Patent Application
20120161345
This invention relates to a method of manufacturing a diffusely reflecting polarizer comprising a film having a first polymer, with a second polymer dispersed therein, the first polymer being a substantially amorphous nano-composite and nearly isotropic material.
Reflective polarizing films transmit light of one polarization and reflect light of the orthogonal polarization. They are useful in an LCD to enhance light efficiency. A variety of films have been disclosed to achieve the function of the reflective polarizing films, among which diffusely reflecting polarizers are more attractive because they may not need a diffuser in a LCD, thus reducing the complexity of the LCD. U.S. Pat. No. 5,783,120 teach a diffusely-reflective polarizing film comprising a film containing an immiscible blend having a first continuous phase (also referred herein as the major phase, i.e., comprising more than 50 weight % of the blend) and a second dispersed phase (also referred herein as the minor phase, i.e., comprising less than 50 weight % of the blend), wherein the first phase has a birefringence of at least 0.05. The film is oriented, typically by stretching, in one or more directions. The size and shape of the dispersed phase particles, the volume fraction of the dispersed phase, the film thickness, and the amount of orientation are chosen to attain a desired degree of diffuse reflection and total transmission of electromagnetic radiation of a desired wavelength in the resulting film. Among 124 examples shown in Table 1 through Table 4, most of which include polyethylene naphthalate (PEN) as a major and birefringent phase, with polymethyl methacrylate (PMMA) (Example 1) or syndiotactic polystyrene (sPS) (other examples) as a minor phase, except for example numbers 6, 8, 10, 42-49, wherein PEN is a minor phase and sPS is a major phase. In all of these 124 examples the major phase comprises a semicrystalline polymer.
Examples 6, 8, and 10 in Table 1 showed that overall transmittance and reflectivity were not satisfactory. A figure of merit (FOM) defined as FOM=Tperp/(1−0.5*(Rperp+Rpara)) was smaller than 1.27. Examples 42-49 in Table 2 did not have the transmittance and reflectivity data, and were not discussed at all.
Films filled with inorganic inclusions with different characteristics can provide unique optical transmission and reflective properties. However, optical films made from polymers filled with inorganic inclusions suffer from a variety of problems. Typically, adhesion between the inorganic particles and the polymer matrix is poor. Consequently, the optical properties of the film decline when stress or strain is applied across the matrix, both because the bond between the matrix and the inclusions is compromised, and because the rigid inorganic inclusions may be fractured. Furthermore, alignment of inorganic inclusions requires process steps and considerations that complicate manufacturing.
Other films consist of a clear light-transmitting continuous polymer matrix, with droplets of light modulating liquid crystals dispersed within. Stretching of the material reportedly results in a distortion of the liquid crystal droplet from a spherical to an ellipsoidal shape, with the long axis of the ellipsoid parallel to the direction of stretch.
There remains a need for an improved diffusely-reflecting polarizer comprising a film having a continuous phase and a disperse phase that avoids the limitations of the prior art. The improved reflecting polarizer should have a major phase that is a relatively inexpensive material and that is amorphous, rather than crystalline or semicrystalline, to minimize haze, so the refractive index mismatch between the two phases along the material's three dimensional axes can be conveniently and permanently manipulated to achieve desirable degrees of diffuse and specular reflection and transmission. The film must also be desirably stable with respect to stress, strain, temperature differences, moisture, and electric and magnetic fields, and wherein the film has an insignificant level of iridescence. Therefore, there remains a need for an improved diffusely-reflecting polarizer comprising a film having a major phase and a dispersed minor phase, the major phase having a controllable refractive index to avoid the limitations of known reflective polarizers.
The present invention provides a method for manufacturing a diffusely reflecting polarizer, comprising the steps of: coextruding first and second polymers through a chaotic mixer and a sheeting die to produce a cast sheet with a desired blend morphology and stretching said cast sheet to produce a composite film containing a first polymer having a birefringence of less than 0.02, with said first polymer being a substantially amorphous nano-composite material, and a second polymer, the first polymer being a major phase, and the second polymer being a dispersed minor phase, wherein said first and second polymers taken together along a first axis for one polarization state of electromagnetic radiation exhibit a diffuse reflectivity R1d, a specular reflectivity R1s, a total reflectivity R1t, a diffuse transmittance T1d, a specular transmittance T1s, and a total transmittance T1t, and along a second axis for another polarization state of electromagnetic radiation exhibit a diffuse reflectivity R2d, a specular reflectivity R2s, a total reflectivity R2t, a diffuse transmittance T2d, a specular transmittance T2s, and a total transmittance T2t, the said first and second axes being orthogonal, wherein the parameters of composition, chaotic mixing, stretch temperature, stretch ratio for the process and Tg, and refractive index of the first and second polymers are selected to satisfy the equations:
R
1d is greater than R1s; and (1)
T
2t/(1−0.5(R1t+R2t))>1.35. (2)
The terms “specular reflectivity”, “specular reflection”, or “specular reflectance” Rs refer to the reflectance of light rays into an emergent cone with a vertex angle of 16 degrees centered around the specular angle. The terms “diffuse reflectivity”, “diffuse reflection”, or “diffuse reflectance” Rd refer to the reflection of rays that are outside the specular cone defined above. The terms “total reflectivity”, “total reflectance”, or “total reflection” Rt refer to the combined reflectance of all light from a surface. Thus, total reflection is the sum of specular and diffuse reflection.
Similarly, the terms “specular transmission” and “specular transmittance” Ts are used herein in reference to the transmission of rays into an emergent cone with a vertex angle of 16 degrees centered around the specular direction. The terms “diffuse transmission” and “diffuse transmittance” Td are used herein in reference to the transmission of all rays that are outside the specular cone defined above. The terms “total transmission” or “total transmittance” Tt refer to the combined transmission of all light through an optical body. Thus, total transmission is the sum of specular and diffuse transmission. In general, each diffusely reflecting polarizer is characterized by a diffuse reflectivity R1d, a specular reflectivity R1s, a total reflectivity R1t, a diffuse transmittance T1d, a specular transmittance T1s, and a total transmittance T1t, along a first axis for one polarization state of electromagnetic radiation, and a diffuse reflectivity R2d, a specular reflectivity R2s, a total reflectivity R2t, a diffuse transmittance T2d, a specular transmittance T2s, and a total transmittance T2t along a second axis for another polarization state of electromagnetic radiation. The first axis and second axis are perpendicular to each other and each is perpendicular to the thickness direction of the diffusely reflecting polarizer. Without the loss of generality, the first axis and the second axis are chosen such as the total reflectivity along the first axis is greater than that along the second axis (i.e., R1t>R2t and the total transmittance along the first axis is less than that along the second axis (i.e., T1t<T2t).
Diffuse reflectivity, specular reflectivity, total reflectivity, diffuse transmittance, specular transmittance, total transmittance, as used herein, generally have the same meanings as defined in U.S. Pat. No. 5,783,120.
The diffusely reflecting polarizers made according to the present invention all satisfy
R
1d>R1s Equation (1)
FOM≡T2t/(1−0.5(R1t+R2t))>1.35 Equation (2)
Equation (1) indicates that the reflecting polarizers of the present invention are more diffusive than specular in reflection. It is noted that a wire grid polarizer (e.g., as available from Moxtek, Inc., Orem, Utah), a multilayer interference-based polarizer such as Vikuiti™ Dual Brightness Enhancement Film, manufactured by 3M, St. Paul, Minn., or a cholesteric liquid crystal based reflective polarizer are more specular than diffusive.
Equation (2) defines the figure of merit for the diffusively reflecting polarizer. This equation states that a film is defined as a reflective polarizer if its figure of merit FOM is greater than 1.35. For polarization recycling, what matters is the total reflection and total transmission, so only total reflection and total transmission are used to compute the FOM for the purpose of ranking different reflective polarizers. The figure of merit describes the total light throughput of a reflective polarizer and an absorptive polarizer such as a back polarizer used in an LCD, and is essentially the same as equation
which applies to LCD systems where the light recycling is effected using a diffusive reflector or its equivalent. It is noted that R accounts for the reflectivity of the recycling reflective film, or the efficiency associated with each light recycling. In an ideal case, R is equal to 1, which means that there is no light loss in the light recycling. When R is less than 1, there is some light loss in the light recycling path. It is also noted that other forms of figure of merit can be used, however, the relative ranking of the reflective polarizers remain the same. For the purpose of quantifying and ranking the performance of a reflective polarizer, FOM≡T2t/(1−0.5(R1t+R2t)) will be used in this application. The extinction ratio T2t/T1t or R1t/R2t may not be proper to describe a reflective polarizer because a reflective polarizer having a higher T2t/T1t or R1t/R2t may not necessarily perform better than one having a lower extinction ratio. For an ideal conventional absorptive polarizer, T2t=1, R1t=R2t=0, so FOM=1. For an ideal reflective polarizer, T2t=1, R1t=1, and R2t=0, so FOM=2. The diffusive reflecting polarizers, as disclosed in example numbers 6, 8, 10 of U.S. Pat. No. 5,783,120 having sPS as the major phase had the FOM<1.27, which were not satisfactory.
By tuning the optical properties of the first and second polymers, the diffusively reflecting polarizers according to the present invention could have FOM values greater than 1.35, more preferably greater than 1.5. Though some diffusive reflecting polarizers as disclosed in U.S. Pat. No. 5,783,120 had a FOM greater than 1.35, they were composed of a major phase with a birefringence of greater than 0.05 and a dispersed phase with lower birefringence.
A polymer that does not produce crystallization (exothermic) or melting (endothermic) peaks during a differential scanning calorimetry (DSC) test over a temperature ranging from below its glass transition temperature (Tg) to Tg+300° C. is said to be amorphous. Conversely, if such peaks are recorded in a DSC test the polymeric material is semi-crystalline. The DSC test is well known to those skilled in the art.
Throughout this application, substantially amorphous nano-composite material refers to bi- or multi-phasic amorphous materials containing at least a continuous amorphous phase as defined above and at least one additional phase, the additional phase being dispersed into small domains within the continuous amorphous phase, the domains having characteristic dimensions in at least one dimension smaller than the wavelength of light, i.e., nano-scale domains, such that these domains do not contribute to scattering of light passing through the material, but can be used to tune the effective optical properties (refractive index and birefringence) of the material. The nano-scale dispersed materials may be amorphous or crystalline, organic or inorganic. These domains may also be spherical or non-spherical in shape.
In the present invention, a semi-crystalline polymer is not well suited for use in the major phase of the diffusely reflective film because of its propensity to thermally crystallize during stretching at elevated temperatures and thus produce an undesirable level of haze. This issue was overcome in the '440 patent, wherein the use of an amorphous polymer in the major phase was proposed. But, unlike the '440 patent, the major phase in the present invention comprises a substantially amorphous nano-composite material rather than a purely amorphous one. This approach allows better control of the optical properties of the major phase in the diffusely reflective film without loss of light transmittance by scattering.
Polymers that are thermodynamically incompatible when mixed together in the melt state are said to be immiscible. Such polymers will separate into distinct phases having coarse morphology and create an inhomogeneous blend, with each phase retaining the distinct characteristics of the polymer components and exhibiting poor adhesion between the phases. Compatible blends on the other hand exhibit fine phase morphology and good adhesion between the polymer domains comprising the blend.
Major and minor phases are thermodynamically distinct phases in a mixture with the phases having different weight fractions. A major phase has a weight fraction greater than 50%, while a minor phase has a weight fraction of less than 50%. Similarly, a continuous phase in a mixture is a thermodynamically distinct phase having a volume fraction greater than 50% and a discrete phase in a mixture is a thermodynamically distinct phase having a volume fraction less than 50%. A dispersed phase is either a minor phase or a phase having a volume fraction less than 50%.
The quantity (nx−ny) is referred to as in-plane birefringence, Δnin, where nx and ny are indices of refraction in the direction of x and y; x is taken as the direction of maximum index of refraction in the x-y plane and y direction is taken perpendicular to it; the x-y plane is parallel to the surface plane of the layer; and d is a thickness of the layer in the z-direction. The value of Δnin is typically given at a wavelength λ=550 nm.
The quantity [nz−(nx+ny)/2] is referred to as out-of-plane birefringence, Δnth, where nz is the index of refraction in the z-direction. If nz>(nx+ny)/2, Δnth is positive (positive birefringence), and if nz<(nx+ny)/2, Δnth is negative (negative birefringence). The value of Δnth is typically given at λ=550 nm.
As used herein, “nearly optically isotropic” or “weakly birefringent” means that after stretching, the material has a birefringence less than 0.02.
The present invention concerns a diffusely reflecting polarizer comprising a first polymer, with the first polymer being a substantially amorphous nano-composite material forming a major phase and a second polymer forming a minor phase, the major phase being nearly isotropic, and a method of making such a diffusely reflecting polarizer is described herein below and by referring to the drawings. The diffusely reflecting polarizers of the invention are effectively employed in a display device such as an LCD to enhance light efficiency.
Referring now to
Optionally, the diffusely reflective polarizer 30 may comprise additional layers (protective layers or “skin” layers 20A and 20B are illustrated in
The concentration of the major phase is at least 51% by weight while the concentration of the dispersed minor phase is less than 50% by weight of the total material in the film 10. Preferably the major phase is at least 60% by weight and the dispersed minor phase is less than 40% by weight.
The first polymer major phase may comprise a single polymer or two or more miscible polymers in addition to the dispersed nano-scale domains. The second polymeric phase that is the dispersed minor phase may also comprise a single polymer or two or more miscible polymers. Typically, each of the two phases comprises only a single polymer. However, a blend of two or more miscible polymers may be effectively employed in either phase in order to optimize or modify various properties such as melt viscosity, Tg, physical properties, thermal properties, refractive index, and the like.
The one or more polymers comprising the major phase are substantially amorphous nano-composite, transmissive and weakly birefringent. The one or more polymers comprising the dispersed minor phase are transmissive and highly birefringent and typically, but not exclusively, semi-crystalline. Low birefringence values in the first polymer are achieved by selecting polymers that have very low stress-optical coefficients, by dispersing nano-scale materials that lower the stress-optical coefficient of the matrix and/or by stretching the film at a temperature (Ts) well above the glass transition temperature of the polymers comprising the major phase, Ts>Tg,1+30° C. (where Tg,1 is the glass transition temperature of the major phase), such that the molecular orientation is allowed to relax sufficiently to reduce the level of in-plane birefringence to below 0.02, preferably below 0.01, and more preferably below 0.005, as the material solidifies after stretching. Examples of polymers for use in the substantially amorphous nano-composite major phase include cyclic block copolymers (CBC) produced by substantially fully hydrogenating anionically polymerized vinyl aromatic-conjugated diene block copolymers using a porous silica supported metal heterogeneous catalyst.
The vinyl aromatic/conjugated diene block copolymer, prior to hydrogenation, may have any known architecture, including distinct block, tapered block, and radial block. Distinct block structures that include alternating vinyl aromatic blocks and conjugated diene blocks yield preferred results, especially when such block structures yield triblock copolymers or pentablock copolymers, in each case with vinyl aromatic end blocks. Pentablock copolymers constitute particularly preferred block copolymers. The vinyl aromatic blocks may have the same or different molecular weights as desired. Similarly, the conjugated diene blocks may have the same or different molecular weights.
Typical vinyl aromatic monomers include styrene, alpha-methylstyrene, all isomers of vinyl toluene (especially paravinyl toluene), all isomers of ethyl styrene, propyl styrene, butyl styrene, vinyl biphenyl, vinyl naphthalene, vinyl anthracene and the like, or mixtures thereof. The block copolymers can contain one or more than one polymerized vinyl aromatic monomer in each vinyl aromatic block. The vinyl aromatic blocks preferably comprise styrene, more preferably consist essentially of styrene, and still more preferably consist of styrene.
The conjugated diene blocks may comprise any monomer that has two conjugated double bonds. Illustrative, but non-limiting, examples of conjugated diene monomers include butadiene, 2-methyl-1,3-butadiene, 2-methyl-1,3-pentadiene, isoprene, or mixtures thereof. As with the vinyl aromatic blocks, the block copolymers may contain one (for example, butadiene or isoprene) or more than one (for example, both butadiene and isoprene). Preferred conjugated diene polymer blocks in the block copolymers may, prior to hydrogenation, comprise polybutadiene blocks, polyisoprene blocks or mixed polybutadiene/polyisoprene blocks. While a block copolymer may, prior to hydrogenation, include one polybutadiene block and one polyisoprene block, preferred results follow with block copolymers that, prior to hydrogenation, have conjugated diene blocks that are solely polybutadiene blocks or solely polyisoprene blocks. A preference for a single diene monomer stems primarily from manufacturing simplicity. In both cases, the microstructure of diene incorporation into the polymer backbone can be controlled to achieve a CBC polymer that is substantially or fully amorphous.
Illustrative preferred vinyl aromatic/conjugated diene block copolymers wherein each vinyl aromatic block comprises styrene (S) and each conjugated diene block comprises butadiene (B) or isoprene (I) include SBS and SIS triblock copolymers and SBSBS and SISIS pentablock copolymers. While the block copolymer may be a triblock copolymer or, more preferably a pentablock copolymer, the block copolymer may be a multiblock that has one or more additional vinyl aromatic polymer blocks, one or more additional conjugated diene polymer blocks or both one or more additional vinyl aromatic polymer blocks and one or more additional conjugated diene polymer blocks, or a star block copolymer (for example, that produced via coupling). One may use a blend of two block copolymers (for example, two triblock copolymers, two pentablock copolymers or one triblock copolymer and one pentablock copolymer) if desired. One may also use two different diene monomers within a single block, which would provide a structure that may be shown as, for example, SIBS. These representative structures illustrate, but do not limit, block copolymers that may be suitable for use as the first polymer in an embodiment of this invention.
“Substantially fully hydrogenated” means that at least 95 percent of the double bonds present in vinyl aromatic blocks prior to hydrogenation are hydrogenated or saturated and at least 97 percent of double bonds present in diene blocks prior to hydrogenation are hydrogenated or saturated. By varying the relative length of the blocks, total molecular weight, block architecture (e.g., diblock, triblock, pentablock, multi-armed radial block, etc) and process conditions, various types of nanostructure morphology can be obtained from this block copolymer and thereby modify the optical properties of the major phase. Specific, non-limiting examples include lamellar morphology, bicontinuous gyroid morphology, cylinder morphology, and spherical morphology, etc. The morphology and microphase separation behavior of a block copolymer is well known and may be found, for example, in The Physics of Block Copolymers by Ian Hamley, Oxford University Press, 1998. Particularly preferred CBC polymers are those having an amount of styrene from 55-80 wt % and an amount of conjugated diene from 20-45 wt %, prior to hydrogenation.
Amorphous, highly transmissive polymers blended with well dispersed inorganic nano-scale particles is another example of a substantially amorphous nano-composite material.
The CBC polymers in the major phase can be blended with a non-block polymer or copolymer with one block. Illustrative non-block polymers and copolymers include, but are not limited to, hydrogenated vinyl aromatic homopolymers or random copolymers, cyclic olefin polymers (COPs), cyclic olefin copolymers (COCs), acrylic polymers, acrylic copolymers or mixtures thereof. The non-block polymer or copolymer, when blended with a CBC material, is miscible with, and sequestered within, one phase of the block copolymer.
COCs and COPs are especially interesting as a miscible blend component for the major phase of this invention. These cyclic olefin materials are remarkably glass-like organic material. COC materials have a luminous transmittance of 91% in the visible region. In addition to their high transmittance and high Abbé number (58), COC resins have very low haze and yellowness, ensuring minimal light loss by scattering or absorption. As amorphous polymers with low optical anisotropy, they also have inherently low birefringence and a very low stress-optic coefficient—as low or even lower than that of PMMA—so that they retain low birefringence under load. Grades of COC resins are available with heat deflection temperatures as high as 170° C. (338° F.) making them tolerant of short term exposures to quite high temperatures. COC resins provide excellent moisture control. It has roughly double the moisture barrier of high-density polyethylene (HDPE) and five times the moisture barrier of low-density polyethylene (LDPE). COC resins are environmentally-friendly and permit easy disposal. On combustion it forms no toxic gases but only water and carbon dioxide.
High values of birefringence in the minor phase are achieved by utilizing materials with relatively high stress-optical coefficient and by stretching the film at a temperature Ts such that: Tg,2<Ts<Tg,2+30° C. (where Tg,2 is the Tg of the disperse minor phase). Examples of polymers suitable for use in the minor phase include, but are not limited to, polyesters, polyamides, and polyester-amides and other classes of semicrystalline polymers. Specific non-limiting examples include poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), and polyesters containing the cyclohexyl dimethylene moiety. In addition to the high birefringence in the minor phase, the overall levels of transmittance and reflectivity are greatly influenced by the domain morphology of the dispersed phase. Generally, a platelet-like morphology, as shown in
In one embodiment of the invention, the minor dispersed phase comprises a compatible polyester blend and a means of substantially inhibiting a transesterification reaction. The polyester blend may comprise one or more polyesters or at least one polyester and a polycarbonate. Transesterification inhibitors are well known in the polymer processing industry and generally comprise a phosphorous compound. Suitable transesterification inhibitors for use in the present invention include, but are not limited to, organophosphites such as triphenyl phosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, di-n-octadecyl phosphite, tris(2,4-di-t-butylphenyl)phosphite, tris(monononylphenyl)phosphite, trimethyl phosphite, triethyl phosphite, and others.
The diffusely reflecting polarizer of the invention may be used in combination with one or more other optical films that are typically employed in display devices such as LCD, including, for example, films that provide the function of antireflection, ambient light suppression, illumination enhancement, light collimation, light directing, light diffusion, light spreading, viewing angle enhancement, polarization, and the like
The diffusely reflecting polarizer of the present invention is produced by a multi-step process. First, the first major phase polymer and the second dispersed phase polymer are fed separately by two or more extruders into a continuous flow chaotic mixing element and a sheeting die, with the chaotic mixing element being operated to create a cast sheet with a desired blend morphology. The chaotic mixing method is known in the art as a way to produce multi-component blends with a wide variety of well-controlled blend morphologies. Studies by Zumbrunen et al. (e.g., Y. H. Liu and D. A. Zumbrunen, J. Mat. Sci., 34, 1921 (1999), O. Kwon and D. A. Zumbrunen, Polym. Eng. Sci., 43, 1443 (2003); See also, U.S. Pat. Nos. 6,770,340 and 6,902,805), focusing mostly on blend applications, describe different morphologies that can be created in multi-phase polymer systems by adjusting the operational parameters of the chaotic mixer whereby the morphology of the two-phase polymer system is controlled not only by the thermodynamics of the blend components but also by the dynamics of the chaotic mixing process.
In the case of the diffusely reflecting polarizer of the present invention, a platelet-like morphology (as shown in
The cast sheet with the desired blend morphology must further undergo a stretching step in order to induce the desired birefringence level in the dispersed minor phase of the film. The stretching, to orient the composition, can be done in line right after the sheet extrusion, or carried out in a separate step off-line. In either case the sheet is first heated to a temperature Ts, such that: Tg,2<Ts<Tg,230° C. and is then stretched uniaxially, along the machine direction or along the cross-machine direction, or both, to produce the desired level of in-plane birefringence in the dispersed minor phase. Thus, the stretching temperature must meet the following conditions:
T
g,1
<T
s
T
g,2
<T
s
<T
g,2+30° C.
Where Tg,1 is the glass transition temperature of the substantially amorphous nano-composite major phase and Tg,2 is the glass transition temperature of the second polymer comprising the minor phase.
If the major phase has a high stress-optical coefficient then Ts−Tg,1>30° C., i.e., the stretching temperature must be relatively high compared to the glass transition temperature of the major phase in order to achieve low birefringence in the major phase (that is, the birefringence of the major phase must be less than 0.02 after stretching and solidification).
Typical extension or stretch ratios range from 3× to 7× although a wider range of stretch ratios may be considered. Stretching can be done using a number of methods well known to those skilled in the art. In some cases the edges of the stretched film can be restrained during the stretching step although unrestrained stretch is preferred. Compared to the approach of U.S. Pat. No. 5,783,120, the present invention provides a wide range of options and materials with the possibility of lower cost and superior optical performance without the limitations defined in the prior art.
After stretching, the total thickness of the reflecting polarizer of the invention is expected to vary from 25 to 1000 microns. Typically, a total thickness from about 100 to 500 microns is sufficient to achieve the desired degree of polarization recycling and dimensional stability.
As mentioned previously, in one embodiment of the invention the reflective polarizer 30 is produced with protective layers 20A and 20B on each side of film 10, see
