TWI854170B - Optical film and display device formed therefrom and evaluation method of resistance to polarizer cracking for optical film - Google Patents

Abstract

An embodiment of the present disclosure provides an optical film, including: a polarizer; a first protective film disposed on the first side of the polarizer; and a second protective film disposed on the second side of the polarizer opposite to the first side of the polarizer. At least one of the first protective film or the second protective film has a coefficient of hygroscopic expansion CHE less than 200 (ppm/ΔRH), and the coefficient of hygroscopic expansion CHE satisfies the following formula: CHE = (S _{( RH90%)}-S _(RH25%))∙10 ⁶/(S _(RH25%)∙ΔRH) (Formula 1), wherein CHE: coefficient of hygroscopic expansion (ppm/ΔRH), S _{( RH90%)}: the size of the protective film at a relative humidity of 90%(mm), S _{( RH25%)}: the size of the protective film at a relative humidity of 25%(mm), ΔRH: variance in relative humidity.

Description

Optical film, display device formed therefrom, and method for evaluating the optical film's resistance to polarized photon cracking

The present disclosure relates to an optical film and its application as well as an evaluation method of the optical film's resistance to polarized photon cracking.

Generally, polarizers are made of polyvinyl alcohol polarizers that are stretched and dyed with iodine and bonded between two protective films. Polarizers may have cracks along the stretching direction of the polarizers under harsh conditions of thermal shock (-30℃ and 80℃).

Previous literature has used methods such as increasing the glass transition temperature and elastic coefficient of the protective film and reducing the shrinkage stress of the polarizer to improve the crack resistance of the polarizer. However, the stress caused by the expansion of the protective film itself when it absorbs moisture in high humidity can easily cause the polarizer to crack, and no research has been done on this in the past.

Therefore, although the existing protective films generally meet various requirements, they are not satisfactory in all aspects, so there is still a need for improvement in the problem of polarization cracking.

The embodiment of the present invention provides an optical film, comprising: a polarizer; a first protective film, disposed on a first side of the polarizer; and a second protective film, disposed on a second side of the polarizer opposite to the first side, wherein at least one of the first protective film or the second protective film has a hygroscopic expansion coefficient CHE of less than 200 (ppm/ΔRH), and the hygroscopic expansion coefficient CHE conforms to the following formula: CHE = (S _(RH90%) -S _(RH25%) )∙10 ⁶ /(S _(RH25%) ∙ΔRH) (Formula 1) wherein CHE: hygroscopic expansion coefficient (ppm/ΔRH), S _(RH90%) : size of the protective film at a relative humidity of 90% (mm), S _(RH25%) : size of the protective film at a relative humidity of 25% (mm), ΔRH: The change in relative humidity.

The present invention provides a display device, comprising: a display panel having a first surface and a second surface opposite to each other; a first polarizing structure disposed on the first surface of the display panel; and a second polarizing structure disposed on the second surface of the display panel, wherein at least one of the first polarizing structure and the second polarizing structure comprises the above-mentioned optical film.

The present invention provides an evaluation method for the polarization cracking resistance of an optical film, comprising: measuring the dimensions of a first protective film and a second protective film at 25°C and at relative humidity of 25% and 90% respectively; calculating the hygroscopic expansion coefficient CHE of the first protective film and the second protective film respectively from the dimensions of the first protective film and the second protective film at relative humidity of 25% and 90% respectively, wherein the hygroscopic expansion coefficient CHE conforms to the following formula: CHE = (S _(RH90%) -S _(RH25%) )∙10 ⁶ /(S _(RH25%) ∙ΔRH) (Formula 1) wherein CHE: hygroscopic expansion coefficient (ppm/ΔRH), S _(RH90%) : dimension of the protective film at relative humidity of 90% (mm), S _(RH25%) : The size of the protective film at a relative humidity of 25% (mm), ΔRH: the change in relative humidity; and the hygroscopic expansion coefficient CHE is used as an indicator to evaluate the anti-polarizer cracking characteristics of the optical film, and the first protective film or the second protective film is judged to be a good product when the hygroscopic expansion coefficient CHE of at least one of the first protective film or the second protective film is less than 200 (ppm/ΔRH); and the first protective film and/or the second protective film judged to be a good product are bonded to the polarizer.

In order to make the features of the present disclosure clear and easy to understand, the following is a detailed description of the embodiments with the accompanying drawings. For other matters, please refer to the technical field.

The following disclosure provides a number of embodiments or examples for implementing different elements of the subject matter provided. Specific examples of each element and its configuration are described below to simplify the description of the embodiments of the present invention. Of course, these are merely examples and are not intended to limit the embodiments of the present invention. For example, if the description refers to a first element formed on a second element, it may include an embodiment in which the first and second elements are directly in contact, and it may also include an embodiment in which additional elements are formed between the first and second elements so that they are not directly in contact. In addition, the embodiments of the present invention may use repeated element symbols in various examples. Such repetition is for the purpose of simplicity and clarity, and is not used to indicate the relationship between the different embodiments and/or configurations discussed.

Furthermore, spatially relative terms such as "under", "below", "lower", "above", "higher" and the like may be used to facilitate describing the relationship between one component or feature and another component or feature in the drawings. Spatially relative terms are used to include different orientations of the device in use or operation, as well as the orientations described in the drawings. When the device is rotated 90 degrees or in other orientations, the spatially relative adjectives used will also be interpreted based on the rotated orientation.

In the text, the terms "about", "approximately", and "substantially" generally mean within 5%, preferably within 3%, more preferably within 1%, or within 2%, or within 1%, or within 0.5% of a given value or range. The quantities given here are approximate quantities, that is, in the absence of specific description of "about", "approximately", and "substantially", the meaning of "about", "approximately", and "substantially" can still be implied.

Research has found that when polarizers are subjected to hot and cold shock tests (-30~80℃), the humidity will be high during the low temperature process (for example, the relative humidity is about 90%). In a high humidity environment, the protective film will absorb moisture and expand, and the moisture absorption and expansion will make the protective film unable to suppress the contraction force of polarizers and squeeze the polarizers, causing the polarizers to crack. To solve this problem, a protective film with small dimensional changes under high humidity must be selected. This disclosure provides a method for calculating the dimensional stability of the protective film under high humidity, and provides a combination of materials to improve the crack resistance of polarizers.

The following is a detailed description of an embodiment of a method for evaluating the polarized photon cracking resistance of an optical film.

[Evaluation method for the resistance of optical films to polarized photon cracking]

The present disclosure provides a method for calculating the dimensional stability of a protective film under high humidity, using the hygroscopic expansion coefficient CHE of the protective film as an indicator to evaluate the anti-polarized photon cracking properties of an optical film.

The first protective film and the second protective film are cut into appropriate sizes as samples for expansion size measurement, such as 15 mm in length and 3 mm in width. The material of the protective film is not particularly limited, and the types of materials will be described in detail later. It should be understood that protective films of other sizes can also be used. In a specific embodiment, acetyl cellulose, polymethyl methacrylate, polyethylene terephthalate, or cycloolefin copolymer can be used as the material of the protective film.

Next, the dimensions of the first protective film and the second protective film at 25°C and relative humidity of 25% and 90% are measured, and the hygroscopic expansion coefficients CHE of the first protective film and the second protective film are calculated from the dimensions of the first protective film and the second protective film at relative humidity of 25% and 90%, respectively. The hygroscopic expansion coefficient CHE conforms to the following formula: CHE = (S _(RH90%) -S _(RH25%) )∙10 ⁶ /(S _(RH25%) ∙ΔRH) (Formula 1) Wherein CHE: hygroscopic expansion coefficient (ppm/ΔRH), S _(RH90%) : dimension of the protective film at relative humidity of 90% (mm), S _(RH25%) : dimension of the protective film at relative humidity of 25% (mm), ΔRH: relative humidity change.

The following method can be used to determine whether the optical film made of the protective film causes cracks in the polarizer: First, the protective film can be attached to the polyvinyl alcohol polarizer that has been stretched by, for example, iodine dyeing to obtain an optical film. The composition and preparation of the optical film will be described in detail later. Then, the optical film is cut into appropriate sizes (for example, A4 size) as a sample for measurement. The sample is placed in an oven at -30°C for 30 minutes, and then placed in an oven at 80°C for 30 minutes. After repeating the above actions 100 times, observe whether cracks are generated at the edge of the polarizer. If cracks are generated at the edge of the polarizer, it cannot be used as an optical film and is discarded; if cracks are not generated at the edge of the polarizer, it can be used as an optical film.

Compare the correlation between the CHE of the first and second protective films and whether cracks occur at the edge of the polarizer. The results are shown in Table 1 below. Since the CHE of the protective film is highly correlated with the cracks of the polarizer, the CHE of the first and second protective films can be used as an indicator to evaluate whether the protective film will cause cracks in the polarizer.

As shown in Table 1, the optical film made from the protective film sample with a lower CHE is less likely to cause polarization cracking due to the smaller dimensional change under high humidity. The optical film made from at least one of the first protective film or the second protective film with a CHE of less than 200 (ppm/ΔRH) will not cause cracking due to polarization, and the optical film can be used as an optical film resistant to polarization cracking; while the optical film made from at least one of the first protective film or the second protective film with a CHE of greater than or equal to 200 (ppm/ΔRH) will cause cracking due to polarization, and the optical film cannot be used as an optical film resistant to polarization cracking and needs to be discarded or reworked or recycled.

In addition, the sum of the hygroscopic expansion coefficients CHE of the first and second protective films can also be used as an indicator to evaluate whether the protective film will cause polarized light cracking. If the sum of the hygroscopic expansion coefficients CHE of the first and second protective films is less than 350 (ppm/ΔRH), the optical film produced by polarized light does not produce cracks, and the optical film can be used as an optical film that resists polarized light cracking; and if the sum of the hygroscopic expansion coefficients CHE of the first and second protective films is greater than or equal to 350 (ppm/ΔRH), the optical film will produce cracks by polarized light, and the optical film cannot be used as an optical film that resists polarized light cracking and needs to be discarded or reworked or recycled.

In addition to using the hygroscopic expansion coefficient of the first protective film and the second protective film as an indicator of the optical film's resistance to polarized light cracking, the protective film will squeeze the polarized light due to the hygroscopic expansion of the protective film in a high humidity environment, causing the polarized light to crack. In order to reduce the squeezing of the polarized light by the protective film, a protective film with low hygroscopic expansion force F is required. Therefore, in some embodiments, a hygroscopic expansion force F measurement step may be further added to the optical film's resistance to polarized light cracking evaluation method, and the hygroscopic expansion force F of the protective film may be used as an indicator to evaluate whether the protective film will cause the polarized light to crack. The following will further describe in detail an embodiment of the hygroscopic expansion force F measurement method.

The first protective film and the second protective film are cut into appropriate sizes as samples for elastic modulus measurement, such as 15 mm in length and 3 mm in width. The material of the protective film is not particularly limited, and the types of materials will be described in detail later. It should be understood that protective films of other sizes may also be used. In a specific embodiment, acetyl cellulose, polymethyl methacrylate, polyethylene terephthalate, or cycloolefin copolymer may be used as the material of the protective film.

Then, at 25 The elastic coefficients of the first protective film and the second protective film are measured respectively, and the hygroscopic expansion force F of the first protective film and the second protective film are calculated respectively from the elastic coefficient and the hygroscopic expansion coefficient CHE, wherein the hygroscopic expansion force F conforms to the following formula: F = E∙T∙CHE∙10 ^-6 ∙ΔRH∙R (Formula 2) Wherein F: hygroscopic expansion force (N) E: elastic coefficient (MPa) T: thickness (mm) CHE: hygroscopic expansion coefficient (ppm/ΔRH) ΔRH: relative humidity change R: width (mm).

Compare the correlation between the moisture expansion force F of the first and second protective films and whether cracks are generated at the edge of the polarizer. The results are shown in Table 1 below. Since the moisture expansion force F of the protective film is highly correlated with the cracks of the polarizer, the moisture expansion force F of the first and second protective films can be used as an indicator to evaluate whether the protective film will cause cracks in the polarizer.

From Table 1, it can be seen that the optical film made of the protective film sample with a lower hygroscopic expansion force F is less likely to cause polarized light cracking because of the smaller squeezing force on polarized light. If the optical film made of at least one of the first protective film or the second protective film with a hygroscopic expansion force F less than 5N does not cause cracking by polarized light, the optical film can be used as an optical film resistant to cracking by polarized light; and if the optical film made of at least one of the first protective film or the second protective film with a hygroscopic expansion force F greater than or equal to 5N causes cracking by polarized light, the optical film cannot be used as an optical film resistant to cracking by polarized light and needs to be discarded or reworked or recycled.

In addition, the sum of the hygroscopic expansion force F of the first protective film and the second protective film can also be used as an indicator to evaluate whether the protective film will cause polarized light cracking. If the sum of the hygroscopic expansion force F of the first protective film and the second protective film is less than 8N, the polarized light does not produce cracks, and the optical film can be used as an optical film that resists polarized light cracking; and if the sum of the hygroscopic expansion force F of the first protective film and the second protective film is greater than or equal to 8N, the polarized light produces cracks, and the optical film cannot be used as an optical film that resists polarized light cracking and needs to be discarded or reworked or recycled.

In the prior art, due to the lack of an evaluation method to determine whether the protective film will cause cracking of the polarizer in a high humidity environment, the protective film will absorb moisture and expand in a high humidity environment, causing cracking of the polarizer, resulting in a decrease in the yield of the optical film produced. However, the optical film obtained in this case is laminated with the polarizer only after the protective film has been determined to be a good product by the above-mentioned cracking evaluation method, so the dimensional stability of the protective film in high humidity can be ensured.

The optical film and display device provided in this case are described in detail below. It should be understood that the specific elements and arrangements described below are only for the purpose of simply and clearly describing some embodiments of the present disclosure, and are not intended to limit the scope of the present disclosure.

[Optical film]

According to some embodiments of the present invention, the optical film includes a polarizer, a first protective film disposed on a first side of the polarizer, and a second protective film disposed on a second side of the polarizer opposite to the first side. The above-mentioned first protective film and second protective film respectively have a first hygroscopic expansion coefficient CHE1 and a second hygroscopic expansion coefficient CHE2, wherein at least one of the first hygroscopic expansion coefficient CHE1 and the second hygroscopic expansion coefficient CHE2 has a hygroscopic expansion coefficient CHE less than 200 (ppm/ΔRH), preferably less than 190 (ppm/ΔRH), so that the dimensional change of the hygroscopic expansion of the protective film is smaller, which can improve the polarization cracking problem caused by low-temperature expansion under high humidity, wherein the definition of the hygroscopic expansion coefficient CHE is as described above and will not be elaborated here. In addition, the sum of the first hygroscopic expansion coefficient CHE1 and the second hygroscopic expansion coefficient CHE2 is less than 350 (ppm/ΔRH), preferably less than 340 (ppm/ΔRH), which can further improve the polarization cracking problem caused by hygroscopic expansion.

In some embodiments, the first protective film and the second protective film have a first hygroscopic expansion force F1 and a second hygroscopic expansion force F2, respectively. At least one of the first hygroscopic expansion force F1 and the second hygroscopic expansion force F2 has a hygroscopic expansion force F less than 5N, preferably less than 4.5N, which can reduce the stress caused by the hygroscopic expansion of the protective film and reduce the polarization cracking caused by stress. The definition of the hygroscopic expansion force F is as described above and will not be elaborated here. The derivation of the formula for the above hygroscopic expansion force will be described in detail later. Furthermore, the sum of the first moisture expansion force F1 and the second moisture expansion force F2 is less than 8N, preferably less than 7.8N, which can improve the problem of polarizer cracking by reducing the stress and shear force caused by the moisture expansion of the first protective film and the second protective film to the polarizer located between the two.

In some embodiments, the material of the first protective film and the second protective film may be, for example, a thermoplastic resin having excellent transparency, mechanical strength, thermal stability, moisture barrier properties, etc. The thermoplastic resin may include acetyl cellulose resin (e.g., triacetate cellulose (TAC), diacetate cellulose (DAC)), acrylic resin (e.g., poly(methyl methacrylate), PMMA), polyester resin (e.g., polyethylene terephthalate (PET), polyethylene naphthalate), olefin resin, polycarbonate resin, cycloolefin resin, oriented-polypropylene (OPP), polyethylene (PE), polypropylene (PP), cyclic olefin polymer (COP), cyclic olefin copolymer (COC), polycarbonate (PC), or any combination thereof. In addition, the materials of the first protective film and the second protective film may be, for example, the following thermosetting resins or UV-curing resins: (meth) acrylic acid series, urethane series (e.g., polyurethane (PU)), acrylic urethane series (e.g., polyacrylic urethane), epoxy series (e.g., epoxy resin), polysilicone series (e.g., polysilicone resin), etc. In addition, the first protective film and the second protective film may be further subjected to surface treatment, for example, anti-glare treatment, anti-reflection treatment, hard coating treatment, antistatic treatment, or anti-fouling treatment.

In some embodiments, the thickness of the first protective layer and the thickness of the second protective layer may be independently 5-90 microns, preferably 35-80 microns.

In some embodiments, the material of the polarizer is, for example, an iodine polarizer, which includes a polyvinyl alcohol (PVA) resin film and iodine adsorbed and directed therein/on it. The polyvinyl alcohol film can be made by saponifying polyvinyl acetate resin. The degree of saponification of polyvinyl alcohol resin is generally above about 85 mol%. Examples of polyvinyl acetate resins include monomers of vinyl acetate (i.e., polyvinyl acetate), copolymers of vinyl acetate, and other monomers that can be copolymerized with vinyl acetate. Examples of other monomers that can be copolymerized with vinyl acetate include unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, ethyl acrylate, n-propyl acrylate, methyl methacrylate), olefins (e.g., ethylene, propylene, 1-butene, 2-methylpropylene), vinyl ethers (e.g., ethyl vinyl ether, methyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether), unsaturated sulfonic acids (e.g., vinyl sulfonic acid, sodium vinyl sulfonate), etc. Specific examples of copolymers of the above-mentioned vinyl acetate and other monomers that can be copolymerized therewith include ethylene-vinyl acetate copolymers, etc. The degree of polymerization of polyvinyl alcohol resins is generally about 1000 to 10000, and preferably about 1500 to 5000. The polyvinyl alcohol resins may be modified, for example, polyvinyl formaldehyde, polyvinyl acetaldehyde, polyvinyl butyral, etc. modified with aldehydes may be used.

In some embodiments, the thickness of the polarizer is 5-35 microns, preferably 20-30 microns, such as about 25 microns.

In some embodiments, the optical film may further include an adhesive layer (not shown) located between the first protective film and the polarizer, and between the second protective film and the polarizer. The adhesive layer may include a water-based adhesive, generally using a polyvinyl alcohol resin or a urethane resin as the main component of the water-based adhesive, and in order to improve the adhesion, a crosslinking agent or a curing compound such as an isocyanate compound or an epoxy compound may be added to form a composition.

In some embodiments, when the main component of the water-based adhesive is a polyvinyl alcohol resin, in addition to partially saponified polyvinyl alcohol and completely saponified polyvinyl alcohol, modified polyvinyl alcohol resins such as carboxyl-modified polyvinyl alcohol, acetyl-modified polyvinyl alcohol, hydroxymethyl-modified polyvinyl alcohol, and amino-modified polyvinyl alcohol can also be used. The aqueous solution of such polyvinyl alcohol resin can be used as a water-based adhesive, and the concentration of the polyvinyl alcohol resin in the water-based adhesive is generally 1-10 parts by mass, preferably 1-5 parts by mass, relative to 100 parts by mass of water.

In some embodiments, a water-based adhesive composed of an aqueous solution of a polyvinyl alcohol-based resin may be combined with a curable compound such as a polyvalent aldehyde, a water-soluble epoxy resin, a melamine-based compound, a zirconium oxide-based compound, and a zinc compound to improve adhesion as described above.

[Display Device]

Another embodiment of the present disclosure is a display device, which may include the optical film of any of the above embodiments, but the present disclosure is not limited thereto. According to some embodiments of the present disclosure, the display device may be a display such as a liquid crystal display device, an organic electroluminescent display device, or a plasma display device.

FIG. 1 is a cross-sectional view of a display device according to some embodiments. As shown in FIG. 1, the display device 1 includes a display panel 10, a first polarizing structure 20, and a second polarizing structure 30. The display panel 10 has a first surface 10A and a second surface 10B opposite to each other. The first polarizing structure 20 is disposed on the first surface 10A of the display panel 10, and the second polarizing structure 30 is disposed on the second surface 10B of the display panel 10, wherein at least one of the first polarizing structure 20 and the second polarizing structure 30 includes an optical film according to any of the aforementioned embodiments.

As shown in FIG. 1 , in some embodiments, the first polarizing structure 20 includes a first adhesive layer 220 and an optical film 210. The first adhesive layer 220 can be adhered to the first surface 10A of the display panel 10, and the optical film 210 is disposed on the first adhesive layer 220. In some embodiments, the total thickness of the first adhesive layer 220 and the optical film 210 can be 15 microns to 160 microns, and the preferred thickness range is 60 microns to 110 microns.

Continuing to refer to FIG. 1, in some embodiments, the second polarizing structure 30 includes a second adhesive layer 320 and an optical film 310. The second adhesive layer 320 can be adhered to the second surface 10B of the display panel 10, and the optical film 310 is disposed on the second adhesive layer 320. In some embodiments, the total thickness of the second adhesive layer 320 and the optical film 310 can be 15 micrometers to 160 micrometers, and the preferred thickness range is 60 micrometers to 110 micrometers.

In some embodiments, the first adhesive layer 220 and the second adhesive layer 320 may be, for example, pressure-sensitive adhesive or optical adhesive. The thickness of the first adhesive layer 220 and the thickness of the second adhesive layer 320 may be 5-35 microns, preferably 20-30 microns, for example, about 25 microns.

In some embodiments, the display panel 10 may be a liquid crystal display panel, for example, an In-Plane-Switching (IPS) liquid crystal display panel or a Vertical Alignment (VA) liquid crystal display panel.

In order to make the above and other purposes, features, and advantages of the present disclosure more clearly understood, the following are test results of several embodiments and comparative examples to illustrate the properties of the optical film prepared by applying the present disclosure. However, the following embodiments and comparative examples are only for illustrative purposes and should not be interpreted as limitations on the implementation of the present disclosure.

[Table 1]

In Table 1, acetyl cellulose 1~3, polycycloolefin, and polymethacrylate represent the following compounds respectively: Acetyl cellulose 1: 60±4um acetyl cellulose film supplied by Fuji Electronic Materials (product name: TG60UL) Acetyl cellulose 2: 38±2um acetyl cellulose film supplied by KONICA (product name: KC3XR) Acetyl cellulose 3: 40um acetyl cellulose film supplied by Fuji Electronic Materials (product name: WVBZ4G42) Polycycloolefin: 53um polycycloolefin film supplied by ZEON (product name: e-ZB122) Polymethyl methacrylate: Sumitomo Chemical Co., Ltd. supplies polymethyl methacrylate with a thickness of 60±3 um (product name: W001AU60)

[Production of optical films]

First, the adhesive layer composition is coated on the upper protective film and the lower protective film shown in Table 1 by a coating device to form an adhesive layer. Then, the adhesive layers of the upper protective film and the lower protective film are respectively attached to the two sides of the polarizer after iodine dyeing and stretching, and an optical film can be obtained.

[Expansion size measurement]

The dimensional changes of the upper and lower protective films (length 15 mm, width 3 mm) from 25% to 90% relative humidity were measured three times at 25°C using a thermomechanical analyzer (TMA, HITACHI TMA7100) and the average value was taken.

[Measurement of moisture expansion coefficient]

Substitute the data of the protective film expansion size into the formula to calculate the hygroscopic expansion coefficient of the protective film: CHE = (S _(RH90%) -S _(RH25%) )∙10 ⁶ /(S _(RH25%) ∙ΔRH) (Formula 1) Where CHE: hygroscopic expansion coefficient (ppm/ΔRH), S _(RH90%) : size of the protective film at 90% relative humidity (mm), S _(RH25%) : size of the protective film at 25% relative humidity (mm), ΔRH: change in relative humidity.

[Elastic coefficient measurement]

The elastic coefficient of the protective film was measured three times using DMA (Dynamic Viscoelastic Mechanical Analyzer, PerkinElmer DMA8000) under the following conditions and the average value was taken: Sample size: 15 mm long and 3 mm wide Temperature: 25

[Measurement of moisture expansion]

The elastic coefficient of the protective film is 25 Substitute the thickness and expansion size data below into the formula to calculate the hygroscopic expansion of the protective film: F = E∙T∙CHE∙10 ^-6 ∙ΔRH∙R F: Hygroscopic expansion (N) E: Elastic coefficient (MPa) T: Thickness (mm) CHE: Hygroscopic expansion coefficient (ppm/ΔRH) ΔRH: Relative humidity change R: Width (mm) The formula for the above hygroscopic expansion F is derived as follows: Elastic coefficient (Mpa) = Stress / Strain = By back-calculation, we can get: Hygroscopic expansion force (N) = elastic coefficient ∙ length change rate ∙ cross-sectional area = elastic coefficient ∙ ( ) ∙ Sample thickness ∙ Sample width = elastic modulus ∙ (CHE ∙ ΔRH ∙ 10 ^-6 ) ∙ Sample thickness ∙ Sample width

[Hot and cold shock test]

A4-sized optical film samples were attached to glass with pressure-sensitive adhesive for hot and cold shock tests. The test conditions were to place the sample in an oven at -30°C for 30 minutes, and then place the sample in an oven at 80°C for 30 minutes. After repeating the above actions 100 times, observe whether cracks are generated at the edge of the polarizer.

From the results in Table 1, it can be seen that the hygroscopic expansion coefficients of the upper and lower protective films in Examples 1-4 are all less than 200 (ppm/ΔRH). Since the dimensional change of the protective films is small under high humidity, the polarizer does not generate cracks under high humidity. In contrast, in Comparative Examples 1-5, at least one of the hygroscopic expansion coefficients of the upper and lower protective films is greater than 200 (ppm/ΔRH). Since the dimensional change of the protective films is large under high humidity, the polarizer generates cracks under high humidity.

In addition, it can be seen from the results in Table 1 that the sum of the moisture expansion coefficients of the upper and lower protective films in Examples 1-4 is less than 350 (ppm/ΔRH), and the squeezing of the polarizers is reduced by reducing the dimensional changes of the upper and lower protective films after moisture expansion, so that the polarizers do not crack. In contrast, in Comparative Examples 1-5, since the sum of the moisture expansion coefficients of the upper and lower protective films is greater than 350 (ppm/ΔRH), the dimensional changes of the upper and lower protective films after moisture expansion are greater, squeezing the polarizers located between the two, causing the polarizers to crack.

Furthermore, it can be seen from the results in Table 1 that the moisture expansion of the upper and lower protective films in Examples 1-4 is less than 5N, which can reduce the stress caused by moisture expansion of the protective film at high humidity, so that the polarizer will not crack due to the stress of the protective film at high humidity. In contrast, in Comparative Examples 1-5, since at least one of the moisture expansion of the upper and lower protective films is greater than 5N, the polarizer is subjected to greater stress from moisture expansion of the protective film and cracks.

Furthermore, it can be seen from the results in Table 1 that the total stress of the upper and lower protective films in Examples 1-4 is less than 8N, and the stress and shear force caused to the polarizer between the upper and lower protective films after the upper and lower protective films absorb moisture and expand are reduced, so that the polarizer does not crack. In contrast, in Comparative Examples 1-5, since the total stress of the upper and lower protective films is greater than 8N, the stress and shear force caused to the polarizer between the upper and lower protective films after the upper and lower protective films absorb moisture and expand are caused, causing the polarizer to crack.

The features of several embodiments are summarized above so that those with ordinary knowledge in the art to which the present invention belongs can better understand the viewpoints of the embodiments of the present invention. Those with ordinary knowledge in the art to which the present invention belongs should understand that other processes and structures can be easily designed or modified based on the embodiments of the present invention to achieve the same purpose and/or advantages as the embodiments introduced herein. Those with ordinary knowledge in the art to which the present invention belongs should also understand that such equivalent structures do not deviate from the spirit and scope of the present invention, and various changes, substitutions and replacements can be made without violating the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be defined as the scope of the attached patent application.

1: Display device 10: Display panel 10A: First surface 10B: Second surface 20: First polarizing structure 30: Second polarizing structure 210: Optical film 212: Protective film 214: Polarizer 216: Protective film 220: Adhesive layer 310: Optical film 312: Protective film 314: Polarizer 316: Protective film 320: Adhesive layer

The following will be described in detail with the accompanying drawings. It should be noted that, in accordance with standard practice in the industry, various features are not drawn to scale. In fact, the size of the components can be arbitrarily enlarged or reduced to clearly show the features of the present disclosure. Figure 1 is a schematic diagram of a display device according to some embodiments of the present disclosure.

1: Display device

10: Display panel

10A: First surface

10B: Second surface

20: First polarization structure

30: Second polarization structure

210: Optical film

212: Protective film

214: Polarized photons

216: Protective film

220: Adhesive layer

310: Optical film

312: Protective film

314: Polarized photons

316: Protective film

320: Adhesive layer

Claims (11)

An optical film includes: a polarizer; a first protective film disposed on a first side of the polarizer; and a second protective film disposed on a second side of the polarizer opposite to the first side, wherein at least one of the first protective film and the second protective film has a hygroscopic expansion coefficient CHE of greater than 33 (ppm/△RH) and less than 200 (ppm/△RH), and the hygroscopic expansion coefficient CHE meets the following formula: CHE=(S _(RH90%) -S ( _RH25%) ). 10 ⁶ /(S _(RH25%) ．△RH) (Formula 1) wherein CHE: hygroscopic expansion coefficient (ppm/△RH), S _(RH90%) : size of the protective film at a relative humidity of 90% (mm), S( _RH25%) : size of the protective film at a relative humidity of 25% (mm), △RH: relative humidity variation, wherein at least one of the first protective film and the second protective film has a hygroscopic expansion force F less than 5N, wherein the sum of the hygroscopic expansion forces F of the first protective film and the second protective film is less than 8N, wherein the hygroscopic expansion force F meets the following formula: F=E． T． CHE． 10 ^-6 ． △RH． R (Formula 2) Where F: Hygroscopic expansion force (N), E: elastic coefficient (MPa), T: thickness (mm), CHE: Hygroscopic expansion coefficient (ppm/△RH), △RH: relative humidity change, R: width (mm). The optical film as described in claim 1, wherein the first protective film has a first hygroscopic expansion coefficient CHE1; and the second protective film has a second hygroscopic expansion coefficient CHE2, and the sum of the first hygroscopic expansion coefficient CHE1 and the second hygroscopic expansion coefficient CHE2 is less than 350 (ppm/△RH). The optical film as described in claim 1, wherein the hygroscopic expansion coefficient CHE is less than or equal to 190 (ppm/△RH). An optical film as described in claim 2, wherein the sum of the first hygroscopic expansion coefficient CHE1 and the second hygroscopic expansion coefficient CHE2 is less than 340 (ppm/△RH). The optical film as described in claim 1, wherein the moisture expansion force F is less than 4.5N. The optical film as described in claim 1, wherein the sum of the moisture absorption expansion force F of the first protective film and the second protective film is less than 7.8N. The optical film as described in claim 1, wherein the first protective film or the second protective film comprises acetyl cellulose, polymethyl methacrylate, polyethylene terephthalate, polyethylene naphthalate, polycarbonate resin, polyethylene, polypropylene, polyurethane, polyacrylic urethane, epoxy resin, polysilicone resin, or any combination thereof. A display device includes: a display panel having a first surface and a second surface opposite to each other; a first polarizing structure disposed on the first surface of the display panel; and a second polarizing structure disposed on the second surface of the display panel, wherein at least one of the first polarizing structure and the second polarizing structure includes an optical film as described in any one of claims 1-7. A display device as described in claim 8, wherein the optical film is bonded to the display panel with an adhesive layer. A method for evaluating the resistance of an optical film to polarized photon cracking comprises: measuring the dimensions of a first protective film and a second protective film at 25°C and relative humidity of 25% and 90% respectively; calculating the hygroscopic expansion coefficient CHE of the first protective film and the second protective film respectively from the dimensions of the first protective film and the second protective film at relative humidity of 25% and 90% respectively, wherein the hygroscopic expansion coefficient CHE conforms to the following formula: CHE=(S _(RH90%) -S _(RH25%) ). 10 ⁶ /(S _(RH25%) ．△RH) (Formula 1) wherein CHE: hygroscopic expansion coefficient (ppm/△RH), S _(RH90%) : size of the protective film at a relative humidity of 90% (mm), S _(RH25%) : size of the protective film at a relative humidity of 25% (mm), △RH: relative humidity change; the hygroscopic expansion coefficient CHE is used as an indicator to evaluate the anti-polarized photon cracking property of the optical film; the elastic coefficients of the first protective film and the second protective film are measured respectively; the hygroscopic expansion force F of the first protective film and the second protective film are calculated respectively from the elastic coefficient and the hygroscopic expansion coefficient, wherein the hygroscopic expansion force F meets the following formula: F=E． T． CHE． 10 ^-6 ． △RH．R (Formula 2) Wherein F: hygroscopic expansion force (N), E: elastic coefficient (MPa), T: thickness (mm), CHE: hygroscopic expansion coefficient (ppm/△RH), △RH: relative humidity change, R: width (mm); and the hygroscopic expansion force F is used as an indicator to evaluate the anti-polarized photon cracking properties of the optical film, wherein when the hygroscopic expansion force of at least one of the first protective film and the second protective film When the coefficient of expansion CHE is greater than 33 (ppm/△RH) and less than 200 (ppm/△RH), and the hygroscopic expansion force F of at least one of the first protective film and the second protective film is less than 5N, and the sum of the hygroscopic expansion forces F of the first protective film and the second protective film is less than 8N, it is judged as a good product; and the first protective film and/or the second protective film judged as a good product are bonded to a polarizer. The method for evaluating the polarized photon crack resistance of an optical film as described in claim 10, wherein the first protective film and the second protective film are judged as good products when the sum of the hygroscopic expansion coefficient CHE of the first protective film and the second protective film is less than 350 (ppm/△RH).

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