CA2261863A1 - Light modulating liquid crystal devices - Google Patents

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

WO ~ 5~3~ PCT/GB97/02202 TITLE
LlGHT MODULATlNG LIQU~D CRYSTAL DEV~CES
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 inct~nce7 as switchable privacy gl~7.ing~.
Background Art Liquid crystal light mod~ ting devices for glazings are known which comprise a thin layer (of the order of 25~m) of liquid crystal material sandwiched bet~veen transparent plastics materials substrates, further l:lmin~ted bet~veen 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 m~tPri~l i.e. clear or scattering, is determined by a voltage applied across the material via the co~ting~
Liquid crystal light motl~ ting devices are increasingly in demand for use in, for example, ar~hitect-lral gla7ing~ 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 gla_ings. 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 nPm~tic liquid crystal m~te.n~l 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.

W O ~g~'~5~5~ PCT/GB97/02202 An inherent feature of PDLC devices, however, is high ha~ in the clear state when the device is viewed at anything other than the normal angle. As the viewing angle deviates from nr~rm~l, mi.cm~trh 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~ or more to the normal.
In ~drliti~n, the process for fabricating PDLC devices has proven problem~tic~l The process may involve coating an aqueous solution of polyvinyl alcohol cont~ining about 50%
dispersed n~m~tic liquid crystal m~t~ri~l onto indium tin oxide ("ITO") coated polyester sheet, allowing the water to evaporate, and roll l~min~ting a second sheet of lTO coated polyester on top of the liquid crystal m~ri~l For architectural applications, the polyester sheet sandwich cannot generally be used on its own, so the polyester sheets must then be l~min~ted 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 ha~ in the clear state, even at relatively large angles to the normal, and preferably operates at lower switching voltages than the 60-lOOV 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 li~uid crystal m~teri~l is (lesr~ibed 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 n~o.m~ti~ layers align with their directors at a slight angle to each other rather than parallel as in a true nemat~ 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 n~.m~tic liquid crystal materials may also adopt a cholesteric ~llu~;lult; in the presence of chiral liquid crystal dopants. Because of their periodic ~t~ u~;lule, cholesteric materials reflect light of a particular wavelength which is defined by the pitch of the helical WO ~X~ J~ PCT/GB97/02202 arr~ng~m~nt of the liquid crystal molecules. With cholesteric liquid crystal m~t~n:~l 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 domain~
of fingerprint texture to be formed. This state is called the focal conic state and is sca~l~A ng becallse 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 m~ten~l 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 a~plop~iate polymers. Both normal mode (off-scattering, on-clear) and reverse mode (off-clear, on-sc~ g) devices of l50mm x 150mm size have been demonstrated. These devices exhibit excellent trans~d.ellcy and low haze in their respective clear states on visual inspection. The opacity in the scattçring state can be increased by increasing the cell thic~nesc, but at the expense of increasing operating voltage at a rate of approximately 1-2V/ym. Previous nematic systems have typically operated in the range of 60-lOOV. 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 moA~ ting devices using cholest~ric liquid crystal m~tPn~l with stabilising polymers have come to be known as Polymer St~bili~eA Cholester c Texture (PSCI') devices. In normal mode PSCI' 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 polym~.ri~atiQn 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 polyimide alignm~nt layer of the type used in twisted nem~tic liquid crystal displays. The sç~t~ring focal conic mode is stabilised in the on-state by the polymer.
While PSCr devices have clear advantages over the original nem~tic technology, PSCr devices have proven rather difficult to manufacture and scale-up.

W O ~XI'~ PCT/GB97/02202 Convention~l PD~C devices are produced by pumping a relatively viscous 50/50 liquid crystaVpolymer solution through a slot die onto a moving sheet of ITO coated polyester sheet.
After allowing the solvent to eva~ e, a second lrrO coated polyester sheet is roll-l~minated on top of the first, with the liquid crystaVpolymer n~i~lure therebetween. The desired uniform spacing of, for example, 25+1 ~Lm is achieved either by adding 25 llm 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 ~ tules are usually 50% liquid crystal wl-e~eas PSCT llfi~ulcs 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 m~ch~nir~lly 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 lOOmm x lOOmm, have been made usingtwo 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 l~ é~lure and for a time (for example at 275QC for sixty mi~ L~,s) 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 ch~mbçr. Evaru~tion of the chamber, lowering the open side of the cell into the liquid crystal mixture and re-pr~ss.., icing the bell jar then results in the cell being unirollnly filled. Sealing the fourth side, UV-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 lTO surfaces completes the cell ~csenlbly.
While adequate for small area devices, for devices greater than about 300 mm x 300 mm the vacuum filling process requires excessively large vacuum ch~n bers, long filling times, and likely non-homogenous filling of the cell due to phase separation of the monomer from the liquid crystal. Known ~lt~rn~tive l~in~ting methods for large area l~rnin~t~s have always been considered to be unsuitable for making large area liquid crystal devices with at least one glass - - r CA 02261863 1999-01-29 7 , ~ ~, 7 7 7 ~ ' 7, 5' ' ''' '' substrate, be~auce 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 mo~ul~ting device from a liquid crystal cont~ining mixture Co~ g a liquid crystal material, either cholesteric or ~ern~tiC~ and a monomer which together are capable of forming polymer st~biliserl PSCI' liquid crystal m~tPri~l which method cornprises sandwiching the liquid crystal cont~ining mixture bel~ two substrates, at least one of which is glass, the liquid crystal cont~ining mixture being applied to a first substrate before the first and second substrate are brought together and coll,l)ressed followed by polym~ ic~tic-n of the monomer in the liquid crystal cont~ining mixture in the plesence of an alternating electric field having a frequency in excess of 1 KHz.
Preferably, the akernating electric field has a frequency in excess of 2 'KHz.
Further preferably, the monomer in the liquid crystal cont~ining mixture is polymeri~ by exposure to IJV radiation. When the liquid crystal material is n~m~ti~ the iiquid crystal cont~ining rnixture further comprises a chiral agent.
The invention offers a sirnple and low cost method suitable for producing large scale devices (typically with one ~i.~.f ..~;on greata than 300mm) of the type used in architectllral gl~7ingc and other applications. The fact that at least one of the substrates in the sandwich is glass rneans that it is readily suitable for use as an a~c~ ec~ al pane in, for example, a window or internal partition, without the need for further l~min~ting to another more suitable architectural sheet n~teri~l What is more, the method according to the invention enables the production of large scale devices without the need for relatively complex and expensive e luipn~ t or techl~ues, such as the use of vacuum apparatus.
- The first substrate is glass and the-second substrate-is pre~bly g]~ss but may be plastics sheet material, such as polyester.
Each substrate preferably has an electri~lty conductive coating on a surface against the liquid crystal cont~ining 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 plcif~led because glass with these coatings is readily available from glass ...A...-~ct--rers. For in~t~nce the applicants produce a range of glasses known as TEC glass which are coated with fluorine doped tin oxide O '~ r.
r.
5a and s~lit~ for use in a nurnber of electrical applications, for eY~mple, heated glazed doors for freezer cabinets.
In order to avoid short circuits in view of the short d~ nce~ only tens of ~lm, belweell the two electrically conductive coatings, each coating may have an inc~ ting or passivation layer, say of silica, applied over it. Some çlPctri~lly conductive coatings may have a rough surface finish which again may increase the chance of short circuits over such a small dist~nce, and it rnay be advantageous to polish these coatings to reduce roughnP.ss.

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CA 02261863 1999-01-29 , ~

The liquid crystal cont~ining m;xture 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.
The ,l~lule may also contain a poly nerisation initi~tor~ preferably a photoinitiator, and, if the liquid crystal mat~ri~l is nPmflt~ a chiral agent. The monomer is preferably polyll~.ised by exposure to W radiation. Typically, the mixture conlai~s in excess of 90 wt% liquid crystal m~teri~l The m~t~n~l available in the UK from the Merck COIll~dlly under the de~igni~tion E48 is an example of a suitable liquid crystal materiaL This has been used in comhin~tion with 4,4'-bis[6-(acryloyloxy)-hexyloxy]-1,1'-biphenylene liquid crystalline monomer, a chiral agent available in the UK from the Merck company under the design~tion R1011 and benzoin methyl ether photoiniator. The proportion of con~tituents in the liquid crystal cont~ining mixture will also vary according to wl~ er a norrnal or reverse mode device is to be produced.
Por normal mode devices, the monomer is poly. ". . ;se~ 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 PO1YT"~ ;c n stage. The mixture may also contain spacer means such as spheres or rods of glass or incompressible plastics m~t~ ri~ , The purpose of the spacer rneans is to m~int~in a gap between the two substrates. Alternatively, the spacer means can be applied separately to the surface of one of the substrates by an appropliale method, such as spraying on a disp~.sion of glass spheres in an alcohol-based carrier "~11;.",. It has also been suggested that adhesive spacer materials could be used which are capable of keeping the two substrates the r~uisi~e n~e apart whilst at the sarne time adhering them together so that, in particular, there is opposition to the possible "bulging" effects of hydrostatic pressu.~. Polymeric adhesive spacers are culle,.~'USe~ in-the production of liquid crystal display panels.
Also prefaably, preS~UlbiS applied to the two sub~hdtes to uni~olll~ly compress the ll~lurt to a desired ll,;,L llGS5, normally in the region of lS~lm. The substrates may be passed be~-. ~n two rollers, placed in a press, placed in an autoclave, put into vacuum bag apparalus or subjected to any other si~lar or equivalent hr~ fnt which involves forcing the substrates .. . . _ W O~ J5~ PCT/GB97/02202 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 inst~nce a pair of rollers, and compl~ting the process by putting the rolled substrates in an autoclave at a much greater pressul~ 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 ~ltPrn~tive adhesive. If the liquid crystal cont~ining mixture is roller coated onto the first substrate, the edges of the sandwich may be sealed prior to applying pl-,s~ule. Roller coating involves applying little if any excess liquid crystal cont~ining mixture, so there is no need to leave the edges unblocked to allow excess m~ten~l to escape during the pressure applying stage. On the other hand, if the material is, for in~t~n~e, 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 del~min~ting than one made later.Operating voltage is applied to the electrically conductive coatings through conductive tracks (Sometim~s called busbars) which are elongate conductive strips attar hed 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 cl~ane(l 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 t~chnology The Drawin~,c Figure 1 is a schPm~tic 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 W O 9~/&~3~ PCT/GB97/02202 Schem~tically illustrated in figure 1 is a production line for making large area liquid crystal light mod~ ting devices. The line, indirat~cl generally at 10, comprises a sandwiching station 20, a pressing station 30 and a polymeri~tion station 40.
With reference also to figure 2 (for ease of understanding flim~ncions have beenexaggerated), the first stage in the production of the device is the sandwiching of a mixture 6 cont~ining liquid crystal m~teri~l 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 ~lectri~ally conducting fluorine doped tin oxide coating 8a, 8b previously applied to it by a ~hP.mic~l 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 l5~m 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 PSCI liquid crystal materi~l 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 m~teri~l; 2.1 wt% 4,4'-bis[6-(acryloyloxy)-hexyloxy]-l,l'-biphenylene liquid crystalline monomer, 3 wt% R1011 (Merck ~esignatinn) chiral agent and 0.15 wt% benzoin methyl ether photoinitiator.
With the first substrate 2 coated with liquid crystal containing rnixture 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 ples~ulc 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 cor~r~ining ~ t~lle 6 to the desired thickness of 15 ,um, limited and maintained by the spacers. Applying pressure also assists in driving out any unwanted trapped air.

W 098~ J~ PCT/GB97/02202 After this, the coml-lcssed sandwich 14 passes onto a polymerisation station 40 at which it is exposed to UV r~ tio~ = 365 nm), thereby polymerising the monomer in the liquid crystal ~ ul~ 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 elec~ric~l 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 sl-rf~ces 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 :~lignment layer (not shown) by spin coating it with a polyimide precursor solution, heating it for a time and at a ~mpela~ul~ 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 (20)

11

1. A method of making a liquid crystal light modulating device from a liquid crystal containing mixture comprising a liquid crystal material, either cholesteric or nematic, and a monomer which together are capable of forming polymer stabilized PSCT liquid crystal material which method comprises sandwiching the liquid crystal containing mixture between two substrates, at least one of which is glass, the liquid crystal containing mixture being applied to a first substrate before the first and second substrate are brought together and compressed followed by polymerisation of the monomer in the liquid crystal containing mixture in the presence of an alternating electric field having a frequency in excess of 1 KHz.

2. A method as claimed in claim 1 wherein the alternating electric field has a frequency in excess of 2 KHz.

3. A method as claimed in claim 1 or claim 2 wherein the monomer in the liquid crystal containing mixture is polymerised by exposure to UV radiation.

4. A method according to any one of claims 1 to 3 wherein the first substrate is glass and the second substrate is glass or a plastics sheet material.

5. A method according to claim 4 wherein the second substrate is a polyester material.

6. A method according to any one of the preceding claims wherein each substrate has an electrically conductive coating on a surface against the liquid crystal containing mixture.

7. A method according to any one of the preceding claims wherein the liquid crystal material is nematic and the mixture further comprises a chiral agent.

8. A method according to claim 7 wherein the mixture further comprises a photoinitiator.

9. A method according to any one of the preceding claims wherein the mixture comprises at least 90 wt% liquid crystal material.

10. A method according to any one of the preceding claims wherein the liquid crystal containing material further comprises spacer means.

11. A method according to any one of claims 1 to 9 wherein the spacer means is separately applied to the first substrate.

12. A method according to claim 10 or claim 11 wherein the spacer means comprises spheres or rods of glass or plastics materials or adhesive materials.

13. 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.

14. A method according to any preceding claim wherein pressure is applied to the two substrates to compress the mixture to a desired thickness.

15. A method according to claim 14 wherein pressure is applied to the substrates using rollers, a press, vacuum bag means or an autoclave.

16. A method according to claim 13 or claim 14 wherein the edges of the sandwich are sealed either before or after applying pressure.

17. A method according to claim 6 wherein conductive tracks are applied to the electrically conductive coatings to facilitate connection to a power supply.

18. A method according to any preceding claim-wherein the at-least one-glass-sheet is toughened and/or coloured.

19. A method according to any preceding claim wherein the device has one major dimension in excess of 300 mm.

20. A liquid crystal light modulating device made by a method according to any preceding claim.