KR20190037960A - Preparation Method for Substrate used in Optical Device - Google Patents

Abstract

In the present application, provided is a method for effectively manufacturing a substrate having a spacer on the surface of the substrate which does not cause light leakage and the like during formation of an optical device, wherein the spacer is fixed in a uniformly dispersed state on an alignment film formed on the substrate.

Description

[0001] The present invention relates to a method of manufacturing a substrate for an optical device,

The present application relates to a method of manufacturing a substrate for an optical device.

An optical device capable of adjusting the transmittance or color of light by disposing a light modulating layer between substrates is known. For example, Patent Document 1 describes a so-called GH cell (Guest host cell) applying a mixture of a liquid crystal host and a dichroic dye guest.

Such an optical device generally comprises a two-layer substrate disposed opposite to the substrate and the light modulating layer present between the substrate and also includes a spacer to maintain the spacing between the substrates.

There is also known a structure in which a spacer is typically referred to as a so-called column spacer and a ball spacer, and a ball spacer is fixed in the alignment film in the case of using a ball spacer. Such a spacer is also referred to as a so-called fixed spacer. Such a structure is particularly useful when an optical device is manufactured using a flexible substrate because it is possible to maintain a uniform cell gap without causing movement or aggregation of the ball spacer when the substrate is bent.

However, in the structure in which the fixed spacer is present, orientation defects tend to occur in the alignment film around the spacer, thereby causing problems such as light leakage.

The present application relates to a method of manufacturing a substrate for an optical device. In the present application, it is an object to manufacture a substrate on which a fixed spacer is present on the surface, and which can manufacture an optical device with excellent performance without defects such as light leakage.

The present application relates to an optical device or a method of manufacturing a substrate for the device. According to the exemplary manufacturing method of the present application, it is possible to provide a substrate that includes orientation films in which spacers are uniformly dispersed and fixed on a substrate layer, and which does not have orientation defects or light leakage caused by the fixed spacers.

The above manufacturing method of the present application may be a manufacturing method of an orienting substrate in another example. The orienting substrate may mean a substrate provided with a function of aligning the liquid crystal material in a predetermined direction.

The term optical device in the present application encompasses two or more different optical states, for example, a high transmittance and a low transmittance state, a high transmittance, a medium transmittance and a low transmittance state, Any type of device formed to be capable of switching may be included.

The manufacturing method of the present application may include a step of forming an alignment film on the base layer. The alignment film may be an alignment film into which a spacer is introduced in one example. The above manufacturing method may include, for example, a step of coating on the base material a coating material comprising an orientation film forming material and a spacer including an orientation film forming material and a solvent.

In this process, the thickness of the alignment layer can be controlled within a range of about 30 to 300 nm. The thickness of the alignment film may be 35 nm or more, 40 nm or more, 45 nm or more, or 50 nm or more in another example. In another example, the thickness of the alignment layer is about 290 nm or less, about 280 nm or less, about 270 nm or less, about 260 nm or less, about 250 nm or less, about 240 nm or less, about 230 nm or less, about 220 nm or less, About 210 nm or less, or about 200 nm or less.

The spacer may be fixed on the orientation film having such a thickness. The inventors of the present invention have confirmed that the orientation defects of the alignment layer around the spacer fixed by the above-described structure can be minimized or eliminated. Further, by adopting an appropriate thickness, applying a solvent having a specific boiling point as described later, and controlling the heat treatment conditions, for example, an optical device having no defect such as doting stain can be effectively .

The coating material may be a mixture of the alignment film forming material and a spacer. The alignment film forming material may have a viscosity within a range of about 0.01 cP to 25 cP. The viscosity may be, for example, not less than 0.1 cP, not less than 0.5 cP, or not less than 1 cP, not more than about 20 cP, not more than 18 cP, not more than 16 cP, not more than 14 cP or not more than 13 cP. By applying the coating material in which the spacer is mixed with the alignment film forming material having such a viscosity, an alignment film that is fixed in a state in which the spacer is uniformly dispersed can be effectively formed on the substrate layer.

In the case where the measured temperature affects the physical properties of the physical properties mentioned in the present application, the physical properties may be physical properties measured at room temperature unless otherwise specified. The term ambient temperature can be any natural, unheated or non-warmed temperature, for example, about any temperature within the range of about 10 ° C to 30 ° C, such as about 23 ° C or about 25 ° C.

In addition, when the measured pressure affects the properties of the physical properties mentioned in the present application, the physical properties may be physical properties measured at normal pressure unless otherwise specified. The term atmospheric pressure is a natural pressure, generally about one atmosphere pressure, such as atmospheric pressure.

The ratio of the alignment film forming material in the alignment film forming material may be within a range of about 0.1 to 10% by weight, but the ratio is preferably within a range in which the above-mentioned viscosity can be achieved depending on the type of the solvent and the alignment film- Lt; / RTI >

The alignment film forming material includes at least an alignment film forming material and a solvent.

As the alignment film forming material, any kind of materials known to be capable of exhibiting alignment performance such as a vertical or horizontal alignment ability to a liquid crystal by an appropriate treatment can be used. Examples of such a material include a polyimide compound, a poly (vinyl alcohol) compound, a poly (amic acid) compound, a polystylene compound, a polyamide compound, A polyimide compound, a polyamic acid compound, a polynorbornene compound, a phenylmaleimide copolymer (such as a polyoxyethylene compound, a polyoxyethylene compound, or the like) phenylmaleimide copolymer compound, a polyvinylcinnamate compound, a polyazobenzene compound, a polyethyleneimide compound, a polyvinylalcohol compound, a polyimide compound, a polyethylene compound, a polystyrene a polystyrene compound, a polyphenylenephthalamide compound, a polyester (pol a substance known to be capable of aligning by light irradiation such as a chloromethylated polyimide compound, a chloromethylated polyimide (CMPI) compound, a polyvinylcinnamate (PVCI) compound, and a polymethyl methacrylate compound, But is not limited thereto.

The alignment film forming material can be prepared by diluting, dispersing and / or dissolving the alignment film forming material as described above in the solvent. The solvent which can be applied at this time is basically not particularly limited. Examples of the solvent include cycloalkane having 3 to 12 carbon atoms or 3 to 8 carbon atoms such as cyclohexane, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), dimethylformamide (DMF), N-methyl- such as 2-butoxyethanol or butanol, having 1 to 12 carbon atoms or 3 to 8 carbon atoms such as pyrrolidone, chloroform (CHCl ₃ ), cyclohexanone or cyclopentanone, 8 alcohol or a glycol having 1 to 12 carbon atoms or 1 to 8 carbon atoms such as ethylene glycol and butylene glycol, or a mixed solvent of two or more selected from the above can be applied.

A mixed solvent may be applied as the solvent for forming a suitable alignment film. In one example, as the solvent, a mixed solvent comprising a first solvent and a second solvent having different viscosities and / or boiling points may be applied.

In one example, the first and second solvents may be selected such that the absolute value of the difference in viscosity of each other is in the range of about 2 to 20 cP. In other embodiments, the absolute value of the difference in viscosity may be at least about 2.5 cP, at least about 3 cP, or at least about 3.5 cP, at least about 19 cP, at most 18 cP, at most 17 cP, at most 16 cP, . When the above difference in viscosity is satisfied, the absolute value of the viscosity of each solvent is not particularly limited, and can be adjusted so that the viscosity of the alignment film forming material described above can be achieved. In one example, the viscosity of the solvent having a high viscosity in the first and second solvents may be in the range of about 5 to 20 cP. In this case, the method of measuring the viscosity is as described above.

In one example, the first and second solvents may be selected such that the absolute value of the difference in boiling point of each other is in the range of about 0 캜 to 20 캜. In another example, the absolute value of the difference in boiling point may be at least about 0.5 DEG C, at least about 1 DEG C, at least about 1.5 DEG C, at least about 2 DEG C, at least about 2.5 DEG C, at least about 3 DEG C, at least about 3.5 DEG C, Or about 4.5 캜 or higher, about 19 캜 or lower, 18 캜 or lower, 17 캜 or lower, or 16 캜 or lower. When the above difference in boiling point is satisfied, the absolute value of the boiling point of each solvent is not particularly limited. In one example, the boiling point of the solvent having a high boiling point in the first and second solvents may be within a range of about 160 ° C to 300 ° C. The boiling point of the solvent may be about 280 캜 or lower, 260 캜 or lower, 240 캜 or lower, 220 캜 or 200 캜 or lower in other examples.

In another example, a solvent having a boiling point of 160 캜 or lower may be used as the solvent contained in the alignment film forming material, and in this case, it may be advantageous to minimize or prevent the dots from being stained when the Dotting process or the like is applied . In another example, the boiling point may be at least about 100 占 폚, at least 110 占 폚, at least 120 占 폚, or at least 125 占 폚.

Examples of the solvent include a cycloalkane having 3 to 12 carbon atoms such as cyclohexane or a cycloalkane having 3 to 8 carbon atoms such as dimethylformamide (DMF), chloroform (CHCl3), cyclohexanone or cyclopentanone 1 to 12 carbon atoms or an alcohol having 1 to 12 carbon atoms or ethylene glycol or butylene glycol such as ketone, 2-butoxyethanol or butanol, having 3 to 8 carbon atoms, or an alkylene group having 1 to 12 carbon atoms or 1 to 8 carbon atoms Glycol, or a solvent that satisfies the above-mentioned boiling point in a solvent selected from the above. If the above-mentioned boiling point is satisfied as a whole, a mixture of two or more of the above-mentioned solvents may be used, or any one of the solvents may be used alone.

The ratio of the first and second solvents is not particularly limited, but may be about 50% by weight to 99% by weight, about 60% by weight to 90% by weight or about 60% by weight based on the total weight of the first and second solvents, May be mixed so as to have a weight ratio of about 80% by weight to about 80% by weight.

The coating material includes a spacer together with the alignment film forming material. The kind of the spacer applied at this time is not particularly limited. In one example, as the spacer, a known ball spacer or a column spacer may be used. The spacer may be of a suitable size, for example, in consideration of a desired cell gap, without any particular limitation on its size. In one example, the diameter of the ball spacer or the height of the column spacer may be greater than about 1 micrometer. In another example, the diameter or height is at least about 2 탆, at least about 3 탆, at least about 4 탆, at least about 5 탆, at least about 6 탆, at least about 7 탆, at least about 8 탆, at least about 9 탆, 30 mu m or less, 29 mu m or less, 28 mu m or less, 27 mu m or less, 26 mu m or less, 25 mu m or less, or 24 mu m or less and 23 mu m or less 21 mu m or less, 20 mu m or less, 19 mu m or less, 18 mu m or less, 17 mu m or less, 16 mu m or less, or 16.5 mu m or less.

The ratio of the spacer in the coating material is not particularly limited and may be, for example, about 0.05 to about 10 parts by weight based on 100 parts by weight of the alignment film forming material.

This ratio can be appropriately controlled in consideration of the desired dispersibility and the like.

The coating material basically includes the alignment film-forming material and the spacer, and if necessary, other known components may be appropriately included as an additive.

In the present application, there is no particular limitation on the kind of the substrate layer on which the coating material is coated as described above. For example, as the substrate layer, any substrate layer used as a substrate in a configuration of a known optical device such as a liquid crystal display (LCD) can be applied. For example, the substrate layer may be an inorganic substrate layer or an organic substrate layer. As the inorganic base layer, a glass base layer and the like can be exemplified. As the organic base layer, various plastic films and the like can be exemplified. Plastic films include TAC (triacetyl cellulose) film; Cycloolefin copolymer (COP) films such as norbornene derivatives; Polyacrylate films such as PMMA (poly (methyl methacrylate), polycarbonate films, polyolefin films such as PE (polyethylene) or PP (polypropylene), polyvinyl alcohol films, DAC (diacetyl cellulose) A polyether sulfone (PES) film, a polyetheretherketone (PES) film, a polyphenylsulfone (PPS) film, a polyetherimide (PEI) film, a polyethylenemaphthatate (PEN) film, a polyethyleneterephtalate (PET) Film or PAR (polyarylate) film, and the like can be exemplified, but are not limited thereto.

The thickness of the base layer in the present application is not particularly limited, and an appropriate range may be selected depending on the application.

In the manufacturing method of the present application, the coating material may be directly coated on the base layer, or may be coated on the other layer or the structure in a state where another layer or structure exists on the base layer.

Examples of other layers or configurations are not particularly limited and include all known layers and configurations necessary for driving and configuring an optical device. Examples of such layer or constitution include an electrode layer and the like.

The electrode layer may be formed using a known material. For example, the electrode layer may include a conductive polymer, a conductive metal, a conductive nanowire, or a metal oxide such as ITO (Indium Tin Oxide). The electrode layer may be formed to have transparency. In this field, various materials and forming methods capable of forming a transparent electrode layer are known, and all of these methods can be applied. If necessary, the electrode layer formed on the surface of the substrate may be appropriately patterned.

In the present application, the coating thickness for coating the coating material on the base material layer may be determined according to the thickness of the intended orientation film.

In the manufacturing method of the present application, a step of heat-treating the coated layer may be further performed after coating the coating material. In such a case, the heat treatment condition can be selected in an appropriate range in which the solvent contained in the coating material can be removed. In one example, the process may be conducted within a range of about 90 ° C to 200 ° C for about 1 minute to 1 hour, but is not limited thereto.

In one example, the heat treatment may be performed at a temperature of about 115 캜 or less. The present inventors have confirmed that dodging unevenness can be minimized or prevented at the above heat treatment temperature. In this case, the heat treatment process may be performed at a temperature of about 50 캜 or more, about 60 캜 or more, about 70 캜 or more, about 80 캜 or more, about 90 캜 or more, or about 95 캜 or more.

The heat treatment process as described above may be performed within a range of, for example, about 1 minute to 2 hours. The heat treatment time may be, in another example, about 110 minutes or less, about 100 minutes or less, about 90 minutes or less, about 80 minutes or less, about 70 minutes or less, about 60 minutes or less, about 50 minutes or less, About 3 minutes or more, about 4 minutes or more, about 5 minutes or more, about 6 minutes or more, about 7 minutes or more, about 5 minutes or less, about 10 minutes or less, about 20 minutes or less, about 15 minutes or less, 8 minutes or about 9 minutes or more.

In the manufacturing method of the present application, it is possible to further carry out a step of carrying out a roughening treatment on the coating material after the coating step. In this case, the orientation treatment method is not particularly limited. For example, when the alignment film forming material is a rubbing alignment film forming material, an appropriate optical alignment treatment may be performed in the case of a photo alignment film forming material by a proper rubbing process. For example, the rubbing process may be a rubbing process using a rubbing cloth made of a material such as cotton, rayon, or nylon, and the light irradiation process may be performed by a suitable straight line A method of irradiating polarized light, or the like can be applied.

The present application also relates to a method of manufacturing an optical device using an oriented substrate formed as described above. The manufacturing method may be performed using a substrate different from the substrate and the substrate (hereinafter referred to as a second substrate). In this case, the second substrate and the above-described substrate layer and the alignment layer formed on the substrate layer may be included. The second substrate, however, may not include a spacer.

The manufacturing method may include forming a light modulating layer including a light modulating material between the alignment film of the substrate for an optical device and the alignment film of the second substrate.

The method of forming the light modulating material is not particularly limited, and various known methods can be applied.

In a case where the doting process is applied in one example, the manufacturing method may include, for example, a step of dotting a light modulating material containing a liquid crystal compound on an alignment film of the substrate formed as described above, and a step of aligning the counter substrate (the second substrate) The method may further include the step of pressing the dotted light modulating material so as to fill the gap between the substrate layer and the counter substrate by placing the light modulating material in a state of being opposed to the substrate layer having the doted alignment film.

The above process can be performed, for example, according to a general method of a dotting process, and the specific process is not particularly limited.

Of course, in addition to the above-mentioned doting process, various methods known in the art for forming a light modulating layer can be used.

The material of the liquid crystal compound or the like to be applied is not particularly limited, and a well-known suitable material is selected as necessary.

In addition, the kind of the opposing substrate which is opposed to the substrate layer is not particularly limited, and a known substrate can be used. For example, the substrate may also include a substrate layer and an orientation film formed on one side thereof, and may include other structures such as an electrode layer, if necessary. The method of forming the alignment film formed on the substrate layer of the counter substrate is not particularly limited, and a known method can be used.

The distance between the substrate layer and the counter substrate, that is, the so-called cell gap, is determined according to the size of the above-mentioned spacer.

If desired, the light modulating material may further comprise a dichroic dye. For example, the dichroic dye can be divided into two types. A dye that absorbs more light in the specific direction than the other direction and absorbs the polarized light in the long axis direction of the molecule is called a positive dichroic dye or p Type dyes, and negative dichroic dyes or n-type dyes that absorb light in the vertical direction. In general, such a dye may have an absorption spectrum in a narrow region around a wavelength causing maximum absorption. In addition, the dyes used in the guest host LCD are characterized by chemical and optical stability, color and absorption spectrum width, dichroism ratio, color order, solubility in hosts, degree of deionization, extinction coefficients and purity and high resistivity. Unless otherwise stated, dichroic dyes are assumed to be positive dyes.

As used herein, the term " dye " may refer to a material that is capable of intensively absorbing and / or deforming light within a visible light region, for example, within a wavelength range of 400 nm to 700 nm, The term " dichroic dye " may mean a material capable of dichroic absorption of light in at least a part or the entire range of the visible light region.

As the dichroic dye, for example, a known dye known to have properties that can be aligned according to the alignment state of the liquid crystal can be selected and used. As the dichroic dye, for example, a black dye can be used. Such dyes are known, for example, as azo dyes or anthraquinone dyes, but are not limited thereto.

The dichroic ratio of dichroic dyes can be appropriately selected in light of the purpose of the present application. For example, the dichroic dye may have a dichroic ratio of 5 or more to 20 or less. In the present specification, the term " dichroic ratio " means, for example, a value obtained by dividing the absorption of polarized light parallel to the long axis direction of the dye by the absorption of polarized light parallel to the direction perpendicular to the long axis direction . The anisotropic dye may have the dichroic ratio at least at some wavelength or at any wavelength within the wavelength range of the visible light region, for example, within the wavelength range of about 380 nm to 700 nm or about 400 nm to 700 nm.

In this application, a substrate in which spacers are present on the surface, the spacers are fixed in a state of being uniformly dispersed on an alignment film formed on the substrate, and a substrate which does not cause light leakage or the like Can be provided.

Figs. 1 to 3 are photographs of the substrate produced in Examples. Fig.
4 is a photograph of a substrate manufactured in a comparative example.

Hereinafter, the present application will be described in detail by way of examples according to the present application and comparative examples not complying with the present application, but the scope of the present application is not limited by the following examples.

1. Confirmation of occurrence of light leakage

After arranging the absorptive linear polarizer on one side of the optical device such that the liquid crystal host in the optical modulator of the optical device manufactured in Example or Comparative Example is horizontally aligned with the optical axis of the liquid crystal host in a state of being horizontally aligned, Respectively.

2. Check the fastening force of the spacer

The adhesive force of the spacer was evaluated by peeling off the adhesive tape having the peeling force of about 10 gf / in on the alignment film on which the spacer was fixed, and checking the degree of loss of the spacer. The peeling force of the adhesive tape is a peeling force measured at room temperature with a peeling speed of 300 mm / min and a peeling angle of 180 degrees with respect to a TAC (triacetyl cellulose) film. As a result of the above evaluation, if the loss was not significant or insignificant, it was judged to be Passed, and if there was a lot of loss, it was judged as Failed.

Example 1.

Butylene glycol (viscosity: about 16.10 cP, boiling point: about 197 ° C) and NMP (N-methyl-pyrrolidone) were used as the alignment film forming material, polyimide (Nissan, SE- (Viscosity: about 1.67 cP, boiling point: about 202 ° C) to a concentration of about 1% by weight to prepare an alignment film-forming material. The weight ratio of butylene glycol to NMP was about 7: 3 (butylene glycol: NMP). The viscosity of the alignment film-forming material formed in the above manner was about 12.47 cP.

Next, a coating material was prepared by mixing a ball spacer having a diameter of about 12 μm with the alignment film-forming material so that the concentration of the ball spacer was about 1 wt% in the whole coating material.

The coating material was coated on a polycarbonate film having an ITO (Indium Tin Oxide) electrode layer formed on one surface thereof to a thickness of about 40 nm as the thickness of the final alignment film, After the heat treatment, the coating layer was subjected to rubbing alignment treatment to form an alignment film. The adhesion force was measured on the alignment film having the spacer formed thereon as described above and evaluated as Passed.

An optical device was manufactured using the above-prepared substrate and the second substrate. As the second substrate, an ITO (Indium Tin Oxide) electrode layer and a known polyimide vertical alignment film (Nissan, SE-5661) are sequentially formed on one surface and a PC film (polycarbonate polymer) Were used.

An adhesive applied to the manufacture of a liquid crystal cell is usually coated on the end of the substrate where the spacer is present and a light modulating material (HNG730200 (ne: 1.551, no: 1.476, ε∥: 9.6, ε⊥ (A mixture of a liquid crystal and an anisotropic dye (BASF, X12)) was adhered to the second substrate and then the second substrate was adhered to the substrate by dotting ) Of the light modulating material spread evenly between the two substrates to produce an optical device. 1 is a result of observation of light leakage for an optical device formed in the above manner.

Example 2.

A substrate and an optical device were prepared in the same manner as in Example 1, except that the thickness of the final alignment film was about 100 nm when the coating material was coated. The adhesion force was measured on the alignment film having the spacer formed thereon as described above and evaluated as Passed. Further, as shown in Fig. 2, no light leakage was observed for the optical device.

Example 3.

A substrate and an optical device were prepared in the same manner as in Example 1, except that the thickness of the final alignment layer was about 200 nm when the coating material was coated. The adhesion force was measured on the alignment film having the spacer formed thereon as described above and evaluated as Passed. Further, as shown in Fig. 3, no light leakage was observed for the optical device.

Comparative Example 1

A substrate and an optical device were prepared in the same manner as in Example 1, except that the thickness of the final alignment layer was about 20 nm when the coating material was coated. In the case of Comparative Example 1, no light leakage was observed in the optical device, but the adhesion was evaluated as Failed as a result of measuring the adhesion by the above-mentioned method.

Comparative Example 2

A substrate and an optical device were prepared in the same manner as in Example 1, except that the thickness of the final alignment film was about 25 nm when the coating material was coated. In the case of Comparative Example 2, no light leakage was observed in the optical device, but the adhesion was evaluated as Failed as a result of measuring the adhesion by the above-mentioned method.

Comparative Example 3

A substrate and an optical device were prepared in the same manner as in Example 1, except that the thickness of the final alignment layer was about 350 nm when the coating was coated. The adhesion force was measured on the alignment film having the spacer formed thereon as described above and evaluated as Passed. However, as shown in Fig. 4, defects due to light leakage were observed with respect to the optical device.

Claims (11)

Coating a coating material comprising an orientation film forming material and a solvent and a coating material including an orientation film forming material and a solvent on a base layer to form an orientation film having a thickness within a range of 30 nm to 300 nm and in which the spacer is fixed A method for manufacturing a substrate for an optical device. The method of manufacturing a substrate for an optical device according to claim 1, wherein the orientation film forming material has a room temperature viscosity within a range of 0.01 to 25 cP. The method of manufacturing a substrate for an optical device according to claim 1, wherein the ratio of the alignment film forming material in the alignment film forming material is in the range of 0.1 to 10% by weight. The method according to claim 1, wherein the alignment film forming material is selected from the group consisting of a polyimide compound, a poly (vinyl alcohol) compound, a poly (amic acid) compound, a polystylene compound, polyamide compounds, polyoxyethylene compounds, polynorbornene compounds, phenylmaleimide copolymer compounds, polyvinylcinnamate compounds, polyazobenzene compounds, polyethyleneimine polyethyleneimide compound, a polyethylene compound, a polyphenylenephthalamide compound, a polyester compound, a chloromethylated polyimide (CMPI) compound, a polyvinylcinnamate (PVCI) compound, and a polymethyl methacrylate compound ≪ / RTI > wherein at least one selected from the group consisting of silicon nitride and silicon nitride is used. The method of manufacturing a substrate for an optical device according to claim 1, wherein the solvent contained in the alignment film forming material has a boiling point of 160 캜 or less. 6. The method according to claim 5, wherein the coating material is coated on the substrate layer so as to form an alignment film having a thickness of 50 nm or more. The method of claim 1, wherein the spacer is a ball spacer or a column spacer. The method according to claim 1, wherein the spacer is a ball spacer or a column spacer having a diameter or height of 1 탆 or more. 2. The method according to claim 1, wherein the coating material comprises 0.05 to about 10 parts by weight of the spacer relative to 100 parts by weight of the alignment film forming material. The method of claim 1, further comprising the step of heat treating the coated substrate layer. 11. The method of claim 10, wherein the heat treatment is performed at a temperature of 115 DEG C or less.

Images