JP7438869B2 - Display devices, polyimide films for display devices, polyimide films and polyimide precursors - Google Patents

Description

The present invention relates to a polyimide film used as a support base material for forming a display device, and a display device using the same.

Organic EL devices used for various display applications, including large displays such as televisions and small displays such as mobile phones, personal computers, and smartphones, generally consist of thin film transistors (hereinafter referred to as TFTs) on a glass substrate, which is a supporting base material. It is made by forming an electrode, a light-emitting layer, and an electrode in sequence thereon, and then hermetically sealing these with a glass substrate, a multilayer thin film, or the like. The structure of organic EL devices includes a bottom emission structure that extracts light from the side of the glass substrate that is the support base material, and a top emission structure that extracts light from the side opposite to the glass substrate that is the support base material, which can be used depending on the application. It is being Further, since it is possible to have a structure that allows outside light to pass through as is, transparent structures have also been proposed through which electronic elements such as TFTs can be seen from the outside. Both can be realized by selecting transparent electrodes and substrate materials.

In addition, by replacing the conventional glass substrate with a resin as the supporting base material of such an organic EL device, it can be made thinner, lighter, and more flexible, and the applications of the organic EL device can be further expanded. However, since resins are generally inferior to glass in dimensional stability, transparency, heat resistance, moisture resistance, film strength, etc., various studies have been made.

For example, Patent Document 1 relates to an invention related to polyimide useful as a plastic substrate for flexible displays and its precursor, and relates to a polyimide obtained by casting a polyimide precursor solution with a specific structure onto an inorganic substrate, drying it, and imidizing it. It discloses a laminate consisting of a film and an inorganic substrate, and reports that it has high light transmittance and little outgassing. However, the coefficient of thermal expansion (CTE) of the polyimide obtained here exceeds 40 ppm/K, so there is a large difference between the coefficient of thermal expansion and that of the glass substrate, so the organic EL substrate will warp, and after device formation. , peeling and cracking occur, making it difficult to obtain an organic EL device with excellent shape stability.

Further, Patent Document 2 relates to an invention related to a polyimide precursor resin composition for forming flexible device substrates such as display devices and light receiving devices manufactured by peeling from a carrier substrate, and has a glass transition temperature of 300° C. or higher and 20 ppm/K. It is described that it exhibits the following thermal expansion coefficient. However, there is a problem that the heat treatment takes a long time of one hour or more, and the productivity is low.

Furthermore, Patent Document 3 has a glass film with a thickness of 20 to 200 μm and a polyimide resin layer, and the polyimide resin layer has a thermal expansion coefficient of 10 ppm/K or less and a light transmittance of 80% at a wavelength of 500 nm. It is described that the above transparent flexible laminate is provided. This transparent flexible laminate has excellent heat resistance and gas barrier properties, and can be suitably used for flexible substrates in display devices and transparent substrates in solar cells. However, in organic EL flexible display devices and organic EL lighting display devices, a resin substrate is formed on a support substrate, a display element is further mounted on the resin substrate, and then the resin substrate is peeled off together with the display element. Since the resin substrate requires particularly strong strength in the case of a manufacturing process, further improvement is considered to be necessary in order to apply it to these uses.

In addition to the above, attempts have been made to reduce weight by using flexible resins for the support base material. For example, in Patent Document 4, polyimide with high transparency and excellent low coefficient of thermal expansion is used as the support base material. An organic EL device has been proposed. However, the polyimide films described in these documents still have a large coefficient of thermal expansion and produce a large amount of outgassing, so there is concern about device contamination during the process.

By the way, organic EL devices have low resistance to moisture, and moisture deteriorates the characteristics of the EL element, which is a light emitting layer. Therefore, when using a resin as the supporting base material, a resin with a low moisture absorption rate is preferred in order to prevent moisture and oxygen from entering the organic EL device. Generally, inorganic materials such as silicon oxide and silicon nitride are used as organic EL substrates, and the coefficient of thermal expansion (CTE) of these materials is usually 0 to 10 ppm/K. On the other hand, transparent polyimide generally has a CTE of about 60 ppm/K, so if transparent polyimide is simply applied to the supporting base material of an organic EL device, it may warp, crack, or peel due to thermal stress. Problems such as this may occur.

Furthermore, the formation of TFTs required for display applications generally requires an annealing process that reaches approximately 400°C. This was not a particular problem when a glass substrate was used as the support base material, but when a resin was used as the support base material, it was necessary to have heat resistance and dimensional stability against the TFT heat treatment temperature. Become. Incidentally, organic EL devices for lighting may not require TFTs, but by increasing the film formation temperature of the transparent electrode adjacent to the supporting base material, the resistance value of the transparent electrode can be lowered and the power consumption of the organic EL device can be reduced. Therefore, support substrates are similarly required to have heat resistance in the case of lighting applications. Metal oxides such as ITO are generally used as such transparent electrodes, and their CTE is 0 to 10 ppm/K, so in order to avoid cracking and peeling problems, the same level of CTE is required. A resin with a CTE of .

In addition, in order to perform color display with an organic EL display device, materials that can emit the three primary colors of red (R), green (G), and blue (B) are deposited for each color using a shadow mask. However, this method has the problem that producing a shadow mask is extremely difficult and expensive. Furthermore, in manufacturing a shadow mask, it is difficult to increase the definition and size. To address these issues, an organic EL display device that displays color by combining a white-emitting organic EL with a color filter has been proposed, but in order to form the color filter, heat treatment of the resist that reaches temperatures of 230°C or higher is required. Generally necessary, and in particular, heat treatment at 300° C. or higher is considered preferable in order to reduce outgassing from the resist. When such high-temperature heat treatment is performed, if the thermal expansion coefficients and humidity expansion coefficients of the TFT substrate and color filter substrate are mismatched, changes in temperature and humidity will cause differences in the dimensional changes of each substrate. This causes problems such as warping of the display device and peeling between the substrates. Therefore, when using a resin as a support base material for a color filter, it is necessary to have heat resistance at the heat treatment temperature of the resist and dimensional stability equivalent to that of a TFT substrate. These problems can be solved by using the same material for the supporting base material of the TFT and the supporting base material of the color filter.

Furthermore, since polyimide films are generally colored yellow-brown, there is a problem in that if minute foreign matter is mixed into the polyimide film, it is difficult to detect it with the naked eye or with a visual inspection device. In particular, it is extremely difficult to detect foreign substances such as rust on metals that are similar in color to polyimide. The presence of foreign matter in a polyimide film causes defects in the gas barrier layer formed on its surface, short circuits between electrodes, and other causes of defects. In this regard, by using a transparent polyimide film, it is easier to find foreign substances and contribute to preventing a decrease in yield, so electronic paper that does not require transparency in the supporting base material for the display device function. It is also useful to use a transparent polyimide film as a supporting base material in display devices such as the above. Incidentally, the transmittance of glass in the visible light region is generally about 90%, and when using resin as a support base material, it is necessary to make it as close to this as possible. Since the wavelength of light emitted from the light emitting layer of organic EL is mainly from 440 nm to 780 nm, the supporting base material used in organic EL devices should have an average transmittance of at least 80% in this wavelength range. Desired. In addition, it is desirable that the resin itself forming the supporting base material also has moisture resistance.

In addition, when peeling a film from an inorganic substrate, it is necessary for the film to have mechanical properties such as mechanical strength and elongation above a certain level. In particular, if the tear propagation resistance is low, the film will break when peeled. There's a problem. Therefore, films used as supporting base materials are required to have high tear propagation resistance.

In this regard, various methods have been proposed for peeling off the film from the inorganic substrate (Patent Documents 5 and 6), but many of them involve complicated processes and are costly, and the point of suppressing film breakage has not been focused on. It has not been. Further, Patent Document 7 discloses a display device including a gas barrier layer on a support base material made of a polyimide film, and the polyimide film has a transmittance of 80% or more in a wavelength range from 440 nm to 780 nm, and a thermal expansion coefficient. Discloses a display device characterized in that the thermal expansion coefficient is 15 ppm/K or less, and the difference in thermal expansion coefficient between the gas barrier layer and the gas barrier layer is 10 ppm/K or less. It can be made thin, lightweight, and flexible, has no problems with cracking or peeling due to thermal stress, has excellent dimensional stability, and prevents defects in the manufacturing process, resulting in a long-life and good-quality device. It is something that can show characteristics. However, in the manufacturing process of display devices, for example, no attention has been paid to suppressing the breakage of the polyimide film when peeling it off from the inorganic substrate, and no further attention has been paid to improving the yield of the manufacturing process. Further improvements are desired.

Considering the above points, when replacing a glass substrate with a resin substrate as a support base material for a display device, it is necessary to simultaneously satisfy at least low CTE, heat resistance, transparency, and high tear propagation resistance. There was no resin substrate that could satisfy all of these requirements.

Japanese Patent Application Publication No. 2012-40836 Japanese Patent Application Publication No. 2010-202729 Japanese Patent Application Publication No. 2013-163304 WO2013/146460 pamphlet Japanese Patent Application Publication No. 2011-65173 Patent Application No. 2011-142168 WO2013/191180 pamphlet

Therefore, the present inventors conducted intensive research to solve the above problem, and surprisingly, they used a polyimide film made of polyimide containing predetermined repeating structural units. It was discovered that by specifying the ratio, a polyimide film that can simultaneously satisfy low CTE, heat resistance, transparency, and high tear propagation resistance, and a display device equipped with the same, could be obtained, and the present invention was completed.

Therefore, an object of the present invention is to have a material that has low CTE, heat resistance, low moisture absorption, strength, and transparency and is suitable as a support base material for forming display devices such as organic EL displays, organic EL lighting, electronic paper, and touch panels. Our objective is to provide polyimide films.

Another object of the present invention is to provide a display device using the above polyimide film as a supporting base material. In the display device of the present invention, since the resin substrate is not easily damaged during the manufacturing process, it is not easily torn during the device formation process and the subsequent process of peeling from the inorganic substrate, resulting in high productivity.

That is, the present invention forms a polyimide film by casting a polyimide precursor or a solution of polyimide onto an inorganic substrate and performing heat treatment, further mounting a display element on the polyimide film, and then distributing the polyimide from the inorganic substrate. A display device obtained by peeling the polyimide film together with the display element, wherein the polyimide film has a glass transition temperature of 300°C or higher, a 5% thermal decomposition temperature of 530°C or higher, and a total light transmission in a film thickness of 30 μm or less. The present invention relates to a display device having a coefficient of thermal expansion of 80% or more, a coefficient of thermal expansion of 40 ppm/K or less, and a tear propagation resistance of 1.3 mN/μm or more.

Further, in the display device, the polyimide component constituting the polyimide film preferably consists of structural units represented by the following formulas (1) and (2).

当該式（１）及び（２）において、Ｘは単結合または－Ｃ－もしくは－Ｏ－を含む置換基である。ただし、ｍ／（ｍ＋ｎ）＝０．１１～０．４０ である。

In the formulas (1) and (2), X is a single bond or a substituent containing -C- or -O-. However, m/(m+n)=0.11 to 0.40.

Further, in the display device, it is preferable that the X is a single bond or a divalent substituent represented by the following formula (3), (4), or (5).

Further, in the display device, it is preferable that the polyimide film has a moisture absorption rate of 0.8 wt% or less.

Further, it is preferable that the display device is a touch panel.

Further, the present invention provides a method in which a polyimide film is formed by casting a polyimide precursor or a solution of polyimide onto an inorganic substrate and performing heat treatment, and further, a display element is mounted on the polyimide film, and a display element is mounted on the polyimide film. A polyimide film for a display device obtained by peeling the polyimide film together with the display element, the polyimide film having a glass transition temperature of 300°C or higher, a 5% thermal decomposition temperature of 530°C or higher, and a film thickness of 30 μm or less. The present invention relates to a polyimide film for display devices, which has a total light transmittance of 80% or more, a thermal expansion coefficient of 40 ppm/K or less, and a tear propagation resistance of 1.3 mN/μm or more.

ここで、当該式（１）及び（２）において、Ｘは単結合または下記式（４）で表される置換基である。ただし、ｍ／（ｍ＋ｎ）＝０．１１～０．４０である。

Furthermore, the present invention relates to a polyimide and a polyimide precursor having a weight average molecular weight of 100,000 to 400,000 and comprising structural units represented by the following formulas (1) and (2).

Here, in the formulas (1) and (2), X is a single bond or a substituent represented by the following formula (4). However, m/(m+n)=0.11 to 0.40.

The polyimide film of the present invention has low thermal expansion, high transmittance in the visible region, excellent transparency, excellent dimensional stability, high heat resistance, low moisture absorption, and high strength. high and highly productive. Therefore, it can be suitably used as a support base material for display devices such as organic EL displays, organic EL lighting, electronic paper, and touch panels.
Furthermore, the display device of the present invention has high productivity because the resin substrate is less likely to be damaged during the manufacturing process.

The present invention will be further explained below.
In the display device of the present invention, a polyimide film is formed by casting a polyimide precursor or a solution of polyimide onto an inorganic substrate and performing heat treatment, and further, a display element is mounted on the polyimide film, and a polyimide film is formed on the inorganic substrate. A display device obtained by peeling off the polyimide film together with the display element, wherein the polyimide film has a glass transition temperature of 300°C or higher, a 5% thermal decomposition temperature of 530°C or higher, and a total light beam of 30 μm or less in film thickness. The transmittance is 80% or more, the thermal expansion coefficient is 40 ppm/K or less, and the tear propagation resistance is 2.0 mN/μm or more.

Polyimide film is produced by polymerizing the raw materials diamine and acid anhydride in the presence of a solvent to form a polyimide precursor solution, then casting this onto an inorganic substrate and imidizing it by heat treatment. I can do it. Alternatively, it can be manufactured by casting a polyimide solution onto an inorganic substrate and imidizing it by heat treatment.
The molecular weight of the polyimide film can be controlled mainly by changing the molar ratio of the raw materials diamine and acid anhydride, and usually the molar ratio is 1:1. It can be adjusted from 0.985 to 1.025 as needed. The range of molecular weight Mw (weight average molecular weight) is preferably 80,000 or more. Further, it is more preferable to adjust it to a range of 80,000 to 400,000. More preferably, it is adjusted to a range of more than 100,000 and less than 400,000, even more preferably from 120,000 to less than 400,000. Here, the molecular weight Mw is a polystyrene-equivalent molecular weight measured and calculated by GPC (Gel Permeation Chromatography).

The solution of the polyimide precursor is prepared by first dissolving a diamine in an organic solvent, and then adding an acid dianhydride to the solution to produce a polyamic acid, which is a polyimide precursor. Examples of organic solvents include dimethylacetamide, dimethylformamide, n-methylpyrrolidinone, 2-butanone, diglyme, xylene, butyrolactone, triethylene glycol dimethyl ether, etc., and these may be used alone or in combination of two or more. can.

When the obtained polyimide precursor solution is cast onto an inorganic substrate, the viscosity of the solution is preferably in the range of 500 to 70,000 cps by adjusting the concentration and molecular weight of the polyimide precursor. More preferably 2,000 to 20,000 cps.

Alternatively, the casting may be performed after appropriately surface-treating the surface of the substrate or base material, which will be the surface to which the resin solution is applied. In the above, it is appropriate that the drying conditions are 150° C. or lower for 2 to 10 minutes, and the heat treatment for imidization is performed at a temperature of about 130 to 360° C. for about 2 to 60 minutes. As the casting method, a known method can be used, but preferably a spin coating method or a coater method is used.

The subsequent step of imidization by heat treatment can be carried out not only by thermal imidization by thermal dehydration but also by chemical imidization by adding a dehydrating agent and a catalyst to the polyimide precursor and causing the mixture to react. In the case of thermal imidization, the heating temperature is preferably 90 to 360°C. On the other hand, in the case of chemical imidization, the heating temperature is preferably 50 to 250°C. Note that when a polyimide solution is cast onto an inorganic substrate and a polyimide film is formed by heat treatment, the temperature may be sufficient to volatilize the solvent in the solution, and is preferably 100 to 250°C.

As the inorganic substrate, any known substrate can be used without limitation, but glass, SUS, and aluminum are preferable, and glass is more preferable because of its smoothness, high heat resistance, and low dimensional change rate.

By the above heat treatment, a polyimide film having a total light transmittance (in the present invention, means transmittance in a wavelength range of 380 nm to 780 nm) of 80% or more at a film thickness of 30 μm or less can be obtained on an inorganic substrate. In particular, the heating time (hereinafter referred to as high-temperature holding time) in the high-temperature heating temperature range from a temperature 20°C lower than the maximum heating temperature (maximum achieved temperature) to the maximum achieved temperature during temperature increase in the above heat treatment (hereinafter referred to as high-temperature holding time) is within 60 minutes. It is preferable to do so. If this high temperature holding time exceeds 60 minutes, the production efficiency of the process may deteriorate and the transparency of the polyimide film may decrease due to coloring or the like. In order to maintain transparency, the shorter the high temperature holding time, the better; however, if the holding time is too short, the effect of heat treatment may not be sufficiently obtained. The optimum high temperature holding time varies depending on the heating method, heat capacity of the base material, thickness of the polyimide film, etc., but is preferably 0.5 minutes or more and 120 minutes or less. More preferably, the time is 0.5 minutes or more and 60 minutes or less.

In addition, the polyimide film obtained by the above heat treatment has a total light transmittance of 80% or more at a film thickness of 30 μm or less, a glass transition temperature of 300°C or more, a 5% thermal decomposition temperature of 530°C or more, and a thermal expansion. The coefficient is 40 ppm/K or less and the tear propagation resistance is 1.3 mN/μm or more.
When the total light transmittance is less than 80%, when an organic EL element is used as a display element, the light emitted from the organic EL light emitting layer (wavelength is mainly from 380 nm to 780 nm) is sufficiently transmitted through the polyimide film. do not. Therefore, for example, in the case of a bottom emission structure, light emission from the light emitting layer cannot be sufficiently extracted. More preferably, the total light transmittance is 85% or more. Further, when a transparent conductive film for a touch panel is used as a display element, the total light transmittance is 85% or more to ensure sufficient visibility.
Further, the thickness of the polyimide film is not limited as long as it has a total light transmittance of 80% or more at a thickness of 30 μm or less, but it is preferably 5 to 30 μm, more preferably 10 to 20 μm.

Further, the glass transition temperature of the polyimide film is 300°C or higher. Preferably it is 330°C or higher, more preferably 350°C or higher. If the glass transition temperature is less than 300° C., the polyimide film may be deformed by heat during mounting of the display element.

Further, the 5% thermal decomposition temperature (Td5%) of the polyimide film is 530°C or higher. If the temperature is lower than 530° C., there is a risk that the polyimide film may partially decompose due to the heat generated when display elements such as TFTs and transparent conductive films are mounted. More preferably, the weight loss rate at 530°C is 3% or less.

Further, as described above, the thermal expansion coefficient of the polyimide film is 40 ppm/K or less, preferably between -10 ppm/K and 40 ppm/K. -If it is less than 10 ppm/K, or if it exceeds 40 ppm/K, problems such as warping, cracking, or peeling of the display device will occur due to thermal stress when the display element is mounted. Sometimes. More preferably it is 0 ppm/K to 30 ppm/K.

Further, the tear propagation resistance of the polyimide film is 1.3 mN/μm or more. If it is less than 1.3 mN/μm, there is a risk that the polyimide film will break, for example, in the process of mounting a display element on the polyimide film and peeling off the polyimide film from the inorganic substrate. A more preferable range is 1.5 mN/μm or more. A more preferable range is 2.0 mN/μm or more.

Furthermore, in organic EL devices and touch panel devices, it is preferable that the supporting base material has low hygroscopicity in order to prevent moisture from being adsorbed into the device. Therefore, it is preferable that the polyimide film has a moisture absorption rate of 0.8 wt% or less. More preferably, it is 0.6 wt% or less. Further, when the moisture absorption rate is 0.8 wt % or less, no swelling or foaming occurs when mounting a TFT or laminating a transparent conductive film, and the reliability as a display device is improved.

Further, in a polyimide film that satisfies the above-mentioned performance, it is preferable that the polyimide component constituting the polyimide film consists of structural units represented by the above formulas (1) and (2).

In the above formula (2), X is a single bond or a substituent containing -C- or -O-. However, m/(m+n)=0.11 to 0.40. More preferably 0.12 to 0.40, still more preferably 0.15 to 0.40.
Here, examples of the substituent containing -C- include -CO-, -C(CF ₃ ) ₂ -, and -C(CH ₃ ) ₂ -. More preferred is a substituent represented by the above formula (3) or (5).
Furthermore, substituents containing -O- are, for example, -O-Ph-O-, -O-Ph-Ph-O-, -O-, -O-Ph-C(CF ₃ ) ₂ -Ph-O - is mentioned. Here, Ph is a phenylene group. More preferred is a substituent represented by the above formula (3) or (4).

Here, the structural unit of formula (1) above mainly improves properties such as low thermal expansion and high heat resistance, and the structural unit of formula (2) above mainly improves properties such as high transparency and tear propagation resistance. It is effective for Therefore, if m/(m+n) is smaller than 0.11, the coefficient of thermal expansion will be large, the glass transition temperature will be low, and the material will tend to be unable to withstand the process of mounting a TFT. More preferably, it is 0.20 or more. On the other hand, when it exceeds 0.40, tear propagation becomes low, and the polyimide film tends to be easily broken, for example, in the process of mounting a display element on a polyimide film and peeling off the polyimide film from an inorganic substrate. Furthermore, transparency tends to decrease.

In order to obtain a high molecular weight resin, in the present invention, the molar ratio of acid anhydride and diamine is preferably adjusted in the range of 0.985 to 1.025, more preferably in the range of 1.000 to 1.020. It is. More preferred is 1.002 to 1.015. At other molar ratios, the molecular weight will be low and the tear propagation resistance will be low.

Furthermore, inorganic fine particles such as silica, alumina, boron nitride, and aluminum nitride may be added to the polyimide film for the purpose of improving slipperiness and thermal conductivity.

Further, the polyimide film may be made of multiple layers of polyimide. In the case of a single layer, it is preferable to have a thickness of 3 μm to 50 μm. On the other hand, in the case of multiple layers, it is sufficient if the main polyimide layer is a polyimide film having the above-mentioned thickness. Here, the main polyimide layer refers to a polyimide layer having the largest thickness among multiple polyimide layers, and preferably has a thickness of 3 μm to 50 μm, more preferably 4 μm to 30 μm. be.

After the heat treatment is completed, a display element is mounted on the polyimide film. Here, the type of display element is not particularly limited, but includes display devices such as liquid crystal display devices, organic EL display devices, electronic paper, and component parts of display devices such as color filters. In addition, the display devices include organic EL lighting devices, touch panel devices, conductive films laminated with ITO, etc., gas barrier films that prevent penetration of moisture, oxygen, etc., components of flexible circuit boards, etc. The various functional devices used are also included. That is, the display element referred to in the present invention includes not only component parts such as a liquid crystal display device, an organic EL display device, and a color filter, but also an organic EL lighting device, a touch panel device, an electrode layer or a light emitting layer of an organic EL display device, It also includes one or a combination of two or more of gas barrier films, adhesive films, thin film transistors (TFTs), wiring layers of liquid crystal display devices, transparent conductive layers, and the like.

In addition, the method for forming a display element includes forming a display element (for example, in the case of an organic EL display device, a barrier layer, TFT, ITO, an organic EL light emitting layer, and a color filter layer), and in the case of a touch panel, a transparent conductive layer. Formation conditions are set as appropriate depending on the electrode layer such as a film, metal mesh, etc.), but in general, after forming a film of metal etc. on a polyimide film, it is formed into a predetermined shape as necessary. It can be obtained using known methods such as patterning or heat treatment. That is, the means for forming these display elements is not particularly limited, and may be appropriately selected, such as sputtering, vapor deposition, CVD, printing, exposure, or dipping, and if necessary, may be formed in a vacuum chamber, etc. These processes may be performed in the following manner. The inorganic substrate and the polyimide film may be separated immediately after forming the display element through various processes, or may be integrated with the inorganic substrate for a certain period of time, for example, immediately before being used as a display device. It may be separated and removed separately.

After mounting the display element, the polyimide film is peeled off from the inorganic substrate together with the display element. When a polyimide film is stretched when peeled from an inorganic substrate, retardation increases. For this reason, it is preferable to use a method of peeling such that the stress applied to the polyimide film during peeling is reduced.

In order to prevent stretching of the polyimide film, after forming another layer on the inorganic substrate, then forming polyimide on it, and mounting the display element thereon, the polyimide film is attached to the other layer and the display element. Preferred is a method in which the entire layer is peeled off and the stress necessary for peeling is distributed to the other layer. This is particularly effective when the polyimide film is thin. In this case, the polyimide film including the other layers is considered to be the polyimide film of the present invention. Examples of methods for forming other layers include laminating a resin film or metal foil with an adhesive, coating, vapor deposition, and the like.

Furthermore, other known methods can also be applied to facilitate the peeling of the polyimide film from the base material and prevent stretching. For example, in Japanese Patent Publication No. 2007-512568, a yellow film of polyimide or the like is formed on glass, a thin film electronic element is formed on this yellow film, and then UV laser light is irradiated through the glass to the bottom of the yellow film. , discloses that it is possible to peel a yellow film from glass. According to this method, since the polyimide film is separated from the glass by UV laser light, no stress is generated during peeling, and this method is one of the preferred peeling processes of the present invention. However, it is also disclosed that unlike the yellow film, the transparent plastic does not absorb UV laser light, so an absorbing/release layer such as amorphous silicon must be provided beforehand under the film.

Furthermore, Japanese Patent Publication No. 2012-511173 discloses that a laser within the spectrum range of 300 to 410 nm is used to peel a polyimide film from glass by irradiation with UV laser light. Incidentally, the peeling method is to irradiate a laser from the glass side to separate the resin base material with the display part from the glass, or to form a peeling layer by applying it to the glass substrate, and then applying polyimide on top of the peeling layer. After applying a resin and completing the manufacturing process of an organic EL display device, the polyimide film layer is peeled off from the release layer, and after the surface of the inorganic layer is treated with a coupling agent, the pattern of this coupling agent is formed by UV irradiation, etc. For example, a method of forming a laminate having a good adhesion part and an easily peelable part with different peel strengths by carrying out a chemical treatment, and then peeling it off.

Since the wavelength of light emitted from the light emitting layer of an organic EL device is mainly from 440 nm to 780 nm, the supporting base material used for the organic EL device must have an average transmittance of at least 80% in this wavelength range. is required. On the other hand, when peeling glass and polyimide film by irradiation with UV laser light as described above, if the transmittance at the wavelength of UV laser light is high, it is necessary to provide an absorption/release layer under the film. This reduces productivity. In order to perform peeling without providing an absorption/release layer, the polyimide film itself must absorb laser light sufficiently, and the transmittance of the polyimide film at 400 nm is preferably 60% or less, and Preferably it is 40% or less.

Further, a gas barrier layer may be formed on the polyimide film to prevent moisture and oxygen from entering the organic EL device or the touch panel device. In this case, the polyimide film including the gas barrier layer is considered to be the polyimide film of the present invention. It may be formed as a single polyimide film, or as a laminate of a polyimide film and a base material such as glass or metal foil. Any known gas barrier layer can be used, but silicon oxide, aluminum oxide, silicon carbide, silicon oxide carbide, silicon carbonitride, silicon nitride, silicon nitride oxide, etc. can be used as a gas barrier layer with barrier properties against oxygen, water vapor, etc. An inorganic oxide film is preferably exemplified, and may be composed of only one type of composition, or a film having a mixture of two or more types of composition may be selected.

Hereinafter, the content of the present invention will be explained in more detail based on Examples, etc., but the present invention is not limited to the scope of these Examples.

First, the abbreviations of monomers and solvents used in synthesizing polyimide, and the methods and conditions for measuring various physical properties in Examples are shown below.

TFMB: 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl PMDA: Pyromellitic dianhydride DMAc: N,N-dimethylacetamide 6FDA: 2,2'-bis(3,4- dicarboxyphenyl)hexafluoropropane dianhydride ODPA: 4,4'-oxydiphthalic dianhydride BPADA: 2,2'-bis(4-(3,4-dicarboxyphenoxy)propanoic dianhydride)
BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride m-TB: 2,2'-dimethylbenzidine TPE-R: 1,3-bis(4-aminophenoxy)benzene support substrate: Glass substrate (made by Corning, 0.7mm thick)
Solvent: dimethylacetamide (DMAc)

[Coefficient of thermal expansion (CTE)]
A polyimide film with a size of 3 mm x 15 mm was heated from 30 °C to 280 °C at a constant heating rate (20 °C/min) while applying a load of 5.0 g using a SEIKO thermomechanical analysis (TMA) device TMA100. The temperature was raised, then lowered to 30°C, a tensile test was conducted in this temperature range, and the coefficient of thermal expansion (ppm/K) was measured from the amount of elongation of the polyimide film with respect to the temperature change from 250°C to 100°C. .

[Tear propagation resistance]
A test piece of polyimide film (63.5 mm x 50 mm) was prepared, a cut with a length of 12.7 mm was made in the test piece, and the test piece was measured at room temperature using a light load tear tester manufactured by Toyo Seiki. The measured tear propagation resistance value was expressed as a resistance value per unit thickness (mN/μm).

[Glass transition temperature Tg]
The behavior of polyimide film (10 mm x 22.6 mm) when heated from 20°C to 500°C at a rate of 5°C/min using a dynamic rheological analysis (DMA) device RSA3 manufactured by TA Instruments Japan. The glass transition temperature Tg (tan δ maximum value) was determined.

[Light transmittance]
The average light transmittance of a polyimide film (50 mm x 50 mm) from 440 nm to 780 nm was determined using a U4000 spectrophotometer.

[Total light transmittance]
The total light transmittance of the polyimide film (5 cm square) was measured using HAZE METER NDH-5000 manufactured by Nippon Denshoku Kogyo.

[Thermal decomposition temperature (Td5%)]
Measure the weight change when a polyimide film weighing 10 to 20 mg is heated at a constant rate from 30°C to 550°C under a nitrogen atmosphere using a SEIKO thermogravimetric analysis (TG) device TG/DTA6200. The temperature at which the weight loss rate was 5% was defined as the thermal decomposition temperature. In addition, if the weight loss rate did not reach 5% in the above temperature range, any temperature of 530° C. or higher and the weight loss rate at that temperature were described.

[Moisture absorption rate]
After drying a polyimide film (4 cm x 20 cm) at 120°C for 2 hours, it was left to stand for 24 hours in a constant temperature and humidity machine at 23°C/50% RH, and the weight change before and after was determined by the following formula.
Moisture absorption rate (%) = [(weight after moisture absorption - weight after drying) / weight after drying] x 100

[Releasability]
If the polyimide film could be peeled off from the support substrate without breakage or tearing, it was judged as ○. In addition, if breakage or tearing occurs during the peeling process and peeling becomes impossible, it was judged as "×".

[Humidity expansion coefficient (CHE)]
The viscosity of the polyamic acid solution was adjusted to about 10,000 cP by adding DMAc before coating. Then, it was applied onto a copper foil with a thickness of 18 μm using an applicator so that the film thickness after drying was about 10 μm, dried at 50 to 130°C for 2 to 60 minutes, and then heated from 130°C to 360°C. Heat treatment was performed for 30 minutes to form a polyimide layer on the copper foil to obtain a laminate. This laminate was cut into a size of 25 cm x 25 cm, an etching resist layer was provided on the copper foil side, and this was formed into a pattern in which 16 points with a diameter of 1 mm were arranged at 10 cm intervals on the four sides of a square with sides of 30 cm. The exposed portion of the etching resist opening was etched to obtain a polyimide film for CHE measurement having 16 copper foil remaining points. After drying this film at 120°C for 2 hours, it was left to stand for 24 hours in environments of 23°C/30% RH, 23°C/50% RH, and 23°C/70%. CHE (ppm/%RH) was determined from the dimensional change. Note that the dimensional change was measured using a two-dimensional length measuring machine.

[warp]
A laminate of copper foil and polyimide film was cut into a size of 10 cm x 10 cm, placed on a flat place, and the degree of warpage of the laminate was determined. If the warpage was less than 1 mm, it was marked as ○, if it was 1 mm or more and less than 3 mm, it was marked as Δ, and if it was 5 mm or more, it was marked as ×.

[Weight average molecular weight Mw]
The weight average molecular weight Mw was measured and calculated using a GPC device (TOSOHHLC-8220GPC manufactured by Tosoh Corporation). The column used was Tskgel GMHHR-M x 2, manufactured by Tosoh Corporation. The standard sample is polystyrene and the detector type is RI. A DMAc-based developing solvent was used as the solvent.

[Synthesis example 1]
(Polyamic acid A)
TFMB25.907 was dissolved in 200 g of DMAc as a solvent while stirring in a 500 ml separable flask under a nitrogen stream. Next, 14.0 g of PMDA and 5.019 g of ODPA were added to this solution. The molar ratio of acid anhydride and diamine was set to 1.005. Thereafter, 55 g of DMAc was added so that the solid content was 15 wt%. Thereafter, the solution was continuously stirred at room temperature for 5 hours to carry out a polymerization reaction, and was maintained overnight. A viscous colorless polyamic acid solution was obtained, and it was confirmed that polyamic acid A with a high degree of polymerization was produced. Table 1 shows the weight composition of the monomers in polyamic acid A.

[Synthesis examples 2 to 16]
(Polyamic acid B to P)
Polyamic acids B to P according to Synthesis Examples 2 to 16 were obtained in the same manner as Synthesis Example 1 except that the compositions shown in Tables 1 to 3 were used. Mw of polyamic acid E was 211,596. Moreover, Mw of polyamic acid N was 195,170.

[Example 1]
DMAc was added to the polyamic acid A solution obtained above to dilute it to a viscosity of 5000 cP. It was applied onto a 0.5 mm thick, 10 mm square glass substrate using an applicator so that the film thickness after heat treatment would be approximately 25 μm, and the temperature was raised from 90°C to 360°C over 30 minutes, and then the film was placed on the glass substrate. Polyimide was formed to obtain a laminate A. Next, the polyimide film was peeled off from the glass substrate to obtain a colorless polyimide film A. Table 4 shows the results of various evaluations performed on the obtained polyimide film A and laminate A.
Note that the evaluation of the humidity expansion coefficient (CHE) and warpage was performed according to the procedures described in the above-mentioned [Humidity expansion coefficient (CHE)] and [warp], respectively. The evaluation results are shown in Table 4.

[Example 4 and Reference Examples 2, 3 , 5, 6 and 7 ]
Example 4, and laminates B to G and Reference Examples 2, 3 , 5, 6, and 7 were prepared in the same manner as in Example 1 , except that polyamic acids B to G were used instead of polyamic acid A. Polyimide films B to G were produced. The evaluation results are also shown in Table 4.

[Reference Examples 8, 9, 11]
The laminated layer according to Reference Example 8 was prepared in the same manner as in Example 1, except that polyamic acid C, E, or N was used instead of polyamic acid A, and the film was coated so that the film thickness after heat treatment was about 10 μm. A laminate E-2 and a polyimide film C-2, a laminate E-2 and a polyimide E-2 according to Reference Example 9, and a laminate N and a polyimide film N according to Reference Example 11 were produced. The evaluation results are also shown in Tables 4 and 6, respectively.

[Reference example 12]
Laminate O and polyimide film O according to Reference Example 12 were prepared in the same manner as in Example 1, except that polyamic acid O was used instead of polyamic acid A and the film thickness after heat treatment was about 8 μm. was created. The evaluation results are also shown in Table 6.

[Example 10]
A laminate A was obtained in the same manner as in Example 1 above, and a color filter layer was further formed on the polyimide surface of the laminate A. Subsequently, the polyimide on which the color filter layer was formed was peeled off from the glass to obtain a color filter substrate using polyimide film A as a flexible substrate. In the manufacturing process of the color filter substrate, the warpage of the laminate was less than 1 mm. Moreover, the polyimide could be peeled cleanly from the glass. Moreover, no damage to the flexible substrate was observed in the obtained color filter substrate.

[Comparative Examples 1 to 6]
Laminated bodies H to M and polyimide films H to M according to Comparative Examples 1 to 6 were produced in the same manner as in Example 1, except that polyamic acids H to M were used instead of polyamic acid A. The evaluation results are shown in Table 5.

[Comparative Examples 7-8]
Laminated bodies P and Comparative Example 7 were prepared in the same manner as in Example 1, except that polyamic acid P or I was used instead of polyamic acid A, and the film thickness after heat treatment was about 10 μm. Polyimide film P, laminate I-2 and polyimide film I-2 according to Comparative Example 8 were produced. The evaluation results are shown in Table 6.

※ 重量減少率が５％に達しなかった。なお、カッコ内の数値は、表に記載の温度における重量減少率を表す。

* Weight reduction rate did not reach 5%. Note that the numbers in parentheses represent the weight loss rate at the temperatures listed in the table.

※重量減少率が５％に達しなかった。なお、カッコ内の数値は、表に記載の温度における重量減少率を表す。

*The weight reduction rate did not reach 5%. Note that the numbers in parentheses represent the weight loss rate at the temperatures listed in the table.

As shown in Tables 4 and 6, the polyimide films of Examples 1 and 4 and Reference Examples 2, 3, 5, 6, 7, 8, 9, 11, and 12 that met the conditions of the present invention had heat resistance. The film had excellent transparency, little warping, was strong, and could be peeled off cleanly from the support substrate while retaining its properties. Therefore, such a polyimide film can be suitably used as a supporting base material for forming display devices such as organic EL displays, organic EL lighting, and electronic paper.
On the other hand, as shown in Tables 5 and 6, those made of polyimide films according to Comparative Examples 1 to 8 that do not meet the conditions of the present invention have a large coefficient of thermal expansion, and the laminate with glass has a large warp and the film is brittle. Therefore, the polyimide film could not be peeled cleanly from the glass substrate and was not suitable as a polyimide film for forming a display device.

Claims (4)

A display device comprising a display element mounted on a polyimide film, wherein the polyimide film has a glass transition temperature of 300°C or higher, a 5% thermal decomposition temperature of 530°C or higher, and a total light transmittance at a film thickness of 30 μm or less. is 80% or more, the thermal expansion coefficient is 40 ppm/K or less, the tear propagation resistance is 1.3 mN/μm or more, the moisture absorption is 0.6 wt% or less, and the polyimide component constituting the polyimide film has a weight average molecular weight of 100,000. 400,000 and is characterized by comprising structural units represented by the following formulas (1) and (2).

In formulas (1) and (2), X is a substituent represented by formula (4) below. However, m/(m+n)=0.11 to 0.40.

The display device according to claim 1, wherein the display device is a touch panel. A polyimide film for a display device comprising a display element mounted on a polyimide film, wherein the polyimide film has a glass transition temperature of 300°C or higher, a 5% thermal decomposition temperature of 530°C or higher, and a film thickness of 30 μm or less. The total light transmittance is 80% or more, the thermal expansion coefficient is 40 ppm/K or less, the tear propagation resistance is 1.3 mN/μm or more, the moisture absorption is 0.6 wt% or less, and the polyimide component constituting the polyimide film is A polyimide film for display devices, having an average molecular weight of 100,000 to 400,000 and comprising structural units represented by the following formulas (1) and (2).

In formulas (1) and (2), X is a substituent represented by formula (4) below. However, m/(m+n)=0.11 to 0.40.

A polyimide having a weight average molecular weight of 100,000 to 400,000, consisting of structural units represented by the following formulas (1) and (2), and obtained by thermal imidization .

In formulas (1) and (2), X is a substituent represented by formula (4) below. However, m/(m+n)=0.11 to 0.40.