CA2208234A1 - Multilayered optical film - Google Patents

Abstract

Birefringent optical films (10, 12, 14) have a Brewster angle (the angle at which reflectance of p-polarized light goes to zero) which is very large or is nonexistant. This allows for the construction of multilayer mirrors and polarizers whose reflectivity for p-polarized light decreases slowly with angle of incidence, are independent of angle of incidence, or increase with angle of incidence away from the normal. As a result, multilayer films (10) having high reflectivity (for both planes of polarization for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers) over a wide bandwidth, can be achieved.

Description

J 49837PCTlB CA 02208234 1997-06-19 MULTILAYERED OPIICAL FILM

BACKGROUND
The present invention relates to optical films useful, e.g., as polarizers 5 and/or mirrors.
Light-re-flP~ting devices based upon multiple polymeric layers are known.
Examples of such devices include polarizers made of alternating polymeric layersin which the layers have different refractive indices.
Q~e ~
SUMMARY
The optical pr~el~ies and design considerations of birefringent optical films described herein allow the construction of multilayer stacks for which theBrewster angle (the angle at which reflectance of p-polarized light goes to zero) is very large or is nonexistant. This allows for the construction of multilayer mirrors and polarizers whose reflectivity for p-po1arized light decreases slowlywith angle of incir1~-nce, are independent of angle of incidence, or increase with angle of incidence away from the normal. As a result, multilayer films having high reflectivity (for both s and p polarized light for any incident direction in the case of mirrors, and for the selecte~ direction in the case of polarizers) over a wide bandwidth, can be achieved.
Briefly, in one aspect the present invention provides a multilayered polymer film comprising layers of a crystalline or semi-crystalline naphth~1~n~
dic~l~o~ylic acid polyester, for example a 2,6-polyethylene naphth~1~t~ (UPEN") or a copolymer derived from ethylene glycol, naphth~1~ne dicarboxylic acid and some other acids such as terephth~l~te ("co-PEN"), with a positive stress optical coefficient, i.e. upon stretching its index of refraction in the stretch direction increases, having an average th~ nçss of not more than 0.5 microns; and layers of a sel~te~ second polymer, for example a polyethylene terephth~1~te ("PET") or a co-PEN, having an average thicl~nçcc of not more than 0.5 microns.
Preferably, after sLre~ching of the films of this invention in at least one direction, the layers of said naphthalene dicarboxylic acid polyester have a higher index of IPE~

CA 0 2 2 0 8 2 3 4 1 9 97 - 0 6 - 1 9 ~ T~
PA~ r~ 'W4LTE
~Efi~.-'r-~TSTF~. 4 ~ ~. J~n. 1997 MINNESOTA MINING & MANUFACTURING CO.
Our Ref: A 2151 PCT

- la -EP-A-0 404 463 relates to a multi-layered polymeric body which reflects light and which can be fabricated to have a silvery or hued metallic or non-conventional hued appearance. The multi-layered reflective body comprises alternating layers of diverse polymeric materials which differ in refractive index and which are either optically thick or optically very thin.

EP-A-0 488 544 relates to a multi-layered birefringent interference polarizer and more particularly to a multi-layered co-extruded polymeric device which can be designed to polarize selected wavelengths of light by constructive optical interference.

AMENDE~ SHEET
IPEAIEP /

WO 96/19347 PCT/US95/16!;55 refr~ction associated with at least one in-plane axis than the layers of the second polymer. The film of this invention can be used to pl~are multilayer films having an average reflectivity of at least 50% over at least a 100 nm wide band.In another aspect, the present invention provides a multilayered polymer 5 film compri~in$ layers of a crystalline or semi-crystalline polyester, for eY~mple a PET, having an average thickn~ss of not more than 0.5 microns; and layers of a s~lPcted second polymer, for example a polyester or a polystyrene, having an average thickn~s of not more than 0.5 microns; wherein said film has been stretched in at least one direction to at least twice that direction's unstretched 10 rlim~n~ion. The film of this invention can be used to prepare multilayer films having an average reflectivity of at least 50% over at least a 100 nm wide band.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further explained with reference to the drawings.
Figures la and lb are diagr~mm~tical views of the polarizer of the present invention.
Figure 2 is a graphical view illustrating the refractive indices char~ct~ tics of the PEN and coPEN layers of the present invention.
Figure 3 is a graphical view of computer simulated data of percent 20 tr~n.~mi~ion of a 50-layer PEN/coPEN film stack based on the indices shown in Figure 2.
Figure 4 is a graphical view of computer simulated data of percent tr~n~mi~ion of an equally biaxially stretched 300-layer PEN/coPET mirror.
Figure 5 is a graphical view of percent measured tr~n~mi~ion of a 25 51-layer I.R. polarizer of the present invention with the first order peak near 1,300 nm.
Figure 6 is a graphical view of percent measured tr~nsmi.~ion of eight 51-layer polarizers of the present invention l~min~ted together.
Figure 7 is a graphical view of percent measured tr~n~mi~ion of a 30 204-layer polarizer of the present invention.

Figure 8 is a graphical view of percent measured tr~n~mi.c~ion of two 204-layer polarizers of the present invention l~min~t~d together.
Figure 9 is a schem~tic view of an overhead projector of the present invention.
Figure 10 shows a two layer stack of films forming a single interf~ce.
Figures 11 and 12 show reflectivity versus angle curves for a uni~xi~l birefringent system in a m~linm of index 1.60.
Figure 13 shows reflectivity versus angle curves for a uni~xi~l birefringent system in a m~inm of index 1Ø
Figures 14, 15 and 16 show various relationships between in-plane indices and z-index for a llni~xi~l birefringent system.
Figure 17 shows off axis reflectivity versus wavelength for two different biaxial birefringent systems.
Figure 18 shows the effect of introducing a y-index difference in a biaxial birefringent film with a large z-index difference.
Figure 19 shows the effect of introducing a y-index dirre.t:nce in a biaxial birefringent film with a smaller z-index difference.
Figure 20 shows a contour plot s~-mm~ri7ing the information from Figures 18 and 19;
Figures 21-26 show optical pelrol"lance of multilayer nlil~ol~ given in Examples 3-6;
Figures 27-31 show optical pe.ro mance of multilayer polarizers given in PY~mples 7-11;
Figure 32 shows the optical pelrormance of the multilayer mirror given in Example 12;
Figure 33 shows the optical pe rolmance of the AR coated multilayer reflective polarizer of Example 13;
~ Figure 34 shows the optical performance of the multilayer reflective polarizer of Example 14; and WO 96/19347 PCT/US95/165!j5 Figures 35a-c show optical performance of multilayer polarizers given in Example 15.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The present invention as illustrated in Figures la and lb includes a multilayered polymeric sheet 10 having ~ltçrn~ting layers of a crystalline naphth~lP,n~, dicarboxylic acid polyester such as 2,6 polyethylene naphth~l~te (PEN) 12 and a sPlpcte~ polymer 14 useful as a reflective polarizer or mirror.
By sl-e~cl~ g PEN/selected polymer over a range of uniaxial to biaxial orit~nt~tion, a film is created with a range of reflectivities for differently oriented plane-pol~ri7~d incide,nt light. If stretched biaxially, the sheet can be stretched asymm~,tric~lly along orthogonal axes or symmetric~lly along orthogonal axes to obtain desired pol~ri7ing and reflecting properties.
For the polarizer, the sheet is preferably oriented by stretching in a single direction and the index of refraction of the PEN layer exhibits a large difference between incid~,nt light rays with the plane of pnl~ri7~tion parallel to the oriented and transverse directions. The index of refraction associated with an in-plane axis (an axis parallel to the surface of the film) is the effective index of refraction for plane-polarized incident light whose plane of pol~ri7~tion is parallel to that axis. By oriented direction is meant the direction in which the film is stretched.
By transverse direction is meant that direction orthogonal in the plane of the film to the direction in which the film is oriented.
PEN is a ~rer~lled m~teri~l because of its high positive stress optical coefficient and permanent birefringence after stretching, with the refractive index for pol~ri7PA incillent light of 550 nm wavelength increasing when the plane of pol~ri7~tion is parallel to the stretch direction from about 1.64 to as high as about 1.9. The differences in refractive indices associated with different in-plane axes exhibited by PEN and a 70-naphthalate/30- terephth~l~te copolyester (coPEN) for a 5:1 stretch ratio are illustrated in Figure 2. In Figure 2, the data on the lower curve represent the index of refraction of PEN in the transverse direction and the WO 96/19347 PCTIUS95/165S~

coPEN while the upper curve represents the index of refraction of PEN in the stretch direction. PEN exhibits a dirÇerellce in refractive index of 0.25 to 0.40 in the visible s~e~;L.uln. The bi,erlillgence (difference in refractive index) can be increased by increasing the molecular orient~tion. PEN is heat stable from about155~C up to àbout 230~C depentling upon ~hrink~ge requirements of the applic~tion. Although PEN has been spe~ific~lly discussed above as the plere~l~d polymer for the birefringent layer, polybutylene naphth~l~te is also asuitable m~tPri~l as well as other crystalline naphth~lPne dicarboxylic polyesters.
The crystalline naphth~lPne dicarboxylic polyester should exhibit a difference in refractive indices associated with dirrelellt in-plane axes of at least 0.05 andpreferably above 0.20.
Minor amounts of comonomers may be substituted into the naphthalene dicarboxylic acid polyester so long as the high refractive index in the stretch direction(s) is not substantially compromised. A drop in refractive index (and therefore decreased reflectivity) may be counter balanced by advantages in any of the following: adhesion to the selected polymer layer, lowered temperature of extrusion, better match of melt viscosities, better match of glass tr~n~ition ~en.pel~Lul~s for sllelchillg. Suitable monomers include those based on isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7- naphth~l~neicarboxylic, 2,6-naphth~l~ne dicarboxylic or cyclohexanedicarboxylic acids.
The PEN/selected polymer resins of the present invention preferably have similar melt viscosities so as to obtain uniform multilayer coextrusion. The twopolymers preferably have a melt viscosity within a factor of 5 at typical shear rates.
The PEN and the preferred selected polymer layers of the present invention exhibit good adhesion properties to each other while still rem~ining as discrete layers within the multilayered sheet.
The glass transition temperatures of the polymers of the present invention are comp~tible so adverse effects such as cracking of one set of polymer layers during stretching does not occur. By compatible is meant that the glass transition te~ dlulc of the selected polymer is lower than the glass transition Lelllpe aLIlre of the PEN layer. The glass transition temperature of the selected polymer layerIc;,ll~el~tule may be slightly higher than the glass transition temperature of the PEN layer, but by no more than 40~C.
S Preferably, the layers have a 1/4 wavelength thickness with dirrelellt sets of layers decign~d to reflect different wavelength ranges. Each layer does not have to be exactly 1/4 wavelength thick. The overriding requirement is that the ~dj~cPnt low-high index film pair have a total optical thickn~ss of O.S
wavelength. The bandwidth of a 50-layer stack of PEN/coPEN layers having the index dirr~ tial in-lic~ted in Figure 2, with layer thicknesses chosen to be a 1/4 wavelength of 550 nm, is about S0 nm. This 50-layer stack provides roughly a 99 percent average reflectivity in this wavelength range with no measurable absorption. A co",pu~l-modeled curve showing less than 1 percent tr~n~mi~ion (99 percent reflectivity) is illustrated in Figure 3. Figures 3-8 include data ch~r~cteri7ed as percent tr~n~mi~ion. It should be understood that since there is no me~m~hle absorbance by the film of the present invention that percent reflectivity is approxim~ted by the following relationship:
100 - (percent tr~n~mi~ciQn) = (percent reflectivity).
The pl~rell~d s~-lPcted polymer layer 14 remains isotropic in refractive index and subst~nti~lly m~t~hes the refIactive index of the PEN layer associatedwith the transverse axis as illustrated in Figure la. Light with its plane of pol~ri7~tion in this direction will be predomin~ntly tr~n~mitted by the polarizer while light with its plane of pol~ri7~tion in the oriented direction will be reflected as illustrated in Figure lb.
The reflective polarizer of the present invention is useful in optical elem~nt~ such as ophth~lmic lenses, mirrors and windows. The polarizer is characterized by a mirror-like look which is considered stylish in sl-ngl~ses. In ~-lrlition, PEN is a very good ultraviolet filter, absorbing ultraviolet efficiently up to the edge of the visible spectrum. The reflective polarizer of the present invention would also be useful as a thin infrared sheet polarizer.

For the polarizer, the PEN/selectéd polymer layers have at least one axis for which the associated indices of refraction are preferably substantially e~ual.
The match of refractive indices associated with that axis, which typically is the transverse axis, results in subst~nti~lly no reflection of light in that plane of 5 pol~ri7~tion The selected polymer layer may also exhibit a decrease in the refractive index ~sori~t~A with the stretch direction. A negative birefringence of the s~Qlectçd polymer has the advantage of increasing the dirrerence between indices of refraction of adjoining layers associated with the orientation axis while the reflection of light with its plane of pol~ri7~tion parallel to the transverse 10 direction is still negligible. Differences between the transverse-axis-associated indices of refraction of adjoining layers after stretching should be less than 0.05 and preferably less than 0.02. Another possibility is that the selected polymer exhibits some positive birefringence due to stretching, but this can be relaxed to match the refractive index of the transverse axis of the PEN layers in a heat 15 tre~tmP-nt The lel"p~l~ture of this heat tre~tmçnt should not be so high as to relax the birefringence in the PEN layers.
The plere~led selected polymer for the polarizer of the present invention is a copolyester of the reaction product of a naphthalene dicarboxylic acid or its ester such as dimethyl naphth~l~te ranging from 20 mole percent to 80 mole 20 percent and isophthalic or terephthalic acid or their esters such as dimethylterephth~l~te r~nging from 20 mole percent to 80 mole percent reacted with ethylene glycol. Other copolyesters within the scope of the present invention have the Ll~elLies discussed above and have a refractive index associated with the transverse axis of approxim~tçly 1.59 to 1.69. Of course, the copolyester 25 must be coextrudable with PEN. Other suitable copolyesters are based on isoI~hth~lic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7- naphth~l~ne dic~l,o~ylic, 2,6-naphth~lçne dicarboxylic or cyclohexanedicarboxylic acids.
Other suitable variations in the copolyester include the use of ethylene glycol,propane diol, butane diol, neopentyl glycol, polyethylene glycol, tetramethylene30 glycol, diethylene glycol, cyclohex~ne~limethanol, 4-hydroxy diphenol, propane WO 96/19347 PCT/US9~;/16555 diol, bisphenol A, and 1,8-dihydroxy biphenyl, or 1,3-bis(2-hydlv~y~Llloxy)benzene as the diol reactant. A volume average of the refractive indices of the monomers would be a good guide in plel)a~ g useful copolyesters. In addition, copolycarbonates having a glass transition S le~ ature compatible with the glass transition temperature of PEN and with a refractive index associated with the transverse axis of approximately 1.59 to 1.69 are also useful as a selected polymer in the present invention. Formation of thecopolyester or copolycarbonate by tr~ncesterification of two or more polymers inthe extrusion system is another possible route to a viable selected polymer.
To make a mirror, two uniaxially stretched pol~ri7ing sheets 10 are positioned with their respective orientation axes rotated 90~, or the sheet 10 is biaxially stretched. In the latter case, both PEN refractive indices in the plane of the sheet increase and the selected polymer should be chosen with as low of a refractive index as possible to reflect light of both planes of pol~ri7~tion.
15 Biaxially stretching the multilayered sheet will result in differences between refractive indices of adjoining layers for planes parallel to both axes thereby res--lting in reflection of light in both planes of polarization directions. Biaxially ~L etchillg PEN will increase the refractive indices associated with those axes of elongation from 1.64 to only 1.75, compared to the uniaxial value of 1.9.
20 Therefore to create a ~lielectric mirror with 99 percent reflectivity (and thus with no noticeable iridescence) a low refractive index coPET is prerelled as the selected polymer. Optical modeling indicates this is possible with an index of about 1.55. A 300-layer film with a 5 percent standard deviation in layer thickn~sc, decigned to cover half of the visible spectrum with six overlapping 25 4u~L~ e stacks, has the predicted performance shown in Figure 4. A greater degree of symmetry of stretching yields an article that exhibits relatively moresymmetric reflective plupellies and relatively less pol~ri7ing properties.
If desired, two or more sheets of the invention may be used in a col"L~osite to increase reflectivity, optical band width, or both. If the optical 30 thicknesses of pairs of layers within the sheets are substantially equal, the col,.posile will reflect, at somewhat greater efficiency, subst~nti~lly the sameband width and spectr~l range of reflectivity (i.e., "band") as the individual sheets. If the optical thicknes~es of pairs of layers within the sheets are not subst~nti~lly equal, the composite will reflect across a broader band width thanS the individual sheets. A composite combining mirror sheets with polarizer sheets is useful for increasing total reflect~nce while still pol~ri7ing tr~n~mitted light.
~ltP~"~ ely, a single sheet may be asymm~tric~lly biaxially stretched to produce a film having selective reflective and polarizing properties.
The ~rert;lled select~l polymer for use in a biaxially stretched mirror 10 application is based on terephthalic, isophthalic, sebacic, azelaic or cyclohex~n~licarboxylic acid to attain the lowest possible refractive index while still maintaining adhesion to the PEN layers. Naphthalene dicarboxylic acid may still be employed in minor amounts to improve the ~-lheciQn to PEN. The diol component may be taken from any that have been previously mentioned.
15 Preferably the selected polymer has an index of refraction of less than 1.65 and more preferably an index of refraction of less than 1.55.
It is not required that the selected polymer be a copolyester or copolycarbonate. Vinyl polymers and copolymers made from monomers such as vinyl naphth~l~nes, styrenes, ethylene, maleic anhydride, acrylates, 20 meth~crylates, might be employed. Condensation polymers other than polyestersand polycarbonates might also be useful, examples include: polysulfones, polyamides, polyu~tl,alles, polyamic acids, polyimides. Naphthalene groups and halogens such as chlorine, bromine and iodine are useful in increasing the refractive index of the selected polymer to the desired level (l.S9 to 1.69) to 25 subst~nti~lly match the refractive index of PEN associated with the transverse direction for a polarizer. Acrylate groups and fluorine are particularly useful in decreasing refractive index for use in a mirror.
Figure 9 illustrates the use of the present invention as a hot mirror in an overhead projector 30. The projector 30 is a tr~n~mi~ive-type projector, and has30 many features of a conventional overhead projector, including a base 32 and a projection head 34. The projection head 34 is 7~tt~chçd to the base 32 by an arm(not shown), which may be raised or lowered thereby moving the head 34 toward or away from the base 32, by conventional adjll~tm~nt means. The base 32 include~ a light source 36, a power supply (not shown) for the light source 36, 5 and a~o~?liate optical components such as a mirror 38 for directing the light toward a projection stage area 40. The stage area 40 in a conventional overhead projector in~ des a tr~ncp~rent sheet such as glass typically having at least one fresnel lens integr~lly formed therein for focusing light toward the head 34. If a tr~ncr~rency having a visual image is placed on the stage 40, the image is 10 collected and projected such as to a nearby projection screen or surface by conventional optics such as a mirror 42 and lens 44 located within the head 34.
A mirror 46 of the present invention is advantageously used in the overhead projector 30 to reflect the heat-producing infrared energy from the light source 36 while tr~ncmitting visible light. When used to reflect infrared energy, 15 the mirror 46 is used as a hot mirror. This is especially illlpOl ~It for inc~ndescçnt light sources where about 85 percent of the emitted energy is in the infrared wavelength. The infrared energy, if uncontrolled, can cause excessive heating of dense tr~ncr~rencies or LCD projection panels that are placed on the projection stage 40. When used as a hot mirror, the mirror 46 is normally 20 positioned between the light source 36 and the projection stage 40. The mirror 46 can be a separate element or the mirror can be applied to an optical component as a coating in the light path between the light source and the projection stage.
~lt~rn~tively, the mirror 46 can be used in the overhead projector 30 as a 25 cold mirror, that is a mirror that reflects visible light, while tr~ncmitting infrared energy. The mirror of the present invention may also be positioned as a folding mirror (not shown) between the light source 36 and the projection stage 40.
R~oflect~nce of a multilayer cold mirror can easily approach 95 percent for visible light. The mirror of the present invention can be applied as a cold mirror 30 coating to a spherical concave reflector such as reflector 38 that is placed behind the light source 36 to collect and redirect visible light emitted from the lightsource while tr~ncmitting infrared energy.
Orient~tion of the extruded film was done by ~LIeLching individual sheets of the m~teri~l in heated air. For economical prod~lc.tion, stretching may be accomr)li.ched on a continuous basis in a standard length orienter, tenter oven, or both. Economies of scale and line speeds of standard polymer film production may be achieved thereby achieving manuf~ctllring costs that are subst~nti~lly lower than costs associated with commercially available absorptive polarizers.
T~min~tinn of two or more sheets together is advantageous, to improve reflectivity or to broaden the bandwidth, or to form a mirror from two polarizers. Amorphous copolyesters are useful as l~min~ting m~teri~lc, with VITEL Brand 3000 and 3300 from the Goodyear Tire and Rubber Co. of Akron, Ohio, noted as m~teri~lc that have been tried. The choice of l~min~ting m~t~ri~lis broad, with adhesion to the sheets 10, optical clarity and exclusion of air being the primary guiding principles.
It may be desirable to add to one or more of the layers, one or more inorganic or organic adjuvants such as an antioxidant, extrusion aid, heat stabilizer, ultraviolet ray absorber, nucleator, surface projection forming agent, and the like in normal quantities so long as the addition does not substantiallyinterfere with the pelrol~ ce of the present invention.

Optical Behavior and Desi~n Considerations of Multilayer Stacks The optical behavior of a multilayer stack 10 such as that shown above in Figs. la and lb will now be described in more general terms.
The optical properties and design considerations of multilayer stacks described below allow the construction of multilayer stacks for which the Brew~lel angle (the angle at which reflect~nce goes to zero) is very large or is~ nonexistant. This allows for the construction of multilayer mirrors and polarizers whose reflectivity for p pol~ri7ed light decrease slowly with angle of incidence, are independent of angle of incidence, or increase with angle of incidence away WO 96/19347 PCT/US95/16~5 from the normal. As a result, multilayer stacks having high reflectivity for both s and p pr)l~ri7~d light over a wide bandwidth, and over a wide range of angles can be achieved.
~he average tr~n.cmiccion at normal incidPn~.e for a multilayer stack, (for S light pol~ri7ed in the plane of the extinction axis in the case of polarizers, or for both pol~ri7~tions in the case of mirrors), is desirably less than 50 % (reflectivity of 0.5) over the intentletl bandwidth. (It shall be understood that for the purposes of the present application, all tr~n.cmi.ccion or reflection values given include front and back surface reflections). Other multilayer stacks exhibit lower average tr~ncmiccion and/or a larger intended bandwidth, and/or over a larger range of angles from the normal. If the intended bandwidth is to be centered around one color only, such as red, green or blue, each of which has an effective bandwidth of about 100 nm each, a multilayer stack with an average tr~ncmi.c.cion of less than 50% is desirable. A multilayer stack having an average tr~ncmi.c.cion of less than 10% over a bandwidth of 100 nm is also prefelled. Other exemplary pl~rt;lled mutlilayer stacks have an average tr~ncmi.~cion of less than 30% over a bandwidth of 200 nm. Yet another prefelled multilayer stack exhibits an average t~n.cmiccion of less than 10% over the bandwidth of the visible spectrum (400-700 nm). Most prerelled is a multilayer stack that exhibits an average tr~n.cmi.ccion of less than 10% over a bandwidth of 380 to 740 nm. The extended bandwidth is useful even in visible light applications in order to accommodate spectral shifts with angle, and variations in the multiiayer stack and overall film caliper.
The multilayer stack 10 can include tens, hundreds or thousands of layers, and each layer can be made from any of a number of different materials. The char~.tt~.ri.ctics which determine the choice of m~t~ri~lc for a particular stack depend upon the desired optical ~lrol-l-ance of the stack.
The stack can contain as many materials as there are layers in the stack.
For ease of manufacture, prerelled optical thin film stacks contain only a few CA 02208234 1997-06-l9 dirrt;lc~t m~teri~l~ For purposes of illustration, the present discussion will describe multilayer stacks inclu-ling two m~teri~
The-bol-n-l~ries belween the m~tPri~l~, or chçmiç~lly ;denti~l m~teri~l~
with different physical p.~;,lies, can be abrupt or gradual. Except for some 5 simple cases with analytical solutions, analysis of the latter type of stratified media with continuously varying index is usually treated as a much larger number of thinner unifo~ layers having abrupt bolln~l~ries but with only a smallchange in pr~elLies between adjacent layers.
Several parameters may affect the maximum reflectivity achievable in any 10 multilayer stack. These include basic stack design, optical absorption, layerthickmPss control and the relationship between indices of refraction of the layers in the stack. For high reflectivity and/or sharp bandedges, the basic stack design should incorporate optical intelrereilce effects using standard thin film opticsdesign. This typically involves using optically thin layers, m~ning layers having 15 an optical thickne~ in the range of 0.1 to 1.0 times the wavelength of interest.
The basic building blocks for high reflectivity multilayer films are low/high index pairs of film layers, wherein each low/high index pair of layers has a combined optical thicknPss of 1/2 the center wavelength of the band it is desi~ned to reflect. Stacks of such films are commonly referred to as 20 ~lu~ ~l w~e stacks.
To minimi7e optical absorption, the ~r~f~ d multilayer stack ensures that wavelengths that would be most strongly absorbed by the stack are the firstwavelengths reflertPA by the stz~c-k. Por most clo~r OptlC I m.telizil-" ir,Llu.ling most polymers, absorption increases toward the blue end of the visible spectrum.25 Thus, it is prerelled to tune the multilayer stack such that the "blue" layers are on the incident side of the multilayer stack.
A multilayer construction of alternative low and high index thick films, often referred to as a "pile of plates", has no tuned wavelengths nor bandwidth constraints, and no wavelength is selectively reflected at any particular layer in 30 the stack. With such a construction, the blue reflectivity suffers due to higher penPtr~tic)n into the stack, res~ ing in higher absorption than for the p~ ed ~lu~ w~e stack design. Arbitrarily increasing the number of layers in a "pile of plates" will not always give high reflectivity, even with zero absorption.
Also, a-biL,dlily increasing the number of layers in any stack may not give the 5 desired reflectivity, due to the increased absorption which would occur.
The relationships between the indices of refraction in each film layer to each other and to those of the other layers in the film stack detPrmine the reflectance behavior of the multilayer stack at any angle of incidence, from anyazimuthal direction. ~llming that all layers of the same material have the same 10 indices, then a single interface of a two component quarterwave stack can be analyzed to understand the behavior of the entire stack as a function of angle.
For simplicity of discussion, therefore, the optical behavior of a single intPrf~e will be described. It shall be understood, however, that an actual multilayer stack according to the principles described herein could be made of 15 tens, hundreds or thousands of layers. To describe the optical behavior of a single interface, such as the one shown in Fig. 10, the reflectivity as a function of angle of incidence for s and p polarized light for a plane of incidçnce inclll~ing the z-axis and one in-plane optic axis will be plotted.
Fig. 10 shows two m~tPri~l film layers forming a single interface, with 20 both immersed in an isotropic medium of index no. For simplicity of illustr~tion, the present discussion will be directed toward an orthogonal multilayer birefringent system with the optical axes of the two materials ~ nPd,and with one optic axis (z) perpendicular to the film plane, and the other opticaxes along the x and y axis. It shall be understood, however, that the optic axes 25 need not be orthogonal, and that nonorthogonal systems are well within the spirit and scope of the present invention. It shall be further understood that the optic axes also need not be aligned with the film axes to fall within the intended scope of the present invention.
The reflectivity of a dielectric interface varies as a function of angle of 30 incid~Pnce, and for isotropic m~tPri~l~, is different for p and s polarized light.

WO 96/19347 PCT/US9~/16555 The reflectivity ~inh~u~ for p polarized light is due to the so called Brew~Ler effect, and the angle at which the reflectance goes to zero is referred to as Brt;~v~Lel 's angle.
The reflectance behavior of any film stack, at any angle of incjdence~ is S deterrnined by the dielectric tensors of all films involved. A general theoretical tre~tmPnt of this topic is given in the text by R.M.A. Azzam and N.M. R~.ch~r~, "Ellipsometry and Po1~ri7~ Light", published by North-Holland, 1987.
The reflectivity for a single interface of a system is calculated by squaring the absolute value of the reflection coefficients for p and s pol~ri7ed light, given lO by equations l and 2, respectively. Equations l and 2 are vaIid for uniaxial orthogonal systems, with the axes of the two components aligned.

1) rpp = n2z * n20 ~i(nlz2 - no2sin2~) - nlz * nlo ~(n2z2 - no2sin2~) n2z * n20 ~I(nlz2 - no2sin2~) + nlz * nlo ~I(n2z2 - no2sin2~) 2) rS5 = ~1(nlo2 - no2sin2~ 1(n202 - no2sin2~) ~1(nlo2 - no2sin2~) + ~(n202 - no2sin2~) where ~ is measured in the isotropic mylium.
In a uni~xi~l birefringent system, nlx = nly = nlo, and n2x = n2y =
20.
For a biaxial birefringent system, equations l and 2 are valid only for light with its plane of polarization parallel to the x-z or y-z planes, as defined in Fig. lO. So, for a biaxial system, for light incident in the x-z plane, nlo = nlx and n20 = n2x in equation l (for p-po1~ri7~1 light), and nlo = nly and n20 =
n2y in equation 2 (for s-polarized light). For light incident in the y-z plane, nlo = nly and n20 = n2y in equation l (for p-polarized light), and nlo = nlx and n20 = n2x in equation 2 (for s-polarized light).
Equations l and 2 show that reflectivity depends upon the indices of refraction in the x, y (in-plane) and z directions of each material in the stack. In an isotropic m~t~ l, all three indices are equal, thus nx = ny = nz. The -relationship between nx, ny and nz determine the optical characteri~tics of the m~t~ri~l. Dirrtrt;l~t rel~tiQn~hiI)s between the three indices lead to three general c~Legolies of m~teri~ isotropic, uniaxially birefringent, and biaxially birefringent. Equations 1 and 2 describe biaxially birefringent cases only along5 the x or y axis, and then only if considered s~L)an~t~ly for the x and y directions.
A uniaxially birefringent material is defined as one in which the index of refraction in one direction is different from the indices in the other two directions. For purposes of the present discussion, the convention for describing llni~xi~lly birefrin~e-nt systems is for the condition nx = ny ~ nz. The x and y10 axes are defined as the in-plane axes and the respective indices, nx and ny, will be referred to as the in-plane indices.
One method of creating a uniaxial birefringent system is to biaxially stretch (e.g., stretch along two ~limen~ions) a multilayer stack in which at least one of the m~teri~l~ in the stack has its index of refraction affected by the lS stretching process (e.g., the index either increases or decreases). Biaxial tclling of the multilayer stack may result in differences between refractive indices of adjoining layers for planes parallel to both axes thus res--ltin~ in reflectiQn of light in both planes of pol~ri7~tic~n.
A llni~xi~l birefringent m~t~ri~l can have either positive or negative 20 uni~xi~l birefringence. Positive uniaxial birefringence occurs when the z-index is greater than the in-plane indices (nz > nx and ny). Negative uniaxial birefringence occurs when the z-index is less than the in-plane indices (nz < nx ~ and ny).
A biaxial birefringent m~t~ri~l is defined as one in which the indices of 25 refraction in all three axes are dirrerellt, e.g., nx ny f nz. Again, the nx and ny indices will be referred to as the in-plane indices. A biaxial birefringent system can be made by ~lletcllillg the multilayer stack in one direction. In other words the stack is uniaxially stretched. For purposes of the present discussion,the x direction will be referred to as the stretch direction for biaxial birefringent 30 stacks.

Uniaxial Birefringent Systems (Mirrors) The optical Llo~lLies and design concide.r~tions of uniaxial birefringent systems will now be discussed. As discussed above, the general conditions for a l-ni~ l birefringent m~t~ri~l are nx = ny ~ nz. Thus if each layer 102 and 104 in Fig. 10 is ~Ini~xi~lly birefringent, nlx = nly and n2x = n2y. For purposes of the present ~1iccllCcion, assume that layer 102 has larger in-plane indices than layer 104, and that thus nl > n2 in both the x and y directions. The optical behavior of a uniaxial birefringent multilayer system can be adjusted by varyingthe values of nlz and n2z to introduce different levels of positive or negative birefringence. The relationship between the various indices of refraction can bemeasured directly, or, the general relationship may be indirectly observed by analysis of the spectra of the reslllting film as described herein.
In the case of mirrors, the desired average tr~n.cmi.scion for light of each lS pol~ri7~tion and plane of incidence generally depends upon the intended use of the mirror. The average tr~n.cmiccion along each stretch direction at normal incidence for a narrow bandwidth mirror across a 100 nm bandwidth within the visible spectrum is desirably less than 30%, preferably less than 20~ and more preferably less than 10%. A desirable average tr~n.cmiccion along each stretch direction at normal incidence for a partial mirror ranges anywhere from, for example, 10% to 50%, and can cover a bandwidth of anywhere between, for example, 100 nm and 450 nm, depending upon the particular application. For a high efficiency mirror, average tr~n.cmiccion along each stretch direction at normal incidence over the visible spectrum (400-700nm) is desirably less than 10%, preferably less than 5%, more preferably less than 2~, and even more preferably less than 1%. In addition, asymmetric ~ lUlS may be desirable for certain applic~tiQn.c. In that case, average tr~n.cmiccion along one stretch direction may be desirably less than, for example, 50%, while the average tr~n.cmiCcion along the other stretch direction may be desirably less than, for example 20%, over a bandwidth of, for example, the visible spectrum WO 96/19347 PCT/US9!i/16555 (400-700 nm), or over the visible spectrum and into the near infrared (e.g, 400-850 nm).
Equation 1 described above can be used to determine the reflectivity of a single interf~e in a uni~xi~l birefringent system composed of two layers such asS that shown in Fig. 10. Equation 2, for s p~ ri~ed light, is i~entic~l to that of the case of isotropic films (nx = ny = nz), so only equation 1 need be ~ minPd. For L,ul~oses of illustration, some specific, although generic, valuesfor the film indices will be ~ign~d Let nlx = nly = 1.75, nlz = variable, n2x = n2y = 1.50, and n2z = variable. In order to illuskate various possible 10 Brc~w~l~r angles in this system, no = 1.60 for the surrounding isotropic media.
Fig. 11 shows reflectivity versus angle curves for p-polarized light inrident from the isotropic medium to the birefringent layers, for cases where nlz is numerically greater than or equal to n2z (nlz 2 n2z~. The curves shown in Fig. 11 are for the following z-index values: a) nlz =1.75, n2z = 1.50; b) nlz = 1.75, n2z = 1.57; c) nlz = 1.70, n2z = 1.60; d) nlz = 1.65, n2z =
1.60; e) nlz = 1.61, n2z = 1.60; and f) nlz = 1.60 = n2z. As nlz approaches n2z, the Brewster angle, the angle at which reflectivity goes to zero, increases.
Curves a - e are strongly angular dependent. However, when nlz = n2z (curve f), there is no angular dependence to reflectivity. In other words, the reflectivity for curve f is constant for all angles of incidence. At that point, equation 1 reduces to the angular independent form: (n2O - nlo)/(n2O ~ nlo). When nlz = n2z, there is no Brewster effect and there is constant reflectivity for all angles of inci~l~nce.
Fig. 12 shows reflectivity versus angle of incidence curves for cases where nlz is numPric~lly less than or equal to n2z. Light is incident from isotropic medium to the birefringent layers. For these cases, the reflectivity monotonically increases with angle of incidence. This is the behavior that wouldbe observed for s-pol~ri7ed light. Curve a in Fig. 12 shows the single case for s polarized light. Curves b-e show cases for p polarized light for various values of nz, in the following order: b) nlz =1.50, n2z = 1.60; c) nlz = 1.55, n2z =

WO 96/19347 PCT/US9~/165S5 1.60; d) nlz =1.59, n2z = 1.60; and e) nlz = 1.60 = n2z. Again, when nlz = n2z (curve e), there is no Brewster effect, and there is constant reflectivity for all angles of incidence.
Fig. 13 shows the same cases as Fig. 11 and 12 but for an incident 5 medium of index no =1.0 (air). The curves in Fig. 13 are plotted for p polarized light at a single interface of a positive uniaxial m~t~ri~l of indices n2x = n2y = 1.50, n2z = 1.60, and a negative uniaxially birefringent tn~teri~l with nlx = nly = 1.75, and values of nlz, in the following order, from top to bottom, of: a) 1.50; jb) 1.55; c) 1.59; d) 1.60; f) 1.61; g) 1.65; h) 1.70; and 10 i) 1.75. Again, as was shown in Figs. 11 and 12, when the values of nlz and n2z match (curve d), there is no angular dependence to reflectivity.
Figs. 11, 12 and 13 show that the cross-over from one type of behavior to another occurs when the z-axis index of one film equals the z-axis index of the other film. This is true for several combinations of negative and positive 15 uniaxially birefringent, and isotropic m~t~ri~l~. Other situations occur in which the Br~w~L~l angle is shifted to larger or smaller angles.
Various possible relationships between in-plane indices and z-axis indices are illustrated in Figs. 14, 15 and 16. The vertical axes indicate relative values of indices and the horizontal axes are used to separate the various conditions.
20 Each Figure begins at the left with two isotropic films, where the z-index equals the in-plane indices. As one proceeds to the right, the in-plane indices are held con~t~nt and the various z-axis indices increase or decrease, indic~ting the relative amount of positive or negative birefringence.
The case described above with respect to Figs. 11, 12, and 13 is 25 illustrated in Fig. 14. The in-plane indices of m~teri~l one are greater than the in-plane indices of m~t~ri~l two, material 1 has negative birefringence (nlz less than in-plane indices), and material two has positive birefringence (n2z greaterthan in-plane indices). The point at which the Brewster angle disappears and reflectivity is constant for all angles of incidence is where the two z-axis indices , CA 02208234 1997-06-l9 WO 96/19347 PCT/US9!;/165~5 are equal. This point corresponds to curve f in Fig. 11, curve e in Fig. 12 or curve d in Fig. 13.
In Fig. 15, m~t~ri~l one has higher in-plane indices than m~ttqri~l two, but m~teri~l one has positive birefringence and m~teri~l two has negative S birefringence. In this case, the Brewster minimum can only shift to lower values of angle.
Both Figs. 14 and 15 are valid for the limiting cases where one of the two films is isotropic. The two cases are where m~t~n~l one is isotropic and m~teri~l two has positive birefringence, or material two is isotropic and m~t~ri~l one has 10 negative birefringence. The point at which there is no Brew~le- effect is where the z-axis index of the birefringent material equals the index of the isotropic film.
Another case is where both films are of the same type, i.e., both negative or both positive birefringent. Fig. 16 shows the case where both films have 15 negative birefringence. However, it shall be understood that the case of two positive birefringent layers is analogous to the case of two negative birefringent layers shown in Fig. 16. As before, the Brewster minimum is elimin~ted only if one z-axis index equals or crosses that of the other film.
Yet another case occurs where the in-plane indices of the two m~t~ ls 20 are equal, but the z-axis indices differ. In this case, which is a subset of all three cases shown in Figs. 14 - 16, no reflection occurs for s polarized light at any angle, and the reflectivity for p pol~ri7ed light increases monotonically with increasing angle of incidence. This type of article has increasing reflectivity for p-pnl~ri7~d light as angle of incidence increases, and is transparent to s-polarized 25 light. This article can be referred to as a "p-polarizer~.
The above described principles and design considerations describing the behavior of uniaxially birefringent systems can be applied to create multilayer stacks having the desired optical effects for a wide variety of circum~t~nce~ and applications. The indices of refraction of the layers in the multilayer stack can 30 be manipulated and tailored to produce devices having the desired optical WO 96/19347 PCT/US9~/16S55 propellies. Many negative and positive uniaxial birefringent systems can be created with a variety of in-plane and z-axis indices, and many useful devices can be dç~ign~d and f~bric~ted using the principl~s described here.

Biaxial Bire~rlin~ent Systems (Polarizers) Referring again to Fig. 10, two component orthogonal biaxial birefringent systems and the design considerations affecting the result~nt optical p~ ies will now be described. Again, the system can have many layers, but an underst~n-iin~ of the optical behavior of the stack is achieved by ex~mining theoptical behavior at one interface.
A biaxial birefringent system can be designed to give high reflectivity for light with its plane of polarization parallel to one axis, for a broad range of angles of incidence, and simultaneously have low reflectivity and high tr~nsmis~ion for light with its plane of polarization parallel to the other axis for a broad range of angles of incidence. As a result, the biaxial birefringent systemacts as a polarizer, tr~n~mitting light of one pol~ri7~tion and reflecting light of the other pol~ri7~tion By controlling the three indices of refraction of each film, nx, ny and nz, the desired polarizer behavior can be obtained. Again, the indices of refraction can be measured directly or can be indirectly observed by analysisof the spectra of the resulting film, as described herein.
Referring again to Fig. 10, the following values to the film indices are ~igned for purposes of illustration: nlx = 1.88, nly = 1.64, nlz = variable, n2x = 1.65, n2y = variable, and n2z = variable. The x direction is referred to as the extinction direction and the y direction as the tr~ncmi~ion direction.
Equation 1 can be used to predict the angular behavior of the biaxial birefringent system for two important cases of light with a plane of incidence in either the stretch (xz plane) or the non-stretch (yz plane) directions. The polarizer is a mirror in one polarization direction and a window in the other direction. In the stretch direction, the large index differential of 1.88 - 1.65 =
0.23 in a multilayer stack with hundreds of layers will yield very high WO 96/19347 PCT/US9!;/1655 reflectivities for s-pnl~ri7~d light. For p-polarized light the reflectance at various angles depends on the nlz/n2z index dirr~lelltial.
In many applications, the ideal reflecting polarizer has high refl~ct~nce along one axis (the so-called extinction axis) and zero reflectance along the other 5 (the so-called tr~ncmic.cion axis), at all angles of incidence. ~or the tr~ncmic.cion axis of a polarizer, it generally desirable to maximize tr~n.cmiccit n of light pol~ri7:~A in the direction of the tr~n.cmi.c.cion axis over the bandwidth of interest and also over the range of angles of interest. Average tr~n.cmic.cion at normal in~.idence for a narrow bandpolarizer across a 100 nm bandwidth is desirably at 10least 50%, preferably at least 70% and more preferably at least 90%. The average tr~ncmi.cc-~ n at 60 degreees from the normal for p-polarized light (measured along the tr~n.cmi.ccion axis) for a narrow band polarizer across a 100 nm bandwidth is desirably at least 50%, preferably at least 70% and more preferably at least 80%.
15The average tr~ncmiccion at normal incidence for a polarizer in the tr~ncmicsion axis across the visible spectrum (400-700 nm for a bandwidth of 300 nm) is desirably at least 50%, preferably at least 70%, more preferably at least 85%, and even more preferably at least 90%. The average tr~ncmic.cion at 60 degrees from the normal (measured along the tr~n~mi.~.cion axis) for a 20 polarizer from 400-700 nm is desirably at least 50%, preferably at least 70%, more preferably at least 80%, and even more preferably at least 90%.
For certain applications, high reflectivity in the tr~n.cmic.cinn axis at off-normal angles are prerelled. The average reflectivity for light polarized along the tr~n.cmiccion axis should be more than 20% at an angle of at least 25 20 degrees from the normal.
If some reflectivity occurs along the tr~nsmiC.cion axis, the efficiency of the polarizer at off-normal angles may be reduced. If the reflectivity along thetr~ncmi.c.cion axis is different for various wavelengths, color may be introduced into the tr~ncmitt~ light. One way to measure the color is to determine the root30 mean square (RMS) value of the tr~n.cmiccivity at a selected angle or angles over WO 96/19347 PCT/US95/16~55 the wavelength range of interest. The % RMS color, R~fS, can be det~ ined according to the equation:

~((T- )2)~12 (~ T

where the range ~1 to ~2 is the wavelength range, or bandwidth, of int~ , T
is the tr~n.cmi.ccivity along the tr~ncmiccion axis, and T is the average tr~ncmiccivity along the tr~nsmicsion axis in the wavelength range of interest.
For applications where a low color polarizer is desirable, the % RMS
color should be less than 10%, preferably less than 8%, more preferably less than 3.5%, and even more preferably less than 2.1% at an angle of at least 30 degrees from the normal, preferably at least 45 degrees from the normal, and even more preferably at least 60 degrees from the normal.
Preferably, a reflective polarizer combines the desired % RMS color along the tr~ncmicsion axis for the particular application with the desired amount of reflectivity along the extinction axis across the bandwidth of interest. For example, for narrow band polarizers having a bandwidth of appru~imately 100 nm, average tr~ncmiccion along the extinction axis at normal incidence is desirably less than 50%, preferably less than 30%, more preferably less than 10%, and even more preferably less than 3%. For polarizers having a bandwidth in the visible range (400-700 nm, or a bandwidth of 300 nm), average tr~ncmiccion along the extinction axis at normal incidence is desirably less than 40%, more desirably less than 25%, preferably less than 15%, more preferably less than 5 % and even more preferably less than 3 % .
Reflectivity at off-normal angles, for light with its plane of pol~ri7~tion p~r~llel to the tr~ncmiccion axis may be caused by a large z-index micm~tch, even if the in-plane y indices are m~trlled. The resulting system thus has large reflectivity for p, and is highly transparent to s polarized light. This case was referred to above in the analysis of the mirror cases as a "p polarizer".
For llni~xi~lly stretched polarizers, p~lror,.lallce depends upon the relationships between the ~lt~."~ g layer indices for all three (x, y, and z) direction~. As described herein, it is desirable to minimi7e the y and z index dirr~c;~lials for a high efficiency polarizer. Introduction of a y-index mi~m~t~h is describe to co---pellsate for a z-index mi~m~t~h. Whether intentionally addedor n~tnr~lly occllrring, any index micm~tch will introduce some reflectivity. AnhllL~olL~l factor thus is making the x-index differential larger than the y- andz-index differentials. Since reflectivity increases rapidly as a function of index dirr~rel.lial in both the stretch and non-stretch directions, the ratios ~ny/~nx and ~nz/~nx should be minimi7~1 to obtain a polarizer having high extinction along one axis across the bandwidth of interest and also over a broad range of angles,while preserving high tr~n~mi~sion along the orthogonal axis; Ratios of less than 0.05, 0.1 or 0.25 are acceptable. Ideally, the ratio ~nz/~nx is 0, but ratios of less than 0.25 or 0.5 also produce a useable polarizer.
Fig. 17 shows the reflectivity (plotted as -Log[l-R]) at 75 ~ for p pt)l~ri7:~d light with its plane of incidence in the non-stretch direction, for an 800 layer stack of PEN/coPEN. The reflectivity is plotted as function of wavelength across the visible spectrum (400 - 700 nm). The relevant indices for curve a at 550 nm are nly =1.64, nlz = 1.52, n2y = 1.64 and n2z = 1.63. The model stack design is a linear thicknt~c~ grade for ~ alL~,~ave pairs, where each pairthickn~ is given by dn = do + do(0.003)n. All layers were assigned a random thicl~npss error with a g~nc~i~n distribution and a 5% standard deviation.
Curve a shows high off-axis reflectivity across the visible spectrum along the tr~n~mi~sitn axis (the y-axis) and that different wavelengths experience dirrerent levels of reflectivity. This is due to the large z-index micm~tch (~nz= 0.11). Since the spectrum is sensitive to layer thickness errors and spatial nonu.liruf...ities, such as film caliper, this gives a biaxial birefringent system 30 with a very nonunifol--- and "colorful" a~L)ea~;~lce. Although a high degree of -WO 96/19347 PCTIUS9!;/16555 color may be desirable for certain applications, it is desirable to control the degree of off-axis color, and minimi7e it for those applications requiring a ullirollll, low color appearance, such as liquid crystal displays or other types of displays.
Off-axis reflectivity, and off-axis color can be minimi7e~ by introducing an index micm~trh to the non-stretch in-plane indices (nly and n2y) that create a Brt;w~Lel con-lition off axis, while keeping the s-pol~ri7~tiQn reflectivity to a minim~m .
Fig. 18 explores the effect of introducing a y-index micm~tch in reducing off-axis reflectivity along the tr~ncmiccion axis of a biaxial birefringent system.
With nlz = 1.52 and n2z = 1.63 (~nz = 0.11), the following conditions are plotted for p pol~ri7ed light: a) nly = n2y = 1.64; b) nly = 1.64, n2y =
1.62; c) nly = 1.64, n2y = 1.66. Curve a shows the reflectivity where the in-plane indices nly and n2y are equal. Curve a has a reflect~nce minimum at lS 0~, but rises steeply after 20~. For curve b, nly > n2y, and reflectivity increases rapidly. Curve c, where nly < n2y, has a reflectance minimum at 38~, but rises steeply thereafter. Considerable reflection occurs as well for s pol~ri7ed light for nly ~ n2y, as shown by curve d. Curves a-d of Fig. 18 indicate that the sign of the y-index micm~t~ll (nly - n2y) should be the same as the z-index micm~trh (nlz- n2z) for a Brewster minimum to exist. For the case of nly = n2y, reflectivity for s polarized light is zero at all angles.
By reduring the z-axis index difference between layers, the off axis reflectivity can be further reduced. If nlz is equal to n2z, Fig. 13 indicates that the extinction axis will still have a high reflectivity off-angle as it does at normal incirlence, and no reflection would occur along the nonstretch axis at any anglebecause both indices are matched (e.g., nly = n2y and nlz = n2z).
Exact m~tching of the two y indices and the two z indices may not be ~ possible in some multilayer systems. If the z-axis indices are not matched in a polarizer construction, introduction of a slight mi~m~tch may ~e desired for in-plane indices nly and n2y. This can be done by blending additional CA 02208234 l997-06-l9 WO 96/19347 PCT/IJS9~/16555 co"lpollents into one or both of the m~ttori~l layers in order to increase or decrease the respective y index as described below in Example 15. Rlenr~ing a second resin into either the polymer that forms the highly birefringent layers or into the polymer that forms the selected polymer layers may be done to modify 5 reflectiQn for the tr~nsmi~ion axis at normal and off-normal angles, or to modify the extin~ tion of the polarizer for light polarized in the extinction axis.
The second, blended resin may accomplish this by modifying the crystallinity and the index of refraction of the polymer layers after orientation.
Another ~Y~mple is plotted in FIG. 19, ~suming nlz = 1.56 and n2z = 1.60 (~nz = 0.04), with the following y indices a) nly = 1.64, n2y = 1.65;
b) nly = 1.64, n2y = 1.63. Curve c is for s-polarized light for either case.
Curve a, where the sign of the y-index mi~m~tch is the same as the z-index mi.~m~t~h, results in the lowest off-angle reflectivity.
The col-l~uled off-axis reflectance of an 800 layer stack of films at 75~
angle of incidence with the conditions of curve a in Fig. 19 is plotted as curve b in Fig. 17. Comparison of curve b with curve a in Fig. 17 shows that there is far less off-axis reflectivity, and therefore lower perceived color and better uniformity, for the conditions plotted in curve b. The relevant indices for curve b at 550 nm are nly = 1.64, nlz = 1.56, n2y = 1.65 and n2z = 1.60.
Fig. 20 shows a contour plot of equation 1 which summarizes the off axis reflectivity discussed in relation to Fig. 10 for p-polarized light. The four independent indices involved in the non-stretch direction have been reduced to two index mi~m~tch~s, ~nz and ~ny. The plot is an average of 6 plots at various angles of incidence from 0~ to 75~ in 15 degree increments. The reflectivity ranges from 0.4 x 10-4 for contour a, to 4.0 x 10-4 for contour j, in constant increments of 0.4 x 10 -4. The plots intlic~te how high reflectivity caused by an index micm~t~h along one optic axis can be offset by a mi~m~trh along the other axis.
Thus, by redu~ing the z-index mi~m~tch between layers of a biaxial birefringent systems, and/or by introducing a y-index mi~m~tcll to produce a Blcw~r effect, off-axis reflectivity, and therefore off-axis color, are minimi7ed along the tr~ncmi~ion axis of a multilayer reflecting polarizer.
It should also be noted that narrow band polarizers operating over a narrow wavelength range can also be designed using the principles described herein. These can be made to produce polarizers in the red, green, blue, cyan, magenta, or yellow bands, for example.
An ideal reflecting polarizer should transmit all light of one pol~ri7~tion, and reflect all light of the other polarization. Unless l~min~t~l on both sides to glass or to another film with a clear optical adhesive, surface reflections at the air/reflecting polarizer interface will reduce the tr~n~micsion of light of the desired polarization. Thus, it may in some cases be useful to add an antireflection (AR) coating to the reflecting polarizer. The AR coating is preferably dçsi~n~d to dereflect a film of index 1.64 for PEN based polarizers in air, because that is the index of all layers in the nonstretch (y) direction. The same coating will have çss~-nti~lly no effect on the stretch direction because the alternating index stack of the stretch direction has a very high reflection coefficient irrespective of the presence or absence of surface reflections. Any AR coating known in the art could be applied, provided that the coating does notoverheat or damage the multilayer film being coated. An exemplary coating would be a qua,lel~ave thick coating of low index m~t~ri~l, ideally with index near the square root of 1.64 (for PEN based materials).

Materials Selection and Processing With the above-described design considerations established, one of o~lillaly skill will readily appreciate that a wide variety of m~t~n~ can be used to form multilayer mirrors or polarizers according to the invention when processed under conditions selected to yield the desired refractive index relationships. The desired refractive index relationships can be achieved in a variety of ways, including stretching during or after film formation (e.g., in the ~ 30 case of organic polymers), extruding (e.g., in the case of liquid crystalline m~teri~l~), or coating. In addition, it is prerelled that the two materials have similar rheological pr~lies (e.g., melt viscosities) such that they can be co-extruded.
In general, a~)r~liate combinations may be achieved by selecting, as the first m~tPri~l, a crystalline or semi-crystalline material, preferably a polymer.
5 The second m~tPri~l, in turn, may be crystalline, semi-crystalline, or amorphous.
The second m~t~-ri~l may have a birefringence opposite to or the same as that ofthe first m~tPri~l. Or, the second material may have no birefringence.
Specific çY~mrles of suitable m~tPri~ls include polyethylene naphth~l~tP
(PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN), 10 polyalkylene terephth~l~tPs (e.g., polyethylene terephthalate, polybutylene terephth~l~tP, and poly-l ,4-cyclohex~ne~limethylene terephth~l~te), polyimides (e.g., polyacrylic imides), polyetherimides, atactic polystyrene, polycarbonates, polym~-th~rrylates (e.g., polyisobutyl meth~rrylate, polypropylmeth~rrylate, polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g., 15 polybutylacrylate and polymethylacrylate), syndiotactic polystyrene (sPS), syndiotactic poly-alpha-methyl styrene, syndiotactic polydichlorostyrene, copolymers and blends of any of these polystyrenes, cellulose derivatives (e.g.,ethyl cellulose, cellulose acetate, cellulose propionate, cellulose acetate butyrate, and cellulose nitrate), polyalkylene polymers (e.g., polyethylene, polypropylene, 20 polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinated polymers (e.g., perfluoroalkoxy resins, polytetrafluoroethylene, fluorinated ethylene-propylene copolymers, polyvinylidene fluoride, and polychlorotrifluoroethylene), chlorinated polymers (e.g., polyvinylidene chloride and polyvinylchloride), polysulfones, polyethersulfones, polyacrylonitrile, 25 poly~mides, silicone resins, epoxy resins, polyvinyl~et~te, polyether-amides,ionomeric resins, elastomers (e.g., polyb~lt~liP-ne, polyisoprene, and neoprene), and polyur~ es. Also suitable are copolymers, e.g., copolymers of PEN
(e.g., copolymers of 2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylicacid, or esters thereof, with (a) terephthalic acid, or esters thereof; (b) 30 isophthalic acid, or esters thereof; (c) phthalic acid, or esters thereof; (d) alkane -CA 02208234 1997-06-l9 glycols; (e) cyçlo~lk~nt~- glycols (e.g., cyclohexane ~limeth~nol diol); (f) alkane dicarboxylic acids; and/or (g) cyclo~lk~ne dicarboxylic acids (e.g., cyclohPY~nedicarboxylic acid)), copolymers of polyalkylene terepht~ tes (e.g., copolymers of terephlllalic acid, or esters thereof, with (a) naphthalene dicarboxylic acid, or 5 esters thereof; (b) isophthalic acid, or esters thereof; (c) phthalic acid, or esters thereof; (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexane ~imtoth~nol diol); (f) alkane dicarboxylic acids; and/or (g) cyclo~lk~ne dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), styrene copolymers (e.g., styrene-but~lipne copolymers and styrene-acrylonitrile copolymers), and 10 copolymers of 4,4'-bibenzoic acid and ethylene glycol. In addition, each individual layer may include blends of two or more of the above-described polymers or copolymers (e.g., blends of SPS and atactic polystyrene). The coPEN described may also be a blend of pellets where at least one component is a polymer based on naphthalene dicarboxylic acid and other components are 15 other polyesters or polycarbonates, such as a PET, a PEN or a co-PEN.
Particularly ~lefel,ed combinations of layers in the case of polarizers include PEN/co-PEN, polyethylene terephthalate (PET)/co-PEN, PEN/sPS, PET/sPS, PEN/Eastar, and PET/Eastar, where "co-PEN" refers to a copolymer or blend based upon naphthalene dicarboxylic acid (as described above) and 20 Eastar is polycyclohexanedimethylene terephthalate commercially available from F~ctm~n Chemical Co.
Particularly pl~fe~led combinations of layers in the case of mirrors include PET/Ecdel, PEN/Ecdel, PEN/sPS, PEN/THV, PEN/co-PET, and PET/sPS, where "co-PET" refers to a copolymer or blend based upon 25 terephthalic acid (as described above), Ecdel is a thermoplastic polyester commercially available from F~ctm~n Chemical Co., and THV is a fluoropolymer commercially available from 3M Co.
The number of layers in the device is selected to achieve the desired optical l,-opelLies using the minimum number of layers for reasons of film 30 thickness, flexibility and economy. In the case of both polarizers and mirrors, - =

the number of layers is preferably less than 10,000, more preferably less than 5,000, and (even more preferably) less than 2,000.
As rli~su~ed above, the ability to achieve the desired relationships among the various indices of refraction (and thus the optical plo~lLies of the multilayer device) is inflllPnced by the proces~ing conditions used to ~re~are the multilayer device. In the case of organic polymers which can be oriented by ~lretclling, the devices are generally prepared by co-extruding the individual polymers to form amultilayer film and then ori~nting the film by stretching at a sPlPctPA
te,~ dLure, optionally followed by heat-setting at a SPlPCtp~d temperature.
~lt~ ely, the extrusion and nrient~tion steps may be performed simultaneously. In the case of polarizers, the film is stretched substantially in one direction (lmi~xi~l orientation), while in the case of mirrors the film is stretched subst~nti~lly in two directions ~biaxial orientation).
The film may be allowed to ~iimen~ionally relax in the cross-stretch direction from the natural reduction in cross-stretch (equal to the square root of the stretch ratio) to being constrained (i.e., no substantial change in cross-stretch tlimPn~ions). The film may be stretched in the machine direction, as with a length ~rienter, in width using a tenter.
The pre-stretch temperature, stretch temperature, stretch rate, stretch ratio, heat set telllpeld~ul~, heat set time, heat set relaxation, and cross-stretch relaxation are SPlPCtpcl to yield a multilayer device having the desired refractive index rel~tion~hir. These variables are imer-dependent; thus, for example, a relatively low stretch rate could be used if coupled with, e.g., a relatively low stretch lelll~ld~ule. It will be apparent to one of ordinary skill how to select the a~?J?r~pliate combination of these variables to achieve the desired multilayer device. In general, however, a stretch ratios in the range from 1:2 to 1:10 (more ~lefeldbly 1:3 to 1:7) in the stretch direction and from 1:0.5 to 1:10 (more preferably from 1:0.5 to 1:7) orthogonal to the stretch direction is prerelred.
Suitable multilayer devices may also be pr~al~d using techniques such as spin coating (e.g., as described in Boese et al., J. Polym. Sci.: Part B, 30:1321 (1992) for birefringent polyimides, and vacuum deposition (e.g., as described byZang et. al., Appl. Phys. Letters, 59:823 (1991) for crystalline organic co,lll ounds; the latter technique is particularly useful for certain combinations of crystalline organic compounds and inorganic m~tPri~
S The invention will now be described by way of the following e7c~mpl~c.
In the examples, because optical absorption is negligihle, reflection equals 1 minus tr~n~mi~ion (R = 1 - T).

EXAMPLE 1 (Polarizer) PEN and a 70 naphthalate/30 terephthalate copolyester (coPEN) were synthPsi7PA in a standard polyester resin kettle using ethylene glycol as the diol.
The intrinsic viscosity of both the PEN and the coPEN was a~r~imately 0.6dl/g. Single layer films of PEN and coPEN were extruded and then uni~xi~lly stretched, with the sides restrained, at approximately 150~C. As extruded, the PEN exhibited an isotropic refractive index of about 1.65, and thecoPEN was characteri7Pd by an isotropic refractive index of about 1.64. By isotropic is meant that the refractive indices associated with all axes in the plane of the film are subst~nti~lly equal. Both refractive index values were observed at 550 nm. After stretching at a 5-1 sketch ratio, the refractive index of the PEN ~c~oci~ted with the oriented axis increased to approximately 1.88. The refractive index associated with the transverse axis dropped slightly to 1.64.
The refractive index of the coPEN film after stretching at a 5:1 stretch ratio r~-rn~inPd isotropic at approximately 1.64.
A ~ti~f~tory multilayer polarizer was then made of alternating layers of PEN and coPEN by coextrusion using a 51-slot feed block which fed a standard extrusion die. The extrusion was run at approximately 295~C. The PEN was extruded at approximately 23 lb/hr and the coPEN was extruded at ~ a~ uxim~tPly 22.3 lb/hr. The PEN skin layers were approximately three times as thick as the layers within the extruded film stack. All internal layers were designPd to have an optical 1/4 wavelength thickness for light of about 1300 nm. The Sl-layer stack was extruded and cast to a thickness of approxim~tely 0.0029 inches, and then uniaxially stretched with the sides restrained at approximately a 5:1 stretch ratio at appru~imately 150~C. The stretched film had a thicknPc~ of approximately 0.0005 inches.
The stretched film was then heat set for 30 seconds at a~,ro,-imately 230~C in an air oven. The optical spectra were e-ssenti~lly the same for film that was stretched and for film that was subsequently heat set.
Figure 5 is a graphical view of percent measured tr~ncmi.~sion of the Sl-layer stack in both an oriented direction 50 and in a transverse direction 52prior to heat setting.
Eight 51-layered polarizers, each made as described above, were combined using a fluid to elimin~te the air gaps forming a polarizer of 408 optical layers. Figure 6 is a graph that characterizes the 408 layers showing percent tr~n~mi~ion from 350 to 1,800 nm in both an oriented direction 54 and in a transverse direction 56.

EXAMPLE 2 (Polarizer) A c~ticf~ctQry 204-layered polarizer was made by extruding PEN and coPEN in the 51-slot feedblock as described in Example 1 and then employing 5 two layer doubling mlllti~liers in series in the extrusion. The m--ltipliers divide the extruded m~t~-ri~l exiting the feed block into two half-width flow streams, then stack the half-width flow streams on top of each other. U.S. Patent 3,565,985 describes similar coextrusion multipliers. The extrusion was rol,.,ed at approxim~t~ly 295~C using PEN at an intrinsic viscosity of 0.50 dl/g at 22.5 lb/hr while the coPEN at an intrinsic viscosity of 0.60 dl/g was run at 16.5 lb/hr. The cast web was approximately 0.0038 inches in thicknesc and was uniaxially stretched at a 5:1 ratio in a longitudinal direction with the sides restrained at an air temperature of 140~C during stretching.
Except for skin layers, all pairs of layers were designed to be 1/2 wavelength 15 optical thickn~-sc for 550 nm light. In the tr~ncmiccion spectra of Figure 7 two reflection peaks in the oriented direction 60 are evident from the tr~ncmiccion spectra, centered about 550 nm. The double peak is most likely a result of film errors introduced in the layer multipliers, and the broad background a result ofcumulative film errors throughout the extrusion and casting process. The 20 tr~ncmiccion spectra in the transverse direction is indicated by 58. Optical extinction of the polarizer can be greatly improved by l~min~ting two of thes films together with an optical adhesive.
Two 204-layer polarizers made as described above were then hand-l~min~t~d using an optical adhesive to produce a 408-layered film stack.
25 Preferably the refractive index of the adhesive should match the index of theisotropic coPEN layer. The reflection peaks evident in Figure 7 are smoothed out for a l~min~t~d sample, as shown in Figure 8. This occurs because the peak reflectivity occurs at dirr~ t wavelengths for different areas of the film, in arandom pattern. This effect is often referred to as "iridescence". T ~min~tiQn of 30 two films reduces iridescence because the random variations in color do not WO 96/19347 PCT/US95/16!j55 match from one film to another, and tend to cancel when the films are overlapped.
Figure 8 illustrates the tr~n.cmiccion data in both the oriented direction 64 and transverse direction 62. Over 80 percent of the light in one plane of S pol~ri7~tit n is reflecte~ for wavelengths in a range from approximately 450 to 650 nm.
The iric~escPnce is eccenti~lly a measure of nonunirollllities in the film layers in one area versus ~ ent areas. With perfect thicknçsc control, a film stack centered at one wavelength would have no color variation across the sample. Multiple stacks decignçd to reflect the entire visible spectrum will have iriclescPnce if ci~nifiç~nt light leaks through random areas at random wavelengths, due to layer thickness errors. The large differential index betweenfilm layers of the polymer systems presented here enable film reflectivities of greater than 99 percent with a modest number of layers. This is a great advantage in P1imin~ting iri~escence if proper layer thicknP-cc control can be achieved in the extrusion process. Computer based optical modeling has shown that greater than 99 percent reflectivity across most of the visible spectrum ispossible with only 600 layers for a PEN/coPEN polarizer if the layer thickn~cc values are controlled with a standard deviation of less than or equal to 10 percent.

EXAMPLE 3 (PET:Ecdel. 601~ Mirror) A coextruded film cont~ining 601 layers was made on a sequential flat-film-making line via a coextrusion process. A Polyethylene terephth~l~t~
(PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt. % phenol/40 wt. %
dichlorobenzene) was delivered by one extruder at a rate of 75 pounds per hour and Ecdel 9966 (a thermoplastic elastomer available from F~ctm~n Chemical) was delivered by another extruder at a rate of 65 pounds per hour. The PET was on the skin layers. The feedblock method (such as that described in U.S. Patent 3,801,429) was used to generate 151 layers which was passed through two multirliers producing an extrudate of 601 layers. U.S. Patent 3,565,985 describes Plr~mpl~ry coextrusion mllltipliers. The web was length nriPnted to a draw ratio of about 3.6 with the web temperature at about 210~F. The film was subsequently preheated to about 235~F in about S0 secQn~ls and drawn in the S transverse direction to a draw ratio of about 4.0 at a rate of about 6% per - second. The film was then relaxed about 5 % of its maximum width in a heat-set oven set at 400~F. The fini~hed film thickness was 2.5 mil.
The cast web produced was rough in texture on the air side, and provided the tr~n~mi~ion as shown in Figure 21. The % tr~n~mi~ion for p-polarized light at a 60~ angle (curve b) is similar the value at normal incidence (curve a) (with a wavelength shift).
For comparison, film made by Mearl Corporation, presumably of isotropic m~tt-ri~ (see Fig. 22) shows a noticeable loss in reflectivity for p-pol~ri7~d light at a 60~ angle (curve b, compared to curve a for normal incidence).

EXAMPLE 4 (PET:Ecdel~ 151. Mirror) A coextruded film containing lS l layers was made on a sequential flat-film-making line via a coextrusion process. A Polyethylene terephth~l~te (PET) with an Intrinsic Viscosity of 0. 6 dl/g (60 wt phenol/40 wt. %
dichlorobenzene) was delivered by one extruder at a rate of 75 pounds per hour and _cdel 9966 (a thermoplastic elastomer available from F~ctm~n Chemical) was delivered by another extruder at a rate of 65 pounds per hour. The PET was on the skin layers. The feedblock method was used to generate 151 layers. The web was length oriented to a draw ratio of about 3.5 with the web telllpe~ure atabout 210~F. The film was subsequently preheated to about 215~F in about 12 seconds and drawn in the transverse direction to a draw ratio of about 4.0 at a ~ rate of about 25% per second. The film was then relaxed about 5% of its maximum width in a heat-set oven set at 400~F in about 6 seconds. The fini~h~
film thickness was about 0.6 mil.

The tr~n~mi~cion of this film is shown in Figure 23. The % tr~ncmi~ion for p-polarized light at a 60~ angle (curve b) is similar the value at normal in~iden~e (curve a) with a wavelength shift. At the same extrusion conl1itionc the web speed was slowed down to make an infrared reflecting film with a thickn~cs of about 0.8 mils. The tr~ncmiccinn is shown in Fig. 24 (curve a at normal inci<l~nce, curve b at 60 degrees).

EXAMPLE 5 (PEN:Ecdel, 225, Mirror) A coextruded film cont~ining 225 layers was made by extruding the cast web in one operation and later orienting the film in a laboratory film-stretching app~d~us. A Polyethylene naphthalate (PEN) with an Intrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by one extruder at a rate of 18 pounds per hour and Ecdel 9966 (a thermoplastic elastomer available from F~ctm~n Ch~mic~l) was delivered by another extruder at a rate of 17 pounds per hour. The PEN was on the skin layers. The feedblock method was used to generate 57 layers which was passed through two mnltirliers producing an extrudate of 225 layers. The cast web was 12 mils thickand 12 inches wide. The web was later biaxially oriented using a laboratory ~ e~cl~ g device that uses a pantograph to grip a square section of film and .cimlllt~nPously stretch it in both directions at a uniform rate. A 7.46 cm square of web was loaded into the stretcher at about 100~C and heated to 130~C in 60 seconds. Stretching then commenced at 100%/sec (based on original t1imPn~ionc) until the sample was stretched to about 3.5x3.5. Tmmedi~tely after the ~L-e~clling the sample was cooled by blowing room temperature air on it.
Figure 25 shows the optical response of this multilayer film (curve a at normal incidence, curve b at 60 degrees). Note that the % tr~ncmiccion for p-pol~ri7ed light at a 60~ angle is similar to what it is at normal incidence (with some wavelength shift).

WO 96/19347 . PCT/US95/16555 EXAMPLE 6 (PEN:THV 500~ 449. Mirror~
A coextruded film cont~ining 449 layers was made by extruding the cast web in one operation and later orienting the film in a laboratory film-s~ cllinga~p~dlus. A Polyethylene naphthalate (PEN) with an Tntrin~ic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by one extruder at a rate of 56 pounds per hour and THV 500 (a fluoropolymer available from Minnesota Mining and Manufacturing Company) was delivered by another extruder at a rate of 11 pounds per hour. The PEN was on the skin layers and 50% of the PEN was present in the two skin layers. The feedblock method was used to generate 57 layers which was passed through three multipliers producing an extrudate of 449 layers. The cast web was 20 mils thick and 12 inches wide.
The web was later biaxially oriented using a laboratory stretching device that uses a pantograph to grip a square section of film and simultaneously stretch it in both directions at a uniform rate. A 7.46 cm square of web was loaded into the stretcher at about 100~C and heated to 140~C in 60 seconds. Stretching then commPnce~ at 10%/sec (based on original ~limen~ions) until the sample was stretched to about 3.5x3.5. Tmme~ t~-ly after the stretching the sample was cooled by blowing room temperature air at it.
Figure 26 shows the tr~n~mic~ion of this multilayer ~1lm. Again, curve a shows the response at normal incidence, while curve b shows the response at 60 degrees.

EXAMPLE 7 (PEN:CoPEN 449--Low Color Polarizer) A coextruded film containing 449 layers was made by extrudlng the cast web in one operation and later orienting the film in a laboratory film-stretching a~ dtus. A Polyethylene naphthalate (PEN) with an Intrinsic Viscosity of 0.56 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by one ~ extruder at a rate of 43 pounds per hour and a CoPEN (70 mol% 2,6 NDC and 30 mol% DMT) with an intrinsic viscosity of 0.52 (60 wt. % phenol/40 wt. ~
dichlorobenzene) was delivered by another extruder at a rate of 25 pounds per hour. The PEN was on the skin layers and 40% of the PEN was present in the two skin layers. The feedblock method was used to generate 57 layers which was passed through three multipliers producing an extrudate of 449 layers. The cast web was 10 mils thick and 12 inches wide. The web was later llni~xi~lly c~ ont~ using a laboratory stretching device that uses a pantograph to grip a square section of film and stretch it in one direction while it is con.s~r~ined in the other at a uniform rate. A 7.46 cm square of web was loaded into the stretcher at about 100~C and heated to 140~C in 60 seconds. Stretching then commencecl at 10%/sec (based on origin~ mPncions) until the sample was stretched to about 5.5xl. TmmeAi~t~.ly after the stretching the sample was cooled by blowing room le~ el~ture air at it.
Figure 27 shows the tr~n.cmi.c.cion of this multilayer film. Curve a shows tr~ncmiccion of light polarized in the non-stretch direction at normal incidence, curve b shows tr~ncmiccinn of p-polarized light at 60~ incidence, and curve c shows tr~ncmi.ccion of light polarized in the stretch direction at normal ineidçnce.
Note the very high tr~ncmiccion of light polarized in the non-stretch direction at both normal and 60~ incidence. Average tr~n.cmiccion for curve a over 400-700 nm is 87.1%, while average tr~ncmi.c.cinn for curve b over 400-700 nm is 97.1%. Tr~ncmi.c.sion is higher for p-polarized light at 60~ incidence because the air/PEN interface has a Brewster angle near 60~, so the tr~ncmiscion at 60~
inci~ence is nearly 100%. Also note the high extinction of light polarized in the stretched direction in the visible range (400-700nm) shown by curve c, where theaverage tr~ncmi.ccion is 21.0%. The % RMS color for curve a is 1.5%. The %
RMS color for curve b is 1.4%.
EXAMPLE 8 (PEN:CoPEN. 601--High Color Polarizer) A coextruded film containing 601 layers was produced by extruding the web and two days later c)rienting the film on a different tenter than described in all the other examples. A Polyethylene Naphth~l~te (PEN) with an Tntrin.cic Viscosity of 0.5 dl/g (60 wt. % phenoll40 wt. % dichlorobenzene) was delivered WO 96/19347 . PCTIUS95/165S5 by one extruder at a rate of 75 pounds per hour and a CoPEN (70 mol~ 2,6 NDC and 30 mol% DMT) with an IV of O.SS dl/g (60 wt. % phenol/40 wt. %
dichlorobenzene) was delivered by another extruder at a rate of 65 pounds per hour. The PEN was on the skin layers. The feedblock method was used to generate lSl layers which was passed through two multipliers producing an extrudate of 601 layers. U.S. Patent 3,565,985 describes similar coextrusion mllltipliers. All sL-elchillg was done in the tenter. The film was pr~hP~t~ to about 280~F in about 20 seconds and drawn in the transverse direction to a draw ratio of about 4.4 at a rate of about 65~ per second. The film was then relaxed about 2% of its maximum width in a heat-set oven set at 460~F. The finiehed film thicknesc was 1.8 mil.
The tr~n~mi~eion of the film is shown in Figure 28. Curve a shows tr~nemi~eion of light polarized in the non-stretch direction at normal incidence, curve b shows tr~n~mi.e~ion of p-pol~ri7ed light at 60~ incidence, and curve c lS shows tr~n~mi~ion of light pol~ri7~d in the stretch direction at normal incidence.
Note the nonuniform tr~n~mi~einn of p-polarized light at both normal and 60~
incidence. The average tr~n~mi.~ion for curve a over 400-700 nm is 84.1%, while the average tr~n~miesion for curve b over 400-700 nm is 68.2%. The average tr~nemi~ion for curve c is 9.1%. The % RMS color for curve a is 1.4%, and the % RMS color for curve b is 11.2%.

EXAMPLE 9 (PET: CoPEN 449~ Polarizer) A coextruded film containing 449 layers was made by extruding the cast web in one operation and later orienting the film in a laboratory film-~L.ctching 25 a~p~tus. A Polyethylene Terephth~l~te (PET) with an Intrinsic Viscosity of 0.60 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by one extruder at a rate of 26 pounds per hour and a CoPEN (70 mol% 2,6 NDC and 30 mol% DMT) with an intrin~ic viscosity of 0.53 (60 wt. % phenol/40 wt. %
dichlorobenzene) was delivered by another extruder at a rate of 24 pounds per 30 hour. The PET was on the skin layers. The feedblock method was used to CA 02208234 l997-06-l9 WO 96/19347 PC'r/US95116555 generate 57 layers which was passed through three multipliers producing an extrudate of 449 layers. U.S. Patent 3,565,985 describes similar coextrusion multipliers. The cast web was 7.5 mils thick and 12 inches wide. The web was later uni~xi~lly oriented using a laboratory stretching device that uses a p~nt~.~ph to grip a square section of film and stretch it in one direction while it is conctr~in~d in the other at a uniform rate. A 7.46 cm square of web was loaded into the stretcher at about 100~C and heated to 120~C in 60 seconds.
Sll~lcl~ g then commenced at 10%/sec (based on ori~in~l ~imenciQns) until the sample was stretched to about 5.0xl. TmmeAi~tely after the stretching the samplewas cooled by blowing room tel-lpel~tllre air at it. The finiched film thicknçsswas about 1.4 mil. This film had sufficient adhesion to survive the orientation process with no d~ min~tion.
Figure 29 shows the tr~ncmi.c.cion of this multilayer film. Curve a shows tr~n.cmi.ccion of light polarized in the non-stretch direction at normal incidence, curve b shows tr~n.cmi.ccion of p-polalized light at 60~ incidence, and curve c shows tr~ncmi.c.cion of light polalized in the stretch direction at normal incidence.
Note the very high tr~ncmi.Ccion of p-p~ ri7~d light at both normal and 60~
int~.itl~.nr.e. The average tr~n.cmi.ccion for curve a over 400-700 nm is 88.0%, and the average tr~ncmicci~n for curve b over 400-700 nm is 91.2%. The average tMncmi.c.ciQn for curve c over 400-700 nm is 27.9%. The % RMS color for curve a is 1.4%, and the % RMS color for curve b is 4.8%.
EXAMPLE 10 (PEN:CoPEN~ 601. Polarizer) A coextruded film con~ h-~ 601 layers was made on a sequential flat-film-making line via a coextrusion process. A Polyethylene naphth~l~te (PEN) with an intrincic viscosity of 0.54 dl/g (60 wt % Phenol plus 40 wt %
dichlorobenzene) was delivered by on extruder at a rate of 75 pounds per hour and the coPEN was delivered by another extruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole % 2,6 naphthalene dicarboxylate methyl ester, 15 % dimethyl isophth~l~te and 15% dimethyl terephthalate with ethylene glycol. The feedblock method was used to generate 151 layers. The feedblock -WO 96/19347 PCTIUS9~/16555 was designed to produce a gr~tlient distribution of layers with a ration of thickn~c~ of the optical layers of 1.22 for the PEN and 1.22 for the coPEN. The PEN skin layers were coextruded on the outside of the optical stack with a totalthickn~c~ of 8% of the coextruded layers. The optical stack was mnltipli~ by two sequential multipliers. The nominal multiplication ratio of the multipliers were 1.2 and 1.27, respectively. The film was subsequently preh~ted to 310~F
in about 40 secon~iC and drawn in the transverse direction to a draw ratio of about 5.0 at a rate of 6% per second. The fini~hed film thicknPc~ was about 2 mils.
Figure 30 shows the tr~n~mi~ion for this multilayer film. Curve a shows tr~n~mi~sion of light polarized in the non-stretch direction at normal incidence, curve b shows tr~ncmi~cion of p-polarized light at 60~ incidence, and curve c shows tr~ncmi~ion of light polarized in the stretch direction at normal incidence.
Note the very high tr~nsmi~ion of p-polarized light at both normal and 60~
incidence (80-100%). Also note the very high extinction of light polarized in the stretched direction in the visible range (400-700nm) shown by curve c.
Extinction is nearly 100% between 500 and 650nm.

EXAMPLE 11 (PEN:sPS. 481. Polarizer) A 481 layer multilayer film was made from a polyethylene naphthalate (PEN) with an intrincic viscosity of 0.56 dl/g measured in 60 wt. % phenol and 40 wt % dichlorobenzene purchased from F~ctm~n Chemicals and a syndiotactic poly~Lylene (sPS) homopolymer (weight average molecular weight -- 200,000 . Daltons, sampled from l~ow Corporation). The PEN was on the outer layers and was extruded at 26 pounds per hour and the sPS at 23 pounds per hour. The feedblock used produced 61 layers with each of the 61 being alJplu~imately the same thickn~ss. After the feedblock three (2x) multipliers were used. Equal thicknes$ skin layers containing the same PEN fed to the feedblock were added ~ after the final multiplier at a total rate of 22 pounds per hour. The web was extruded through a 12" wide die to a thickness or about 0.011 inches (0.276 mm). The extrusion temperature was 290~C.

This web was stored at ambient conditions for nine days and then ~mi~xi~lly orient~d on a tenter. The film was preheated to about 320~F (160~C) in about 25 seconds and drawn in the transverse direction to a draw ratio of about 6:1 at a rate of about 28% per second. No relaxation was allowed in the S stretched direction. The finich~d film thickness was about 0.0018 inches (0.046 mm).
Figure 31 shows the optical p~lrol~l,ance of this PEN:sPS reflective polarizer co~ g 481 layers. Curve a shows tr~ncmiccion of light pol~ri7ed in the non-stretch direction at normal incidence, curve b shows tr~n.cmic.cion of 0 p-pol~ri7ed light at 60~ incidence, and curve c shows tr~n.cmi.c.cion of light pol~ri7:~ in the stretch direction at normal incidence. Note the very high tr~n.cmi.ccion of p-pol~ri7eci light at both normal and 60~ incidence. Average ~n.cmiccion for curve a over 400-700 nm is 86.2%, the average tr~ncmic.cion for curve b over 400-700 nm is 79.7%. Also note the very high extinction of light pol~ri7~A in the stretched direction in the visible range (400-700nm) shown by curve c. The film has an average tr~ncmi.ccion of 1.6~o for curve c between 400 and 700 nm. The % RMS color for curve a is 3.2%, while the % RMS color for curve b is 18.2 % .

EXAMPLE 12 (PET:Ecdel 601~ Mirror) A coextruded film co~ ining 601 layers was made on a sequential flat-film-making line via a coextrusion process. A Polyethylene terephth~l~t~
(PET) with an Tntrincic Viscosity of 0.6 dl/g (60 wt. % phenol/40 wt. %
dichlorobenzene) was delivered to the feedblock at a rate of 75 pounds per hour and Ecdel 9967 (a thermoplastic elastomer available from F~ctm~n Che.mi~
was delivered at a rate of 60 pounds per hour. The PET was on the skin layers.
The feedblock method was used to generate 151 layers which was passed through two multipliers producing an extrudate of 601 layers. The multipliers had a nominal multiplication ratio of 1.2 (next to feedblock) and 1.27. Two skin layers at a total throughput of 24 pounds per hour were added symmetrically between the last multiplier and the die. The skin layers were composed of PET
and were extruded by the same extruder supplying the PET to the feedblock.
The web was length oriented to a draw ratio of about 3.3 with the web ~l,l~r~lule at about 205~F. The film was subsequently pr~h~t~-d to about 205~F in about 35 seconds and drawn in the transverse direction to a draw ratio of about 3.3 at a rate of about 9% per second. The film was then relaxed about 3% of its maximum width in a heat-set oven set at 450~F. The fini~h~A film ~icknto~ was about 0.0027 inches.
The film provided the optical performance as shown in Figure 32.
Tr~n~mic~inn is plotted as curve a and reflectivity is plotted as curve b. The luminous reflectivity for curve b is 91.5%.

EXAMPLE 13 (PEN:CoPEN. 601~ Antireflected Polarizer~
A coextruded film containing 601 layers was made on a sequential flat-~llm-making line via a coextrusion process. A Polyethylene naphth~l~te (PEN~ with an intrin~ic viscosity of 0.54 dl/g (60 wt % Phenol plus 40 wt %
dichlorobenzene) was delivered by on extruder at a rate of 75 pounds per hour and the coPEN was delivered by another extruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole ~ 2,6 naphthalene dicarboxylate methyl ester, 30% dimethyl terephth~l~te with ethylene glycol. The feedblock method was used to generate 151 layers. The PEN skin layers were coextruded on the outside of the optical stack with a total thicknes~ of 8~ of the coextruded layers.
The feedblock was desi~n~1 to make a linear gradient in layer thickness for a 149 layer optical stack with the thinnest layers on one side of the stack. The individual layer thicknP-~es were designed in pairs to make equal thickness layers of the PEN and coPEN for each pair. Each pair thickness, d, was determined by the formula d = do + do*0.003*n, where do is the minimum pair thickness, and n is the pair number between 1 and 75. The optical stack was multiplied by two sequential multipliers. The nominal multiplication ratio of the multipliers were1.2 and 1.27, respectively. The film was subsequently prelle~t~d to 320~F in WO 96tl9347 PCT/US95/16555 about 40 seconds and drawn in the transverse direction to a draw ratio of about 5.0 at a rate of 6% per second. The finished film thickness was about 2 mils.
A silical sol gel coating was then applied to one side of the reflecting polarizer film. The index of refraction of this coating was a~lo,~imately 1.35.
S Two pieces of the AR coated reflecting polarizer film were cut out and the two were l~min~ted to each other with the AR co~tingc on the outside. Tr~ncmicciQn spectra of polarized light in the crossed and parallel directions were obtained.The sample was then rinsed with a 2% solution of ammonium bifluoride (NH4 HF2) in deo~ d water to remove the AR coating. Spectra of the bare multilayer were then taken for comparison to the coated sample.
Figure 33 shows the spectra of the coated and uncoated polarizer. Curves a and b show the tr~ncmicci~,n and extinction, respectively, of the AR coated reflectin~ polarizer, and curves c and d show the tr~ncmiccion and extinction, respectively, of the uncoated reflecting polarizer. Note that the extinction spectrum is çscenti~lly unchanged, but that the trancmiccion values for the AR
coated polarizer are almost 10% higher. Peak gain was 9.9% at 565 nm, while the average gain from 425 to 700 nm was 9.1%. Peak tr~ncmiccion of the AR
coated polarizer was 97.0% at 675 nm. Average tr~ncm-ccions for curve a over 400-700 nm was 95.33%, and average tr~ncmiccion for curve d over 400-700 nm was 5.42%.

EXAMPLE 14 (PET:_cdel. 601. Polarizer) ~ A coextruded film cont~ining 601 layers was made on a sequential flat-film-making line via a coextrusion process. A polyethylene terephth~l~t~
(PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt. % phenol/40 wt. %
dichlorobenzene) was delivered to a feedblock by one extruder at a rate of 75 pounds per hour and Ecdel 9967 (a thermoplastic elastomer available from F~ctm~n Chemical) was delivered to the feedblock by another extruder at a rate of 60 pounds per hour. The PET was on the skin layers. The feedblock method was used to generate lSl layers which passed through two multipliers (2x) CA 02208234 1997-06-l9 WO 96/19347 . PCT/US95/16555 producing an extrudate of 601 layers. A side stream with a throughput of 50 pounds per hour was taken from the PET extruder and used to add two skin layers between the last multiplier and the die. The web was length oriented to adraw ratio of about 5.0 with the web temperature at about 210~F. The film was not len~ d. The fini~hP~ film thicknPss was about 2.7 mil.
Figure 34 shows the tr~ncmi~ion for this film. Curve a shows the tr~n~mi.~it)n of light pol~ri7~d in the stretch direction, while curve b shows the tr~n~mi~icn of light polarized orthogonal to the stretch direction. The average tr~n~mi~ion from 400-700 nm for curve a is 39.16% .
EXAMPLE 15 (PEN:CoPEN. 449. Polarizers) A coextruded film containing 449 layers was made by extruding the cast web in one operation dnd later orienting the film in a laboratory film-stretching a~dlus. A polyethylene naphthalate (PEN) with an Intrinsic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by one extruder at a rate of 26.7 pounds per hour to the feedblock and a different m~tPri~l was delivered by second extruder at a rate of 25 pounds per hour to thefeedblock. The PEN was the skin layers. The feedblock method was used to generate 57 layers which passed through three multipliers producing an extrudateof 449 layers. The cast web was 0.0075 mils thick and 12 inches wide. The web was later uniaxially oriented using a laboratory stretching device that uses a pantograph to grip a square section of film and stretch it in one direction at auniform rate while it is constrained in the other. A 7.46 cm square of web was loaded into the stretcher at about 100~C and heated to 140~C for 60 seconds.
Stre~c}~ing then commPnced at 10%/sec (based on original tlimpn~ions) until the sample was stretched to about 5.5xl. Immediately after stretching, the sample was cooled by blowing room temperature air at it.
The input to the second extruder was varied by blending pellets of the following poly(ethylene esters) three m~tPri~ls~ a CoPEN (70 mol%
2,6-napthalene dicarboxylate and 30 mol% terephth~l~te) with an intrin~ic viscosity of 0.52 (60 wt. % phenol/40 wt. % dichlorobenzene); (ii) the PEN, same m~t~ri~l as input to first extruder; (iii) a PET, with an intrinsic viscosity of 0.95 (60 wt. % phenol/40 wt. % dichlorobenzene). l~F 9506 purchased from Shell.
For the film shown in Figure 35A the input to the second extruder was 80-wt % of the CoPEN and 20 wt % of the PEN; for the film shown in Figure 35B the input to the second extruder was 80 wt% of the CoPEN and 20 wt % of the PET; for the film shown in Figure 35C the input to the second extruder was CoPEN.
Figures 35A, 35B, and 35C show the tr~ncmiccion of these multilayer films where curve a shows tr~ncmi.c~iQn of light polarized in the non-stretch direction at normal incidence, curve b shows tr~ncmicsion of p-polarized light pol~ri7ecl in the non-stretched direction at 60~ incidence, and culve c shows tr~ncmiccion of light pol~ri7ed in the stretch direction at normal incidence. Note that the optical response of these films is sensitive to the chemical composition of the layers from the second extruder. The average tr~ncmiccion for curve c in Figure 35A is 43.89%, the average tr~ncmiccion for curve c in Figure 35B is 21.52%, and the average tr~ncmiccion for curve c in Figure 35C is 12.48%.
Thus, extinction is increase~d from Figure 35A to Figure 35C.
For the eY~mples using the 57 layer feedblock, all layers were de-cigned for only one optical thicknçcc (1/4 of 550nm), but the extrusion equipment introduces deviations in the layer thicknesses throughout the stack reslllting in a fairly bro~-lb~nd optical response. For examples made with the 151 layer feedblock, the feedblock is designed to create a distribution of layer thicknPcces to cover a portion of the visible spectrum. Asymmetric multipliers were then used to broaden the distribution of layer thicknçsses to cover most of the visible spectrum as described in U.S. Patents 5,094,788 and 5,094,793.
Although the present invention has been described with reference to p~ led embodim~-ntc, those of skill in the art will recognize that changes may CA 02208234 l997-06-l9 be made in form and detail without departing from the spirit and scope of the invention.

Claims (29)

WE CLAIM:

1. A multilayered polymer film comprising (A) layers of a crystalline naphthalene dicarboxylic acid polyester, with a positive stress optical coefficient, having an average thickness of not more than 0.5 microns; and (B) layers of a selected second polymer having an average thickness of not more than 0.5 microns, wherein said naphthalene dicarboxylic acid polyester is more positively birefringent than said second polymer.

2. The film of Claim 1 wherein said layers of said naphthalene dicarboxylic acid polyester and said layers of said second polymer are immediately adjacent alternating layers.

3. The film of Claim 2 wherein said layers of said naphthalene dicarboxylic acid polyester and said layers of said second polymer have good interlayer adhesion.

4. The film of Claim 1 wherein there are at least 50 of each of said layers of said naphthalene dicarboxylic acid polyester and said layers of said second polymer.

5. The film of Claim 1 wherein said film has been stretched in at least one direction.

6. The film of Claim 5 wherein said film has been stretched in at least one direction to at least twice that direction's unstretched dimension.

7. The film of Claim 6 wherein said layers of said naphthalene dicarboxylic acid polyester have a higher index of refraction associated with atleast one in-plane axis than the layers of said second polymer.

8. The film of Claim 7 wherein said higher index of refraction is at least 0.05 higher.

9. The film of Claim 7 wherein said higher index of refraction is at least 0.10 higher.

10. The film of Claim 7 wherein said higher index of refraction is at least 0.20 higher.

11. The film of Claim 1 wherein said film has been stretched in at least two directions.

12. The film of Claim 1 wherein said naphthalene dicarboxylic acid polyester is a poly(ethylene naphthalate).

13. The film of Claim 1 wherein said naphthalene dicarboxylic acid polyester is a copolyester comprising naphthalate units and terephthalate units.

14. The film of Claim 1 wherein said second polymer is a polyester.

15. The film of Claim 14 wherein said second polymer comprises naphthalene units.

16. The film of Claim 14 wherein said second polymer is a copolyester comprising naphthalate units and terephthalate units.

17. The film of Claim 1 wherein said second polymer is a polystyrene.

18. The film of Claim 1 wherein said second polymer is a fluoropolymer.

19. The film of Claim 1 wherein said second polymer is a polyacrylate, polymethacrylate, or polyolefin.

20. The film of Claim 1 wherein said film has an average reflectivity, for at least one plane of polarization, of at least 50% over at least a 100 nm wide band.

21. Article comprising the film of Claim 1.

22. A method for preparing a multilayered polymer film according to claim 1 comprising (A) layers of a crystalline <-> polyester [-] having an average thickness of not more than 0.5 microns; and;
(B) layers of a selected second polymer having an average thickness of not more than 0.5 microns;
wherein said film has been stretched in at least one direction to at least twice that direction's unstretched dimension, and wherein said naphtalene dicarboxylic acid polyester is more positively birefringent than said second polymer.

23. The method of claim 22 wherein said second polymer is a polyester.

24. The method of Claim 22 wherein said layers of a polyester and said layers of a second polymer are immediately adjacent alternating layers.

< naphthalene dicarboxylic acid >
[ with a positive stress optical coefficient ]

25. The method of Claim 24 wherein said layers of a polyester and said layers of a second polymer have good interlayer adhesion.

26. The method of Claim 22 wherein there are at least 50 of each of said layers of a polyester and said layers of a second polymer.

27. The method of Claim 22 wherein said film has been stretched in at least two directions.

28. The method of Claim 22 wherein said film has an average reflectivity, for at least one plane of polarization, of at least 50% over at least a 100 nm wide band.

29. Article comprising the film obtainable according to Claim 22.