Cellulose ester films having +c plate optical properties

Abstract

The present application discloses compositions and films comprising cellulose esters having low hydroxyl content and certain hydroxy containing additives. These films exhibit positive C (+C) plate behavior, which make them particularly suitable for use in optical applications, such as in liquid crystal displays (LCD) and organic LED (OLED) displays as protective and compensation films.

Description

[0001]CELLULOSE ESTER FILMS HAVING +C PLATE OPTICAL PROPERTIES BACKGROUND OF THE INVENTION Cellulose esters such as cellulose triacetate (CTA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB), are used in a wide variety of films by the liquid crystal display (LCD) as well as OLED display industry. Most notable is their use as protective and compensator films in conjunction with polarizer sheets. These films are typically made by solvent casting, and then are laminated to either side of an oriented, iodinated polyvinyl alcohol (PVOH) polarizing film to protect the PVOH layer against scratching and moisture ingress, while also increasing structural rigidity. Alternatively, as in the case of compensation films, they can be laminated with the polarizer stack or otherwise included between the polarizer and liquid crystal layers. Cellulose esters have many performance advantages over other materials that see use in display films such as cyclolefins, polycarbonates, polyimides, etc. However, optical birefringence requirements currently often dictate that the latter materials be used instead. In addition to serving a protective role, these films also play a role in improving the contrast ratio, wide viewing angle, and color shift performance of the flat panel displays. For a typical set of crossed polarizers used in an LCD, there is significant light leakage along the diagonals (leading to a poor contrast ratio), particularly as the viewing angle is increased. It is known that various combinations of optical films can be used to correct or “compensate” for this light leakage. These films must have certain well-defined birefringence (or retardations) that vary depending on the type of liquid crystal cell used, since the liquid crystal cell itself will also impart a certain degree of undesirable optical retardation that must be corrected. Some of these compensator films are easier to make than others, so compromises are often made between performance and cost. Also, while most of the compensator and protective films are made by solvent casting, there is a push to make more films by melt extrusion. Compensator and optical films are commonly quantified in terms of birefringence which is, in turn, related to the refractive index n. The refractive index is typically in the range of 1.4 to 1.8 for polymers in general, and approximately 1.46 to 1.50 for cellulose esters. The higher the refractive index, the slower the speed the light wave propagates through that given material. For an unoriented isotropic material, the refractive index will be the same regardless of the polarization state of the entering light wave. As the material becomes oriented, or otherwise anisotropic, the refractive index becomes dependent on material direction. For purposes of the present invention, there are three refractive indices of interest denoted as nx, ny, and nz, which correspond to the machine direction (MD), the transverse direction (TD), and the thickness direction, respectively. As the material becomes more anisotropic (e.g., by stretching it), the difference between any two refractive indices will increase. This difference is referred to as the “birefringence.” Because there are many combinations of material directions to choose from, there are correspondingly different values of birefringence. The two that are the most common, namely the planar birefringence Δeand the thickness birefringence Δth, are defined as: (1a) Δe= nx– ny(1b) Δth = nz – (nx + ny) / 2 The birefringence Δeis a measure of the relative in-plane orientation between the MD and TD directions and is dimensionless. In contrast, Δth gives a measure of the orientation of the thickness direction, relative to the average planar orientation. Another term often used to characterize optical films is the optical retardation R. R is simply the birefringence times the thickness (d) of the film in question. Thus, (2a) Re = Δed = (nx – ny)d (2b) Rth= Δthd = [nz– (nx+ ny) / 2]d Retardation is a direct measure of the relative phase shift between the two orthogonal optical waves and is typically reported in units of nanometers (nm). Note that the definition of Rth varies with some authors particularly with regard to the + / - sign. The actual compensator films that are used in an LCD can take on a variety of forms including biaxial films where all three refractive indices differ and two optical axes exist, and uniaxial films having only one optical axis where two of the three refractive indices are the same. There are also other classes of compensator where the optical axes twist or tilt through the thickness of the film (e.g., discotic films), but these are of lesser importance to understanding the present invention. The important point is that the type of compensation film that can be made is limited by the birefringence characteristics of the polymer (i.e., positive or negative). In the case of uniaxial films, a film having refractive indices such that (3a) nx > ny = nz is denoted as a “+A” plate. In these films, the x direction of the film has a high refractive index while the y and thickness directions are approximately equal in magnitude (and lower than nx). This type of film is also referred to as a positive uniaxial crystal structure with the optic axis along the x-direction. Such films are easy to make by uniaxially stretching a positively birefringent material, using, for example, a film drafter. In contrast, a “-A” plate uniaxial film is defined as (3b) nx < ny = nz where the x-axis refractive index is lower than the other directions (which are approximately equal). The most common method for making a -A plate is to stretch a negative birefringent polymer, or alternatively, by coating a negatively birefringent liquid crystal polymer onto a surface such that the molecules are lined up in a preferred direction. Another class of uniaxial optical film is the C plate, which can also be “+C” or “-C.” The difference between a C plate and an A plate is that in the former, the unique refractive index (or optical axis) is in the thickness direction as opposed to in the plane of the film. Thus, (4a) nz > ny = nx (“+C” plate) (4b) nz< ny= nx(“-C” plate) C-plates can be made by biaxial stretching if the relative stretch in the x and y directions is held constant. Alternatively, they can be made by compression forming. Compressing or equibiaxially stretching an initially isotropic, positive intrinsic birefringent material will result in a -C plate since the effective orientation direction is in the plane of the film. Conversely, a +C plate is made by compressing or equibiaxially stretching an initially isotropic film made with negative intrinsic birefringent material. In the case of biaxial stretching, if the orientation level is not kept the same in the MD and TD directions, then the material is no longer a true C-plate, but instead is a biaxial film with 2 optical axes. A third, and more common option for producing C-plates takes advantage of the stresses that form during solvent casting of a film. Tensile stresses are created in the plane of the film due to the restraint imposed by the casting belt, which are also equi-biaxial in nature. These tend to align the chains in the plane of the film resulting in -C or +C films for positive and negative intrinsic birefringent materials, respectively. As most cellulose ester films used in displays are solvent cast, and all are essentially positive birefringent, it is apparent that solvent cast cellulose esters normally only produce -C plates. These films can also be uniaxially stretched to produce +A plates (assuming the initial as-cast retardation is very low), but the ability to make +C or -A plates with cellulose esters is extremely limited. In order for compensator films to properly eliminate light leakage, they must be combined in certain ways depending on the type of liquid crystal cell used. For example, Fundamentals of Liquid Crystal Displays (D.K. Yang and S.T. Wu, Wiley, New Jersey, 2006, pp 208-237) describes various ways to compensate for IPS (in-plane switching), twisted nematic (TN), and VA (vertical alignment) type cells using combinations of uniaxial plates (biaxial plates are also effective but are more complicated mathematically). In the case of an IPS cell, a +C plate followed by a +A plate is described (also described is +A followed by +C. The Rthvalues of solvent-cast cellulose triacetate films range from about -20 to -70 nm, but with mixed ester systems, we have observed ranges from about -20 to -300 nm depending on the type of cellulose ester involved (which determines its intrinsic birefringence), the time left on the casting belt (which controls the residual stress in the film), and the type of plasticizers and additives used. Note that by “mixed ester,” we are referring to cellulose esters having more than one ester type such as, for example, cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB). Of note, however, is that -A and +C compensator plates cannot be easily made with cellulose esters because of their positive birefringent nature. Thus, other more costly (or poorer performing) materials have to be used instead. Currently, commercial films exhibiting +C plate behavior are made using a nematic liquid crystal coating with subsequent polymerization process. The coating process and liquid crystal material, however, are very expensive and require an additional process step of coating a film to achieve the desired properties. There are no commercial films exhibiting high C+ behavior based on cellulose ester and additives. Thus, there is a need in the art for films exhibiting +C plate behavior without using a liquid crystal material and without requiring an additional coating step. SUMMARY OF THE INVENTION The present application discloses an optical compensation film comprising: (i) a cellulose ester comprising: (a) a plurality of hydroxyl substituents, (b) a plurality of propionyl substituents, and (c) a plurality of acetyl substituents, wherein: the average degree of substitution for the hydroxyl substituents (“DSOH”) is in the range of from 0 to 0.25, the average degree of substitution for the propionyl substituents (“DSPr”) is in the range of from 0-3.0, the average degree of substation for the acetyl substituents (“DSAc”) is in the range of from 0-3.0, and the sum of the DSPr and the DSAc is at least 2.75; (ii) an additive, wherein the additive is an aliphatic diol, an aliphatic triol, or an aliphatic polyol; wherein: the additive is present at from 5-25wt%, based on the total weight of the optical compensation film, and the optical compensation film has an out-of-plane retardation (“Rth”) measured at 589 nm ranging from 15-60 nm in the thickness direction measured at a film thickness of 80 µm or less. The present application also discloses a composition, comprising: (i) a cellulose ester comprising: (a) a plurality of hydroxyl substituents, (b) a plurality of propionyl substituents, and (c) a plurality of acetyl substituents, wherein: the average degree of substitution for the hydroxyl substituents (“DSOH”) is in the range of from 0 to 0.25, the average degree of substitution for the propionyl substituents (“DSPr”) is in the range of from 0-3.0, the average degree of substation for the acetyl substituents (“DSAc”) is in the range of from 0-3.0, and the sum of the DSPr and the DSAc is at least 2.75; (ii) an additive, wherein the additive is an aliphatic diol, an aliphatic triol, or an aliphatic polyol; wherein: the additive is present at from 5-25wt%, based on the total weight of the optical composition. The present application also discloses pellets, dopes, melts, and various articles comprising the compositions disclosed herein. DETAILED DESCRIPTION OF THE INVENTION The present invention may be understood more readily by reference to the following detailed description of the invention, and to the Examples included therein. Before the present compositions of matter and methods are disclosed and described, it is to be understood that this invention is not limited to specific synthetic methods or to particular formulations, unless otherwise indicated, and, as such, may vary from the disclosure. It is also to be understood that the terminology used is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the invention. Definitions Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. Each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Further, the ranges stated in this disclosure and the claims are intended to include the entire range specifically and not just the endpoint(s). For example, a range stated to be 0 to 10 is intended to disclose all whole numbers between 0 and 10 such as, for example 1, 2, 3, 4, etc., all fractional numbers between 0 and 10, for example 1.5, 2.3, 4.57, 6.1113, etc., and the endpoints 0 and 10. Also, a range associated with chemical substituent groups such as, for example, “C1 to C5 hydrocarbons”, is intended to specifically include and disclose C1and C5hydrocarbons as well as C2, C3, and C4 hydrocarbons. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include their plural referents unless the context clearly dictates otherwise. For example, reference a “film,” or a “polarizer,” is intended to include the processing or making of a plurality of films, or polarizers. References to a composition containing or including “an” additive or “a” catalyst is intended to include other ingredients or other additives or other catalysts, respectively, in addition to the one named. By “comprising” or “containing” or “including” we mean that at least the named compound, element, particle, or method step, etc., is present in the composition or article or method, but does not exclude the presence of other compounds, catalysts, materials, particles, method steps, etc, even if the other such compounds, material, particles, method steps, etc., have the same function as what is named, unless expressly excluded in the claims. It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps before or after the combined recited steps or intervening method steps between those steps expressly identified. Moreover, the lettering of process steps or ingredients is a convenient means for identifying discrete activities or ingredients and the recited lettering can be arranged in any sequence, unless otherwise indicated. Throughout this application, where patents or publications are referenced, the disclosures of these references in their entireties are intended to be incorporated by reference into this application, to the extent they are not inconsistent with the present invention, in order to more fully describe the state of the art to which the invention pertains. “Melt extruded film” means is a film that has been made by a extrusion process. “Melt” means polymer composition that is flowable that has been heated above its melting temperature or glass transition temperature. Films It has been surprisingly discovered that a positive C-plate film can be made by using low hydroxyl cellulose esters and certain additives. The present application discloses, in a first aspect, an optical compensation film comprising: (i) a cellulose ester comprising: (a) a plurality of hydroxyl substituents, (b) a plurality of propionyl substituents, and (c) a plurality of acetyl substituents, wherein: the average degree of substitution for the hydroxyl substituents (“DSOH”) is in the range of from 0 to 0.25, the average degree of substitution for the propionyl substituents (“DSPr”) is in the range of from 0-3.0, the average degree of substation for the acetyl substituents (“DSAc”) is in the range of from 0-3.0, and the sum of the DSPrand the DSAcis at least 2.75; (ii) an additive, wherein the additive is an aliphatic diol, an aliphatic triol, or an aliphatic polyol; wherein: the additive is present at from 5- 25wt%, based on the total weight of the optical compensation film, and the optical compensation film has an out-of-plane retardation (“Rth”) measured at 589 nm ranging from 15-60 nm in the thickness direction measured at a film thickness of 80 µm or less. In one embodiment or in combination with any other embodiment of the first aspect, the DSAcis in the range of from 0-3.0, or in the range of from 0- 2.8, or in the range of from 0-2.6, or in the range of from 0-2.4, or in the range of from 0-2.2. In one embodiment or in combination with any other embodiment of the second aspect, the DSPris in the range of from 1.1-3.0, and the DSAcis in the range of from 0-1.6. In one class of this embodiment, the DSPr is in the range of from 1.1-2.7, or in the range of from 1.1-2.4, or in the range of from 1.3-3.0, or in the range of from 1.3-2.7, or in the range of from 1.3-2.4, or in the range of from 1.6-3.0, or in the range of from 1.6-2.7, or in the range of from 1.6-2.4. In one class of this embodiment, the DSAc is in the range of from 0-1.4, or in the range of from 0-1.2, or in the range of from 0-1.0, or in the range of from 0.2-1.6, or in the range of from 0.2-1.2, or in the range of from 0.2-1.0, or in the range of from 0.4-1.6, or in the range of from 0.4-1.4, or in the range of from 0.4-1.2, or in the range of from 0.4-1.0. In one embodiment or in combination with any other embodiment of the first aspect, the DSPr is in the range of from 1.45-1.53, the DSAc is in the range of from 1.4-1.5, and the DSOHis in the range of from 0.03-0.1. In one class of this embodiment, the DSPr is in the range of from 1.45-1.5, or in the range of from 1.5-1.53. In one class of this embodiment, the DSAc is in the range of from1.4-1.45. In one embodiment or in combination with any other embodiment of the first aspect, the DSPris in the range of from 1.8-1.9, the DSAcis in the range of from 1-1.1, and the DSOH is in the range of from 0.1-0.2. In one class of this embodiment, the DSPr is in the range of from 1.8-1.85, or in the range of from 1.85-1.9. In one embodiment or in combination with any other embodiment of the first aspect, the DSPr is in the range of from 1.8-2.4, DSAc is in the range of from 0.5-1.0, and DSOHis in the range of from 0.02-0.08. In one class of this embodiment, the DSPr is in the range of from 1.8-2.2, or in the range of from 1.8-2.0, or in the range of from 2.0-2.4, or in the range of from 2.0-2.2, or in the range of from 2.2-2.4. In one embodiment or in combination with any other embodiment of the first aspect, the DSPr is in the range of from 2.8-3.0, the DSAc is in the range of from 0-0.2, and DSOHis in the range of from 0-0.2. In one class of this embodiment, the DSPr is in the range of 2.9-3.0, or in the range of from 2.8- 2.9. In one class of this embodiment, the DSAc is in the range of from 0.05- 0.2, or in the range of from 0.05-0.1, or in the range of from 0.1 to 0.2. In one embodiment or in combination with any other embodiment of the first aspect, the DSAc is in the range of from 2.8-3.0, and the DSOH is in the range of from 0-0.2. In one class of this embodiment, the DSAc is in the range of from 2.8-2.9, or in the range of from 2.9-3.0. In one embodiment or in combination with any other embodiment of the first aspect, the additives have a melting point that is higher than 15°C. In one embodiment or in combination with any other embodiment of the first aspect, the additive is 1,4-cyclohexanedimethanol (CHDM), neopentyl glycol (NPG), 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD), trimethylolpropane (TMP), xylitol, sorbitol, or combinations thereof. In one embodiment or in combination with any other embodiment of the first aspect, the additive is selected from CHDM, NPG, TMP, or a combination thereof. In one embodiment or in combination with any other embodiment of the first aspect, the additive is present at from 6-25 wt%, or from 6-22wt%, or from 6-20wt%, or from 6-18wt%, or from 6-16wt%, or from 6-14wt%, or from 6- 12wt%, or from 6-10wt%, or from 8-25wt%, or from 8-22wt%, or from 8- 20wt%, or from 8-18wt%, or from 8-16wt%, or from 8-14wt%, or from 8- 12wt%, or from 8-10wt%, or from 10-25wt%, or from 10-22wt%, or from 10- 20wt%, or from 10-18wt%, or from 10-16wt%, or from 10-14wt%, or from 10- 12wt%, or 12-25wt%, or from 12-22wt%, or from 12-20wt%, or from 12- 18wt%, or from 12-16wt%, or from 12-14wt%, or from 14-25wt%, or from 14- 22wt%, or from 14-20wt%, or from 14-18wt%, or from 14-16wt%, or from 16- 25wt%, or from 16-22wt%, or from 16-20wt%, or from 16-18wt%, or from 18- 25wt%, or from 18-22wt%, or from 18-20wt%, or from 20-25wt%, or from 20- 22wt%, based on the total weight of the optical compensation film. In one embodiment or in combination with any other embodiment in the first aspect, the film is a melt extruded film or a solvent cast film. In one class of this embodiment, the film is a melt extruded film. In one class of this embodiment, the film is a solvent cast film. An example of a cast cellulose ester film is disclosed in US Pat. No.7084944. The cellulose ester film is prepared according to a solvent casting method. As the solvent, an organic solvent is used. The solvent cast method comprises the steps of dissolving the cellulose ester in an organic solvent to prepare a solution (dope) and casting the dope onto a substrate to prepare a film. The substrate can be a glass substrate, a metal substrate, a polymer substrate, a belt, or a roll. For melt extrusion techniques, the compositions can be extruded using a single-screw extruder, twin-screw extruder, roll-mill, or any other melt plastic processing equipment. Furthermore, as is more common the materials can be pre-compounded together on one system such as a twin-screw or roll-mill, pelletized, and then processed on a second extruder in conjunction with the film line. For single and twin-screw film extrusion, a gear pump can be used in between the extruder and die to ensure uniform flow rate. Casting equipment (solvent or melt) can include a casting roll or drum, a roll-stack arrangement, or even a casting belt. Rolls and drums are more common. In a roll-stack arrangement, the film is often polished or followed with a smaller nip roll to improve gauge control, although care should be taken not to induce excessive stress in the film. The roll is typically a chrome plated roll with mirror-like finish, but matted rolls or rolls with varied surface finishes / roughness can also be used, if desired. Roll casting whereby the roll is partially immersed in a cooling bath is also possible to increase cooling rates and speed up throughput. To ensure good contact between film and casting equipment, an air knife, vacuum box or electrostatic pinning wire can be used. After casting, the film can be dried and annealed in a forced air oven for 5 to 10 minutes at 100ºC. After annealing at 100ºC, the film can then be annealed at a higher temperature (e.g., 120ºC, 130ºC, 140ºC, or 150ºC) for up to 20 minutes. The primary purpose of the annealing process is to increase the diffusion of residual solvents that might remain in the film from the casting process. However, an additional benefit of annealing is the relaxation of residual stresses that developed during the casting process. As the film adheres to the casting substrate, the solvents evaporate to the open surface creating internal stresses in the film. These stresses depend on material properties, solvent mix, adhesion to the substrate, and solvent evaporation rate. Casting methods and rates can lead to higher stresses, higher birefringence, and higher retardation. Relaxing these process-induced stresses is desirable for dimensional stability and low retardation, +C type films. These annealing times and temperatures can vary, depending on the casting technique used. For example, if a continuous solvent casting line is used instead of a batch process in a laboratory, lower annealing temperatures and shorter times may be used. In one embodiment or in combination with any other embodiment in the first aspect, the film is uniaxially stretched, biaxially stretched, or 45-degree stretched. In one class of this embodiment, the film is uniaxially stretched. In one class of this embodiment, the film is biaxially stretched. In one class of this embodiment, the film is 45-degree stretched. In one embodiment or in combination with any other embodiment in the first aspect, the film has a d that is in the range of from 5-50 microns, or in the range of from 5-40 microns, or in the range of from 5-30 microns, or in the range of form 5-20 microns, or in the range of from 10-50 microns, or in the range of from 10-40 microns, or in the range of from 10-30 microns, or in the range of from 10-20 microns, or in the range of from 15-50 microns, or in the range of from 15-40 microns, or in the range of from 15-30 microns, or in the range of from 15-20 microns, or in the range of from 20-50 microns, or in the range of from 20-40 microns, or in the range of from 20-30 microns, or in the range of from 30-50 microns, or in the range of from 30-40 microns, or in the range of from 40-50 microns, or in the range of from 5-100 microns, or in the range of from 5-80 microns, or in the range o from 5-60 microns, or in the range of from 10-100 microns, or in the range of from 10-80 microns, or in the range of from 10-60 microns, or in the range of from 20-100 microns, or in the range of from 20-80 microns, or in the range of from 20-60 microns, or in the range of from 30-100 microns, or in the range of from 30-80 microns, or in the range of from 30-60 microns, or in the range of from 40-100 microns, or in the range of form 40-80, or in the range of from 40-60, or in the range of from 50- 100 microns, or in the range of from 50-80 microns, or in the range of from 50- 60. In one embodiment of this invention, other additives such as stabilizers, antiblocks, slip agents, lubricants, dyes, pigments, retardation modifiers, etc. may be mixed with the cellulose esters. Examples of these other additives are found in US 2009 / 0050842, US 2009 / 0054638, and US 2009 / 0096962; the contents of which are hereby incorporated by reference. In one embodiment or in combination with any other embodiment in the first aspect, the film further comprises a plasticizer that is different that the additive. In one class of this embodiment, the plasticizer is a phosphoric acid- based plasticizer, a phthalic acid ester-based plasticizer, a glycolate based plasticizer, a citric acid ester-based plasticizer, a carbohydrate ester-based plasticizer, and an alditol ester-based plasticizer. Examples of phosphoric acid ester-based plasticizers include but are not limited to triphenyl phosphate (TPP), tricresyl phosphate, cresyl phenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, and tributyl phosphate. Phthalic acid ester-based plasticizers include but are not limited to diethyl phthalate, dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethyl hexyl phthalate, butyl benzyl phthalate, di-2-ethylhexyl phthalate, butyl benzyl phthalate, and dibenzyl phthalate. Citric acid ester-based plasticizers include but are not limited to acetyl trimethyl citrate, and acetyl tributyl citrate. Glycolate-based plasticizers include but are not limited to alkyl phthalyl alkyl glycolate, such as methyl phthalyl methyl glycolate, ethyl phthalyl ethyl glycolate (EPEG), propyl phthalyl propyl glycolate, butyl phthalyl butyl glycolate, octyl phthalyl octyl glycolate, methyl phthalyl ethyl glycolate, ethyl phthalyl methyl glycolate, ethyl phthalyl propyl glycolate, propyl phthalyl ethyl glycolate, methyl phthalyl propyl glycolate, methyl phthalyl butyl glycolate, ethyl phthalyl butyl glycolate, butyl phthalyl methyl glycolate, butyl phthalyl ethyl glycolate, propyl phthalyl butyl glycolate, butyl phthalyl propyl glycolate, methyl phthalyl octyl glycolate, ethyl phthalyl octyl glycolate, octyl phthalyl methyl glycolate, and octyl phthalyl ethyl glycolate. Other useful plasticizers include, but are not limited to, butyl oleate, methyl acetyl ricinolate, dibutyl sebacate, and triacetin. Carbohydrate ester-based plasticizers include, but are not limited to, esters of 6-carbon aldose sugars, such as glucose pentapropionate, glucose pentaisobutyrate, and glucose pentatbutyrate; esters of 6-carbon ketose sugars such as fructose pentapropionate, fructose pentaisobutyrate, fructose pentatbutyrate; esters of 5-carbon aldose sugars, such as xylose tetrapropionate, xylose tetraisobutyrate, and xylose tetrabutryate. Alditol ester-based plasticizers include but are not limited to 5- carbon alditol esters, such as xylitol pentapropionate, xylitol pentaisobutryate, and xylitol pentabutyrate; 6-carbon alditol esters, such as mannitol hexapropionate, mannitol hexaisobutyrate, and mannitol hexabutyrate. Other useful plasticizers include triphenyl phosphate, xylitol pentaacetate, trimethyl pentanoyl diisobutyrate, 2-naphthyl benzoate or mixtures thereof. The present application also discloses, in a second aspect, a multilayer film comprising any of the films disclosed herein. The multilayer film can be made by solvent co-casting, melt co- extrusion, lamination, or a coating process. These procedures are generally known in the art. Examples of solvent co-casting, melt co-extrusion, lamination, and coating methods to form multilayer structures are found in US 2009 / 0050842, US 2009 / 0054638, and US 2009 / 0096962. Further examples of solvent co-casting, melt coextrusion, lamination, and coating methods to form a multilayer structure are found in U. S. Pat. No. 4, 592, 885; U.S. Pat. No.7,172,713; US 2005 / 0133953; and US 2010 / 0055356, the contents of which are hereby incorporated by reference in their entirety The multilayer film may be configured in an A-B structure or an A-B-A structure. In the case of a bi-layer structure, the layers are made using different cellulose esters. For the tri-layer structure, the top and bottom layers are made using the films disclosed herein and the middle layer is made using a film. Other configurations are possible such as A-X-B where X is an adhesive or tie layer, and B-A-B. The thickness of each layer can be the same or different. By varying the thickness of each layer, the desired optical retardation and reversed optical dispersion can be obtained. The thickness of layer A before stretching can range from 5 µm to 50 µm, and the thickness of layer B before stretching can range from 30 µm to 100 µm. In one embodiment or in combination with any other embodiment in the first aspect, cellulose ester may further comprise other substituents such as butyryl group and other longer-chain groups having more than four carbons. Compositions The present application also discloses, in a second aspect, a composition, comprising: (i) a cellulose ester comprising: (a) a plurality of hydroxyl substituents, (b) a plurality of propionyl substituents, and (c) a plurality of acetyl substituents, wherein: the average degree of substitution for the hydroxyl substituents (“DSOH”) is in the range of from 0 to 0.25, the average degree of substitution for the propionyl substituents (“DSPr”) is in the range of from 0-3.0, the average degree of substation for the acetyl substituents (“DSAc”) is in the range of from 0-3.0, and the sum of the DSPrand the DSAc is at least 2.75; (ii) an additive, wherein the additive is an aliphatic diol, an aliphatic triol, or an aliphatic polyol; wherein: the additive is present at from 5-25wt%, based on the total weight of the composition. In one embodiment or in combination with any other embodiment of the second aspect, the DSAc is in the range of from 0-3.0, or in the range of from 0-2.8, or in the range of from 0-2.6, or in the range of from 0-2.4, or in the range of from 0-2.2. In one embodiment or in combination with any other embodiment of the second aspect, the DSPris in the range of from 1.1-3.0, and the DSAcis in the range of from 0-1.6. In one class of this embodiment, the DSPr is in the range of from 1.1-2.7, or in the range of from 1.1-2.4, or in the range of from 1.3-3.0, or in the range of from 1.3-2.7, or in the range of from 1.3-2.4, or in the range of from 1.6-3.0, or in the range of from 1.6-2.7, or in the range of from 1.6-2.4. In one class of this embodiment, the DSAc is in the range of from 0-1.4, or in the range of from 0-1.2, or in the range of from 0-1.0, or in the range of from 0.2-1.6, or in the range of from 0.2-1.2, or in the range of from 0.2-1.0, or in the range of from 0.4-1.6, or in the range of from 0.4-1.4, or in the range of from 0.4-1.2, or in the range of from 0.4-1.0. In one embodiment or in combination with any other embodiment of the second aspect, the DSPr is in the range of from 1.45-1.53, the DSAc is in the range of from 1.4-1.5, and the DSOHis in the range of from 0.03-0.1. In one class of this embodiment, the DSPr is in the range of from 1.45-1.5, or in the range of from 1.5-1.53. In one class of this embodiment, the DSAc is in the range of from1.4-1.45. In one embodiment or in combination with any other embodiment of the second aspect, the DSPris in the range of from 1.8-1.9, the DSAcis in the range of from 1-1.1, and the DSOH is in the range of from 0.1-0.2. In one class of this embodiment, the DSPr is in the range of from 1.8-1.85, or in the range of from 1.85-1.9. In one embodiment or in combination with any other embodiment of the second aspect, the DSPr is in the range of from 1.8-2.4, DSAc is in the range of from 0.5-1.0, and DSOHis in the range of from 0.02-0.08. In one class of this embodiment, the DSPr is in the range of from 1.8-2.2, or in the range of from 1.8-2.0, or in the range of from 2.0-2.4, or in the range of from 2.0-2.2, or in the range of from 2.2-2.4. In one embodiment or in combination with any other embodiment of the second aspect, the DSPr is in the range of from 2.8-3.0, the DSAc is in the range of from 0-0.2, and DSOHis in the range of from 0-0.2. In one class of this embodiment, the DSPr is in the range of 2.9-3.0, or in the range of from 2.8-2.9. In one class of this embodiment, the DSAc is in the range of from 0.05-0.2, or in the range of from 0.05-0.1, or in the range of from 0.1 to 0.2. In one embodiment or in combination with any other embodiment of the second aspect, the DSAc is in the range of from 2.8-3.0, and the DSOH is in the range of from 0-0.2. In one class of this embodiment, the DSAc is in the range of from 2.8-2.9, or in the range of from 2.9-3.0. In one embodiment or in combination with any other embodiment of the second aspect, the additives have a melting point that is higher than 15°C. In one embodiment or in combination with any other embodiment of the second aspect, the additive is 1,4-cyclohexanedimethanol (CHDM), neopentyl glycol (NPG), 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD), trimethylolpropane (TMP), xylitol, sorbitol, or combinations thereof. In one embodiment or in combination with any other embodiment of the second aspect, the additive is selected from CHDM, NPG, TMP, or a combination thereof. In one embodiment or in combination with any other embodiment of the second aspect, the additive is present at from 6-25 wt%, or from 6-22wt%, or from 6-20wt%, or from 6-18wt%, or from 6-16wt%, or from 6-14wt%, or from 6-12wt%, or from 6-10wt%, or from 8-25wt%, or from 8-22wt%, or from 8- 20wt%, or from 8-18wt%, or from 8-16wt%, or from 8-14wt%, or from 8- 12wt%, or from 8-10wt%, or from 10-25wt%, or from 10-22wt%, or from 10- 20wt%, or from 10-18wt%, or from 10-16wt%, or from 10-14wt%, or from 10- 12wt%, or 12-25wt%, or from 12-22wt%, or from 12-20wt%, or from 12- 18wt%, or from 12-16wt%, or from 12-14wt%, or from 14-25wt%, or from 14- 22wt%, or from 14-20wt%, or from 14-18wt%, or from 14-16wt%, or from 16- 25wt%, or from 16-22wt%, or from 16-20wt%, or from 16-18wt%, or from 18- 25wt%, or from 18-22wt%, or from 18-20wt%, or from 20-25wt%, or from 20- 22wt%, based on the total weight of the composition. In one embodiment or in combination with any other embodiment of the second aspect, other additives such as stabilizers, antiblocks, slip agents, lubricants, dyes, pigments, retardation modifiers, etc. may be mixed with the cellulose esters. Examples of these other additives are found in US 2009 / 0050842, US 2009 / 0054638, and US 2009 / 0096962; the contents of which are hereby incorporated by reference. In one embodiment or in combination with any other embodiment in the second aspect, the film further comprises a plasticizer that is different that the additive. In one class of this embodiment, the plasticizer is a phosphoric acid- based plasticizer, a phthalic acid ester-based plasticizer, a glycolate based plasticizer, a citric acid ester-based plasticizer, a carbohydrate ester-based plasticizer, and an alditol ester-based plasticizer. Examples of phosphoric acid ester-based plasticizers include but are not limited to triphenyl phosphate (TPP), tricresyl phosphate, cresyl phenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, and tributyl phosphate. Phthalic acid ester-based plasticizers include but are not limited to diethyl phthalate, dimethoxyethyl phthalate, dimethyl phthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethyl hexyl phthalate, butyl benzyl phthalate, di-2-ethylhexyl phthalate, butyl benzyl phthalate, and dibenzyl phthalate. Citric acid ester-based plasticizers include but are not limited to acetyl trimethyl citrate, and acetyl tributyl citrate. Glycolate-based plasticizers include but are not limited to alkyl phthalyl alkyl glycolate, such as methyl phthalyl methyl glycolate, ethyl phthalyl ethyl glycolate (EPEG), propyl phthalyl propyl glycolate, butyl phthalyl butyl glycolate, octyl phthalyl octyl glycolate, methyl phthalyl ethyl glycolate, ethyl phthalyl methyl glycolate, ethyl phthalyl propyl glycolate, propyl phthalyl ethyl glycolate, methyl phthalyl propyl glycolate, methyl phthalyl butyl glycolate, ethyl phthalyl butyl glycolate, butyl phthalyl methyl glycolate, butyl phthalyl ethyl glycolate, propyl phthalyl butyl glycolate, butyl phthalyl propyl glycolate, methyl phthalyl octyl glycolate, ethyl phthalyl octyl glycolate, octyl phthalyl methyl glycolate, and octyl phthalyl ethyl glycolate. Other useful plasticizers include, but are not limited to, butyl oleate, methyl acetyl ricinolate, dibutyl sebacate, and triacetin. Carbohydrate ester-based plasticizers include, but are not limited to, esters of 6-carbon aldose sugars, such as glucose pentapropionate, glucose pentaisobutyrate, and glucose pentatbutyrate; esters of 6-carbon ketose sugars such as fructose pentapropionate, fructose pentaisobutyrate, fructose pentatbutyrate; esters of 5-carbon aldose sugars, such as xylose tetrapropionate, xylose tetraisobutyrate, and xylose tetrabutryate. Alditol ester-based plasticizers include but are not limited to 5- carbon alditol esters, such as xylitol pentapropionate, xylitol pentaisobutryate, and xylitol pentabutyrate; 6-carbon alditol esters, such as mannitol hexapropionate, mannitol hexaisobutyrate, and mannitol hexabutyrate. Other useful plasticizers include triphenyl phosphate, xylitol pentaacetate, trimethyl pentanoyl diisobutyrate, 2-naphthyl benzoate or mixtures thereof. In one embodiment or in combination with any other embodiment in the second aspect, cellulose ester may further comprise other substituents such as butyryl group and other longer-chain groups having more than four carbons. In one embodiment or in combination with any other embodiment in the third aspect, the composition further comprising a solvent. In one class of this embodiment, the composition is a dope. Solvents which are useful for preparation of the dope according to the present invention may be employed without any limitations as long as they are capable of simultaneously dissolving the cellulose ester and any additional additives, such as plasticizers. In certain embodiments of the present invention, the organic solvents comprise halogenated solvents and / or non- halogenated solvents. Examples of halogenated solvents include, but are not limited to, methylene chloride, chloroform, dichloroethane, 2,2,2- trifluoroethanol, 2,2,3,3-hexafluoro-l-propanol, 1,3-difluoro-2-propanol, 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol, 1,1,1,3, 3,3-hexafluoro-2- propanol, 2,2,3,3,3-pentafluoro-lpropanol. Examples of non-halogenated solvents include, but are not limited to, methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, sec-butyl acetate, tert-butyl acetate, amyl acetate, acetone, tetrahydrofuran, toluene, 1,3- dioxolane, 1,4-dioxane, cyclohexanone, ethyl formate, nitroethane, or combinations thereof. The present application also discloses, in a fourth aspect, a pellet comprising any of the compositions disclosed herein. The present application also discloses, in a fifth aspect, a melt comprising any of the compositions disclosed herein. This invention can be further illustrated by the following working examples, although it should be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention. EXPERIMENTAL SECTION Abbreviations DS is average degree of substitution; s is second(s);oC is degree(s) Celsius; MHz is Megahertz; mm is millimeter(s); NMR is nuclear magnetic resonance; g is gram(s); wt% is weight percent; mixt is mixture; DCM is dichloromethane; EtOH is ethanol; MeOH is methanol; h is hour(s); RH is relative humidity; min is minute(s); temp is temperature; Ex is example(s); CE is cellulose ester; CAP is cellulose acetate propionate; CP is cellulose propionate; CHDM-D is 1,4-cyclohexyl dimethanol; TMP is trimethylolpropane; TMCD is 2,2,4,4- tetramethyl-1,3-cyclobutanediol; NPG is neopentyl glycol; Measurement Procedures The DS of the cellulose esters were determined by1H NMR using a JEOL Model 600 NMR spectrometer operated at 600 MHz. The sample tube size was 5 mm, and the sample temperature was 80oC. The pulse delay was 5 s and 64 scans were acquired for each experiment. Solvent Casting Procedure Solvent casting of the films was performed according to the following procedure. First, 24 g of solids (resin + additives) were added to 176 g of a 90 / 10 wt% solvent mixt of DCM / EtOH or MeOH. The additives were 10, 12, and 15 based on total solids, and the g of cellulose ester was calculated accordingly. The mixt was sealed, placed on a roller, and mixed for 24 h to create a uniform dope. After mixing, the dope was cast onto a glass plate using a doctor blade adjusted to give a film of desired thickness, in which the thicknesses ranged from 30µm to 100 µm can be obtained. Casting was conducted in a fume hood with RH controlled around 50%, although this was found to vary slightly depending on when the film was cast. After casting, the film on the glass substrate was allowed to dry for 60 min under a cover pan (to minimize rate of solvent evaporation),15 min with cover pan removed. After this initial drying, the film was peeled from the glass substrate and annealed in a forced air oven for 10 min at 100ºC. After annealing at 100ºC, the film was annealed at a higher temp (either 120, 130, 140, or 150ºC) for 5 to 10 min. Ex 1 Films were prepared with the following particulars using the Solvent Casting Procedure described above. Dope Preparation Total solids: 24 g; CE: CAP-1 (DSAc=0.06; DSPr=1.49; DSOH=0.06); Additive (10wt%): CHDM, TMP, NPG; Total Solvent: 176g. The optical retardation value Rth of each the film with normalized thickness of 80 µm at different annealing temperatures and additive (10wt%) concentration is reported in Table 1. Table 1 Film CE Additive Annealing Annealing Rth[589nm] 80 µm (10wt%) temp (C) time (min) Film Normalization x - Films were prepared with the following particulars using the Solvent Casting Procedure described above. Dope Preparation: Total Solids: 24g; CE: CAP-2 (DSAc=1.03, DSPr=1.84, DSOH=0.13), CAP-3 (DSAc=0.85, DSPr=2.10, DSOH=0.05), CP-4 (DSPr=2.95, DSOH=0.05), Additive (wt%): CHDM, Total Solvents: 176g (DCM:158.4g, MeOH:17.6g); The optical retardation value Rthof each the film with normalized thickness of 80 µm annealing at different temp and additive (10wt%) concentration is reported in Table 2. Table 2 Film CE Additive Annealing Annealing Rth[589nm] 80 µm Film CE Additive Annealing Annealing Rth[589nm] 80 µm (10wt%) temp (C) time (min) Film Normalization Ex 5 Films were prepared with the following particulars using the Solvent Casting Procedure described above. Dope Preparation: Total Solids: 24g; CE: CTA-5 (DSAc=2.88, DSOH=0.12), Additive (10wt%, 12.5wt%, 15wt%, 20wt%): CHDM, TMP, NPG, Total Solvents: 176g (DCM:158.4g, MeOH:17.6g); The optical retardation value Rthof each the film with normalized thickness of 80 µm annealing at different temperature and different additive (10wt%) concentration is reported in Table 3. Table 3. Additive Film Annealing Annealing Rth[589nm] 80 µm (wt%) temp (oC) time (min) Film Normalization Additive Film Annealing Annealing Rth[589nm] 80 µm (wt%) temp (oC) time (min) Film Normalization

Claims

CLAIMS What is claimed is:

1. An optical compensation film comprising: (i) a cellulose ester comprising: (a) a plurality of hydroxyl substituents, (b) a plurality of propionyl substituents, and (c) a plurality of acetyl substituents, wherein: the average degree of substitution for the hydroxyl substituents (“DSOH”) is in the range of from 0 to 0.25, the average degree of substitution for the propionyl substituents (“DSPr”) is in the range of from 0-3.0, the average degree of substation for the acetyl substituents (“DSAc”) is in the range of from 0-3.0, and the sum of the DSPr and the DSAc is at least 2.75; (ii) an additive, wherein the additive is an aliphatic diol, an aliphatic triol, or an aliphatic polyol; wherein: the additive is present at from 5-25wt%, based on the total weight of the optical compensation film, and the optical compensation film has an out-of-plane retardation (“Rth”) measured at 589 nm ranging from 15-60 nm in the thickness direction measured at a film thickness of 80 µm or less.

2. The optical compensation film of claim 1, wherein the DSAc is in the range of from 0-3.

0.

3. The optical compensation film of claim 1, wherein the DSPris in the range of from 1.1-3.3, and the DSAc is in the range of from 0-1.6.

4. The optical compensation film of claim 3, wherein the DSPr is in the range of from 1.45-1.53, the DSAcis in the range of from 1.4-1.5, and the DSOHis in the range of from 0.03-0.

1.

5. The optical compensation film of claim 3, wherein the DSPr is in the range of from 1.8-1.9, the DSAcis in the range of from 1-1.1, and the DSOHis in the range of from 0.1-0.

2.

6. The optical compensation film of claim 3, wherein the DSPr is in the range of from 1.8-2.4, DSAcis in the range of from 0.5-1.0, and DSOHis in the range of from 0.02-0.

08.

7. The optical compensation film of claim 1, wherein the DSPr is in the range of from 2.8-3.0, the DSAcis in the range of from 0-0.2, and DSOHis in the range of from 0-0.

2.

8. The optical compensation film of claim 3, wherein the DSAcis in the range of from 2.8-3.0, and the DSOH is in the range of from 0-0.

2.

9. The optical compensation film of any of claims 1-8, wherein the additive is 1,4-cyclohexanedimethanol (CHDM), neopentyl glycol (NPG), 2,2,4,4- tetramethyl-1,3-cyclobutanediol (TMCD), trimethylolpropane (TMP), xylitol, sorbitol, or combinations thereof.

10. The optical compensation film of any of claims 1-8, wherein the additive is selected from CHDM, NPG, TMP, or a combination thereof.

11. The optical compensation film of any of claims 1-10, wherein the additive is present at from 6-25 wt%, or from 6-22wt%, or from 6-20wt%, or from 6- 18wt%, or from 6-16wt%, or from 6-14wt%, or from 6-12wt%, or from 6- 10wt%, or from 8-25wt%, or from 8-22wt%, or from 8-20wt%, or from 8-18wt%, or from 8-16wt%, or from 8-14wt%, or from 8-12wt%, or from 8- 10wt%, or from 10-25wt%, or from 10-22wt%, or from 10-20wt%, or from 10- 18wt%, or from 10-16wt%, or from 10-14wt%, or from 10-12wt%, or 12- 25wt%, or from 12-22wt%, or from 12-20wt%, or from 12-18wt%, or from 12- 16wt%, or from 12-14wt%, or from 14-25wt%, or from 14-22wt%, or from 14- 20wt%, or from 14-18wt%, or from 14-16wt%, or from 16-25wt%, or from 16- 22wt%, or from 16-20wt%, or from 16-18wt%, or from 18-25wt%, or from 18- 22wt%, or from 18-20wt%, or from 20-25wt%, or from 20-22wt%, based on the total weight of the optical compensation film.

12. The optical compensation film of any one of claims 1-11, wherein the film is a melt extruded film or a solvent cast film.

13. The optical compensation film of any one of claim 1-12, wherein the film is uniaxially stretched, biaxially stretched, or 45 degree stretched.

14. The optical compensation film of any one of claims 1-13, wherein d is in the range of from 5-100 microns.

15. A composition, comprising: (i) a cellulose ester comprising: (a) a plurality of hydroxyl substituents, (b) a plurality of propionyl substituents, and (c) a plurality of acetyl substituents, wherein: the average degree of substitution for the hydroxyl substituents (“DSOH”) is in the range of from 0 to 0.25, the average degree of substitution for the propionyl substituents (“DSPr”) is in the range of from 0-3.0, the average degree of substation for the acetyl substituents (“DSAc”) is in the range of from 0-3.0, andthe sum of the DSPr and the DSAc is at least 2.75; (ii) an additive, wherein the additive is an aliphatic diol, an aliphatic triol, or an aliphatic polyol; wherein: the additive is present at from 5-25wt%, based on the total weight of the optical composition.

16. The composition of claim 15, wherein the DSAcis in the range of from 0- 3.

0.

17. The composition of claim 15, wherein the DSPr is in the range of from 1.1- 3.0, and the DSAcis in the range of from 0-1.

6.

18. The composition of claim 17, wherein the DSPris in the range of from 1.45-1.53, the DSAc is in the range of from 1.4-1.5, and the DSOH is in the range of from 0.03-0.

1.

19. The composition of claim 17, wherein the DSPr is in the range of from 1.8- 1.9, the DSAcis in the range of from 1-1.1, and the DSOHis in the range of from 0.1-0.

2.

20. The composition of claim 17, wherein the DSPr is in the range of from 1.8- 2.4, DSAcis in the range of from 0.5-1.0, and DSOHis in the range of from 0.02-0.

08.

21. The composition of claim 17, wherein the DSPr is in the range of from 2.8- 3.0, the DSAcis in the range of from 0-0.2, and DSOHis in the range of from 0- 0.

2.

22. The composition of claim 17, wherein the DSAc is in the range of from 2.8- 3.0, and the DSOHis in the range of from 0-0.2.

23. The composition of any of claims 15-22, wherein the additive is 1,4- cyclohexanedimethanol (CHDM), neopentyl glycol (NPG), 2,2,4,4-tetramethyl- 1,3-cyclobutanediol (TMCD), trimethylolpropane (TMP), xylitol, sorbitol, or combinations thereof.

24. The composition of any of claims 15-23, wherein the additive is selected from CHDM, NPG, TMP, or a combination thereof.

25. The composition of any of claims 15-24, wherein the additive is present at from 6-25 wt%, or from 6-22wt%, or from 6-20wt%, or from 6-18wt%, or from 6-16wt%, or from 6-14wt%, or from 6-12wt%, or from 6-10wt%, or from 8- 25wt%, or from 8-22wt%, or from 8-20wt%, or from 8-18wt%, or from 8- 16wt%, or from 8-14wt%, or from 8-12wt%, or from 8-10wt%, or from 10- 25wt%, or from 10-22wt%, or from 10-20wt%, or from 10-18wt%, or from 10- 16wt%, or from 10-14wt%, or from 10-12wt%, or 12-25wt%, or from 12- 22wt%, or from 12-20wt%, or from 12-18wt%, or from 12-16wt%, or from 12- 14wt%, or from 14-25wt%, or from 14-22wt%, or from 14-20wt%, or from 14- 18wt%, or from 14-16wt%, or from 16-25wt%, or from 16-22wt%, or from 16- 20wt%, or from 16-18wt%, or from 18-25wt%, or from 18-22wt%, or from 18- 20wt%, or from 20-25wt%, or from 20-22wt%.

26. The composition of any one of claims 15-25, wherein the composition further comprises a solvent, wherein the composition is a dope.

27. A pellet comprising any of the compositions of claims 15-26.

28. A melt comprising the composition of any one of claims 15-26.

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