth side, UN-curing the monomer for a few minutes under a UV source as is commonly used in the art, and applying copper tape conductive tracks to the ITO surfaces completes the cell assembly. While adequate for small area devices, for devices greater than about 300 mm x 300 mm the vacuum filling process requires excessively large vacuum chambers, long filling times, .and likely non-homogenous filling of the cell due to phase separation of the monomer from the liquid crystal. Known alternative laminating methods for large area laminates have always been considered to be unsuitable for making large area liquid crystal devices with at least one glass substrate, because of the difficulty in achieving and controlling such thin interlayer spacing over such a large area with a potentially irregular and rigid substrate. The Invention The invention provides a method of making a liquid crystal light modulating device comprising sandwiching a mixture containing liquid crystal material between two substrates, wherein at least one of the substrates is glass, characterised in that the liquid crystal containing mixture is applied to the first substrate before the first and second substrates are brought together. The invention offers a simple and low cost method suitable for producing large scale devices (typically with one dimension greater than 300mm) of the type used in architectural glazings and other applications. The fact that at least one of the substrates in the sandwich is glass means that it is readily suitable for use as an architectural pane in, for example, a window or internal partition, without the need for further laminating to another more suitable architectural sheet material. What is more, the method according to the invention enables the production of large scale devices without the need for relatively complex and expensive equipment or techniques, such as the use of vacuum apparatus. The second substrate is preferably glass but may be plastics sheet material, such as polyester. Each substrate preferably has an electrically conductive coating on a surface against the liquid crystal containing material. The coating may be indium tin oxide or fluorine doped tin oxide or any other suitable conductive coating which may be applied to the substrates. With glass substrates, indium tin oxide or fluorine doped tin oxide are preferred because glass with these coatings is readily available from glass manufacturers. For instance, the applicants produce a range of glasses known as TEC glass which are coated with fluorine doped tin oxide and suitable for use in a number of electrical applications, for example, heated glazed doors for freezer cabinets. In order to avoid short circuits in view of the short distance, only tens of μm, between the two electrically conductive coatings, each coating may have an insulating or passivation layer, say of silica, applied over it. Some electrically conductive coatings may have a rough surface finish which again may increase the chance of short circuits over such a small distance, and it may be advantageous to polish these coatings to reduce roughness. The liquid crystal containing mixture may be applied to the first substrate by pouring it on, roller coating it, spraying it or by any other similar or equivalent technique suitable for the purpose. Further preferably, the mixture contains liquid crystal material, either cholesteric or nematic, and a monomer which together are capable of forming PSCT liquid crystal material. The mixture may also contain a polymerisation initiator, preferably a photoinitiator, and, if the liquid crystal material is nematic, a chiral agent The monomer is preferably polymerised by exposure to UV radiation. Typically, the mixture contains in excess of 90 wt% liquid crystal material. The material available in the UK from the Merck company under the designation E48 is an example of a suitable liquid crystal material. This has been used in combination with 4,4'- bis[6-(acryloyloxy)-hexyloxy]-l,l'-biphenylene liquid crystalline monomer, a chiral agent available in the UK from the Merck company under the designation RIO 11 and benzoin methyl ether photoiniator. The proportion of constituents in the liquid crystal containing mixture will also vary according to whether a normal or reverse mode device is to be produced. For normal mode devices, the monomer is polymerised in the presence of an electric field, preferably an alternating electric field. It has been found that the higher the frequency of the voltage applied to generate the field, the less the likelihood of short circuits occurring during the polymerisation stage. The frequency is preferably in excess of lKHz, more preferably in excess of 2KHz The mixture may also contain spacer means such as spheres or rods of glass or incompressible plastics materials. The purpose of the spacer means is to maintain a gap between the two substrates. Alternatively, the spacer means can be applied separately to the surface of one of the substrates by an appropriate method, such as spraying on a dispersion of glass spheres in an alcohol-based carrier medium. It has also been suggested that adhesive spacer materials could be used which are capable of keeping the two substrates the requisite distance apart whilst at the same time adhering them together so that, in particular, there is opposition to the possible "bulging" effects of hydrostatic pressure. Polymeric adhesive spacers are currently used in the production of liquid crystal display panels. Also preferably, pressure is applied to the two substrates to uniformly compress the mixture to a desired thickness, normally in the region of 15μm. The substrates may be passed between two rollers, placed in a press, placed in an autoclave, put into vacuum bag apparatus or subjected to any other simil,ar or equivalent treatment which involves forcing the substrates together uniformly to compress the mixture. The pressure may be applied in a two stage process; achieving an initial degree of compression using one method, for instance a pair of rollers, and completing the process by putting the rolled substrates in an autoclave at a much greater pressure than the rollers or initial compressing device may be capable of applying. The edges of the sandwich may be sealed with a UV-curable or epoxy or equivalent or alternative adhesive. If the liquid crystal containing mixture is roller coated onto the first substrate, the edges of the sandwich may be sealed prior to applying pressure. Roller coating involves applying little if any excess liquid crystal containing mixture, so there is no need to leave the edges unblocked to allow excess material to escape during the pressure applying stage. On the other hand, if the material is, for instance, poured onto the first substrate, an excess is required to ensure that the whole surface of the substrate is covered. In this case, the edges need to left unblocked and so are not sealed until after pressure is applied or may be only partially sealed. However, it has been found that a seal made prior to the pressure applying step works better and more effectively resists delaminating than one made later. Operating voltage is applied to the electrically conductive coatings through conductive tracks (sometimes called busbars) which are elongate conductive strips attached to the edge of each of the coated surfaces of the substrates, preferably opposite edges. Adhesive copper strips are easy to apply as busbars, but there are many other known types which may be used. Before the liquid crystal mixture is applied, the substrates may be cleaned. The coated substrates may also have their coated surfaces treated, say with silane, to improve adhesion and, for reverse mode devices, may have an alignment layer applied using conventional technology. The Drawings Figure 1 is a schematic diagram of the stations involved in making a light modulating liquid crystal device in accordance with the invention; Figure 2 is a partial cross-sectional view of the sandwiching stage in the process of making a liquid crystal device in accordance with the invention; and Figure 3 is a partial cross-sectional view of the pressure applying stage in the process of making a liquid crystal device in accordance with the invention. Best Mode Schematically illustrated in figure 1 is a production line for making large area liquid crystal light modulating devices. The line, indicated generally at 10, comprises a sandwiching station 20, a pressing station 30 and a polymerisation station 40. With reference also to figure 2 (for ease of understanding dimensions have been exaggerated), the first stage in the production of the device is the sandwiching of a mixture 6 containing liquid crystal material between two substrates 2, 4 at the sandwiching station 10. Each of the substrates 2, 4 is a panel of 6 mm thick sodium silicate glass (made by the float production process) measuring lm x lm. One of the major faces of each of the substrates has an electrically conducting fluorine doped tin oxide coating 8a, 8b previously applied to it by a chemical vapour deposition technique during the glass production process. If desired, a colour suppression undercoating (not shown), such as that described in US Patent Nos. 4,187,336 and 4,419,386 which are incorporated herein by reference, may be provided between the glass and the tin oxide. First of all, each of the two glass substrates 2, 4 is carefully cleaned. A dispersion of glass spheres, each 15μm in diameter, in an alcohol carrier medium is then spray coated onto the coated surface of the first glass substrate 2. A liquid crystal containing mixture 6 capable of forming PSCT liquid crystal material is then applied to the same surface of the first glass substrate 2 by roller coating. One example of a suitable liquid crystal containing mixture for normal mode devices is: 94.75 wt% E48 (Merck designation) nematic liquid crystal material; 2.1 wt% 4,4'-bis[6- (acryloyloxy)-hexyloxy]-l,r-biphenylene liquid crystalline monomer; 3 wt% R1011 (Merck designation) chiral agent and 0.15 wt% benzoin methyl ether photoinitiator. With the first substrate 2 coated with liquid crystal containing mixture 6, the first and second substrates 2, 4 are brought together. The second substrate 4 is lowered into contact with the first substrate 2 from a position above and at a slight angle to the first substrate 4 so as to sandwich the liquid crystal/monomer mixture between the two. Referring also to figure 3 (again with dimensions exaggerated), next, all sides of the resulting sandwich 14 are sealed using an epoxy adhesive 16 applied along the open edges. Then, at the pressing station 30 pressure is applied to the two substrates 2, 4 by passing the sandwich 14 between a pair of sprung rollers 18, 22 so as uniformly to compress the liquid crystal containing mixture 6 to the desired thickness of 15 μm, limited and maintained by the spacers. Applying pressure also assists in driving out any unwanted trapped air. After this, the compressed sandwich 14 passes onto a polymerisation station 40 at which it is exposed to UV radiation (λ = 365 nm), thereby polymerising the monomer in the liquid crystal mixture 6. For a forward mode device the polymerisation is carried out in the presence of a 2KHz alternating electric field. Finally, conductive tracks of adhesive copper tape (not shown) are then applied to provide electrical connections to the electrically conductive coatings 8a, 8b. During the sandwiching stage, the two substrates 2, 4 are brought together in a slightly staggered fashion so that opposed ends of the coated surfaces 8a, 8b protrude slightly and make the application of the tracks that much easier. For reverse mode devices, before the spacers and liquid crystal containing mixture are applied, the first of the coated glass substrates 2 is also provided with an alignment layer (not shown) by spin coating it with a polyimide precursor solution, heating it for a time and at a temperature sufficient to convert the solution to polyimide (for example at 275°C for sixty minutes), and rubbing it unidirectionally using a velour cloth (not shown).

Claims: CLAIMS 1. A method of making a liquid crystal light modulating device comprising sandwiching a liquid crystal containing mixture between two substrates, wherein at least one of the substrates is glass characterised in that the liquid crystal containing mixture is applied to the first substrate before the two substrates are brought together. 2. A method according to claim 1 wherein the second substrate is glass. 3. A method according to claim 1 or claim 2 wherein each substrate has an electrically conductive coating on a surface against the liquid crystal containing mixture. 4. A method according to any of claims 1 to 3 wherein the mixture comprises liquid crystal material and a monomer capable of forming PSCT liquid crystal material. 5. A method according to claim 4 wherein the liquid crystal material is nematic and the mixture further comprises a chiral agent 6. A method according to claim 4 or claim 5 wherein the mixture further comprises a photoinitiator. 7. A method according to any of claims 4 to 6 wherein the mixture comprises at least 90 wt% liquid crystal material. 8. A method according to any of claims 1 to 7 wherein the liquid crystal containing material further comprises spacer means. 9. A method according to any of claims 1 to 7 wherein the spacer means is separately applied to the first substrate. 10. A method according to claim 8 or claim 9 wherein the spacer means comprises spheres or rods of glass or plastics materials or adhesive materials. 11. A method according to claim 4 wherein the compressed sandwich is passed through a polymerisation station to polymerise the liquid crystal/monomer mixture. 12. A method according to claim 11 wherein at the polymerisation station the sandwich is exposed to UV radiation. 13. A method according to claim 11 or claim 12 wherein polymerisation takes place in the presence of an electric field. 14. A method according to claim 13 wherein the electric field is alternating. 15. A method according to claim 14 wherein the frequency of the alternating field is in excess of lKHz. 16. A method according to claim 14 wherein the frequency of the alternating field is in excess of 2KHz. 17. A method according to any preceding claim wherein the liquid crystal containing mixture is applied to the first substrate by pouring, roller coating or spraying. 18. A method according to any preceding claim wherein pressure is applied to the two substrates to compress the mixture to a desired thickness. 19. A method according to claim 18 wherein pressure is applied to the substrates using rollers, a press, vacuum bag means or an autoclave. 20. A method according to claim 17 or claim 18 wherein the edges of the sandwich are sealed either before or after applying pressure. 21. A method according to claim 3 wherein conductive tracks are applied to the electrically conductive coatings to facilitate connection to a power supply. 22. A method according to any preceding claim wherein the at least one glass sheet is toughened and/or coloured. 23. A method according to any preceding claim wherein the device has one major dimension in excess of 300 mm. 24. A liquid crystal light modulating device made by a method according to any preceding claim.

Inventors: HOLMES PAUL ARTHUR, SERGEANT SHIRLEY ANNE, HOLMES, PAUL, ARTHUR, SERGEANT, SHIRLEY, ANNE
Applicants: HOLMES PAUL ARTHUR, PILKINGTON PLC, SERGEANT SHIRLEY ANNE, HOLMES, PAUL, ARTHUR, SERGEANT, SHIRLEY, ANNE
Application year: 1997
Main classification: G02F1
IPC codes: G02F1