Method for evaluating the breaking tension of stretched film

Abstract

To provide a method for evaluating the resistance to a heat cycle property of a polarizer and a polarization film with a simple operation and a high reliability, without manufacturing the polarizer.SOLUTION: The present invention relates to a method for evaluating the resistance to a heat cycle performance of a polarizer, the method sequentially including the steps of: obtaining a test piece which is 5 cm in a stretch direction and 3 cm in a vertical direction (obtaining a test piece 1) from a stretched film (step A); cutting the center part of the test piece 1 by 1 mm to 20 mm in the direction of stretch (obtaining a test piece 2) (step B); and setting the distance between zippers of a tension test machine to be 10 mm and stretching the test piece 2 vertically relative to the direction of stretch of the stretched film at the rate of 1 mm to 15 mm per minute.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for evaluating the breaking strength of a stretched film.

Background Art

[0002] Plastic films are processed into stretched films with various functions by stretching, and stretched films are used in a wide range of fields such as optical applications and food packaging applications.

[0003] A typical example of an optical application is a polarizing film. A polarizing film is processed into a polarizing plate by laminating a protective film such as a cellulose triacetate (TAC) film on its surface, and since it has a light transmission and shielding function, it is used as a basic component of a liquid crystal display device (LCD). LCDs are used in a wide range of fields such as small devices such as calculators and wristwatches, notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal TVs, in-vehicle navigation systems, mobile phones, smartphones, and other measuring devices used indoors and outdoors.

[0004] As the polarizing film, a matrix obtained by uniaxially stretching a polyvinyl alcohol (hereinafter sometimes referred to as PVA) film (a stretched film that is uniaxially stretched and oriented) adsorbed with a dichroic dye such as an iodine-based dye (I3 - or I5 - etc.) is the mainstream. Such a polarizing film is manufactured by, for example, uniaxially stretching a PVA film containing a dichroic dye in advance, adsorbing the dichroic dye simultaneously with the uniaxial stretching of the PVA film, or adsorbing the dichroic dye after uniaxially stretching the PVA film.

[0005] When LCDs are used in harsh environments, the polarizing plates contained within the LCD are subjected to rapid temperature changes. This can cause the polarizing film to fail to keep up with the dimensional changes of the protective film, leading to cracking of the polarizing film and resulting in light leakage from the polarizing plate. For this reason, in recent years, there has been a growing demand for greater heat cycle resistance (durability against rapid temperature changes) in polarizing plates and polarizing films. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-71314 [Patent Document 2] Japanese Patent Publication No. 2009-104062 [Patent Document 3] Japanese Patent Publication No. 2011-248293 [Patent Document 4] Japanese Patent Publication No. 2016-4218 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Conventionally, there were no simple evaluation systems or alternative indicators for evaluating the heat cycle resistance of polarizing plates, and tests were conducted by attaching polarizing plates to panels or glass plates. This resulted in problems such as time-consuming testing, high material costs, and the inability to evaluate large quantities of samples (Patent Documents 1 and 2).

[0008] Furthermore, methods for evaluating the tear strength of plastic films are known. For example, tear strength is known as a measure of how easily a plastic film can be torn, and a method is known in which a cut is made in the stretched film parallel to the stretching direction, the stretched film is torn from the starting point of the cut, and the tear strength at that time is measured using a tensile testing machine (Patent Document 3). In addition, a method is known in which the puncture strength of a stretched film is measured using a puncture tool of a specific shape (Patent Document 4). However, these are not methods for evaluating the heat cycle resistance of polarizing plates or polarizing films.

[0009] Therefore, the present invention has been made in view of the above-mentioned conventional problems, and aims to provide a method for evaluating the heat cycle resistance of polarizing plates and polarizing films with high reliability, using simple operations, and without the need to manufacture polarizing plates. [Means for solving the problem]

[0010] As a result of diligent research, the inventors found that the results of tensile tests performed on stretched films using specific operations correlated with the heat cycle resistance of polarizing films. Based on this finding, they conducted further studies and completed the present invention.

[0011] The present invention provides [1] to [3]. [1] A method for evaluating the heat cycle resistance of a polarizing plate, which involves performing operations A to C below in order. Operation A: Obtain a test specimen from the stretched film with dimensions of 5 cm in the stretching direction and 3 cm in the perpendicular direction (this will be called test specimen 1). Operation B: Make a cut of 1 to 20 mm in the stretching direction in the center of the test piece 1 (this will be called test piece 2). Operation C: Set the chuck distance of the tensile testing machine to 10 mm and pull the test piece 2 perpendicular to the stretching direction of the stretched film at a tensile speed of 1 to 15 mm / min. [2] The evaluation method according to [1], wherein the thickness of the stretched film is 10 to 40 μm. [3] The evaluation method according to [1] or [2], wherein the stretched film is a polarizing film. [Effects of the Invention]

[0012] According to the present invention, the heat cycle resistance of polarizing plates and polarizing films can be evaluated with high reliability through simple operations without the need to manufacture polarizing plates. [Brief explanation of the drawing]

[0013] [Figure 1] This is a diagram showing test specimen 2. [Modes for carrying out the invention]

[0014] The method for evaluating the heat cycle resistance of the polarizing plate and polarizing film of the present invention involves performing the following operations in order. Operation A: Obtain a test specimen from the stretched film with dimensions of 5 cm in the stretching direction and 3 cm in the perpendicular direction (this will be called test specimen 1). Operation B: Make a cut of 1 to 20 mm in the stretching direction in the center of the test piece 1 (this will be called test piece 2). Operation C: Set the chuck distance of the tensile testing machine to 10 mm and pull the test piece 2 perpendicular to the stretching direction of the stretched film at a tensile speed of 1 to 15 mm / min. In this invention, "perpendicular direction" refers to the direction perpendicular to the stretching direction of the stretched film, and is sometimes simply abbreviated as "perpendicular direction." Furthermore, in this specification, when a numerical range such as "XX~YY" is mentioned, it means "XX or greater and YY or less."

[0015] Although the reason for obtaining the effects of the present invention is not necessarily clear, it can be considered as follows. The amount of dimensional change during the heat cycle test is different for the polarizing film, the adhesive, and the protective film, respectively. These members repeat dimensional changes while following each other during the heat cycle test. As a result, repeated stresses are applied in the stretching direction of the polarizing film and in the direction perpendicular to it. On the other hand, generally, it is known that PVA is highly oriented in the stretching direction of the polarizing film, and the polarizing film is significantly weaker against stress in the direction perpendicular to the stretching direction than in the stretching direction. It is considered that the influence of the direction perpendicular to the stretching direction is greater than that of the stretching direction as a factor of cracks after the heat cycle test. Therefore, it is considered that the heat cycle resistance of the polarizing film, and thus the polarizing plate, can be evaluated by pulling a test piece having specific cuts in the stretching direction and the direction perpendicular to it and evaluating its fracture characteristics.

[0016] The stretched film used in the present invention is not particularly limited, and a known stretched film can be used. Examples of the stretched film include a stretched PET film, a stretched PC film, a stretched COP film, a stretched EVOH film, a stretched acrylic film, a stretched PVA film, and the like. The stretched film is preferably a stretched PVA film.

[0017] In the present invention, when a stretched PVA film is used as the stretched film, a polarizing film produced from the stretched film may be used. For the production methods of the stretched PVA film and the polarizing film, for example, they are described in JP-A-2017-106969, JP-A-2012-47799, JP-A-5-2109, and the like.

[0018] The thickness of the stretched film used in the present invention is not particularly limited, but from the viewpoint of reproducibility, the thickness of the stretched film is preferably 10 to 40 μm.

[0019] Operation A is the operation to obtain a test piece from the stretched film with dimensions of 5 cm in the stretching direction and 3 cm in the perpendicular direction (this will be called test piece 1). There are no particular restrictions on where test piece 1 is obtained, but from the viewpoint of reproducibility of evaluation, it is preferable to obtain it from the remaining 80% when the length of the stretched film in the perpendicular direction is set to 100%, excluding the 10% at both ends.

[0020] In operation A, there are no particular restrictions on the jig used to cut out test piece 1; a cutter or the like can be used.

[0021] Operation B is the operation of making a cut of 1 to 20 mm in the stretching direction in the center of the test piece 1 (this will be called test piece 2). The center is the intersection of the diagonals connecting two pairs of opposite vertices of test piece 1. After determining the center, the cut is made in the stretching direction, but the cut is made in the stretching direction so that the intersection coincides with the center of the cut. For example, if the cut is 5 mm, the cut is made 2.5 mm in the direction of stretching from the intersection and 2.5 mm in the opposite direction, for a total of 5 mm. From the viewpoint of improving the reproducibility of the tensile test described later, a cut of 1 to 15 mm is preferred, a cut of 1 to 10 mm is more preferred, and a cut of 1 to 5 mm is even more preferred. If the length of the cut is less than 1 mm, the function of the cut is insufficient, the fracture initiation position cannot be fixed consistently, and it becomes difficult to measure accurately. On the other hand, if the length of the cut is greater than 20 mm, the fracture tension becomes small, and test piece 2 will break even with a small impact, making it difficult to measure accurately. Furthermore, the handling of test specimen 2 becomes more difficult, which also reduces measurement efficiency.

[0022] In operation B, there are no particular restrictions on the jig used to make the cuts in test piece 1; a cutter or the like can be used.

[0023] The specimen 2 obtained in operation B may be used directly in operation C, but from the viewpoint of reproducibility of the tensile test, it is preferable to conditioned it for 16 hours in the range of 20-30°C / 20%RH-60%RH. Conditioning at a humidity lower than 20%RH is undesirable because it is economically disadvantageous, and conditioning at a humidity higher than 60%RH is undesirable because it may cause the specimen 2 to absorb more moisture and change its condition. In terms of temperature, temperatures lower than 20°C are undesirable because they are economically disadvantageous, and temperatures above 30°C are undesirable because they reduce workability.

[0024] Operation C involves setting the distance between the chucks of the tensile testing machine to 10 mm and pulling the test piece 2 perpendicular to the stretching direction of the stretched film at a tensile speed of 1 to 15 mm / min.

[0025] The tensile testing machine used in Operation C is not particularly limited as long as it is a device capable of measuring stress-strain curves (SS curves), but examples include universal material testing machines (Instron Co., Ltd.) and tensile testing machines (Autograph, Shimadzu Corporation).

[0026] In operation C, there are no particular restrictions on the load cell used in the tensile testing machine as long as it can measure a test force of 1 N or more. However, since the tension at which the stretched film breaks is not large, load cells with a capacity of 10 kN or more are unsuitable, and load cells with a capacity of 1 kN or less are preferred.

[0027] In operation C, the distance between the chucks of the tensile testing machine is 10 mm. The test piece 2 is attached to the chuck so that the direction perpendicular to the stretching direction of the stretched film of the test piece 2 is parallel to the tensile direction.

[0028] In operation C, the tensile speed of the tensile testing machine is 1 to 15 mm / min. To improve the reproducibility of the measurements, a tensile speed of 1 to 10 mm / min is preferable, and 1 to 6 mm / min is more preferable.

[0029] The tension at which the stretched film broke (breaking tension) was measured five times by repeating operations A through C five times, and the average value and standard deviation were calculated. Next, the coefficient of variation of the breaking tension was calculated by dividing the calculated standard deviation by the average value. In addition, the value obtained by dividing the breaking tension by the cross-sectional area of ​​test piece 2 was calculated as the breaking stress (N / mm²). 2 The average value and coefficient of variation of the fracture stress were calculated. The average values ​​and coefficients of variation of the calculated fracture tension and fracture stress were used as the measurement results.

[0030] The fracture stress of the polarizing film obtained by the evaluation method of the present invention correlated with the number of cracks that occurred after a heat cycle test of the polarizing film actually performed using that film. In other words, films with a high fracture stress had fewer cracks that occurred in the heat cycle test. In addition, because the coefficient of variation of the fracture stress was small, the heat cycle resistance of polarizing films and polarizing plates can be evaluated with high reliability and reproducibility. [Examples]

[0031] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way by these examples. The evaluation methods used in the following examples and comparative examples are shown below.

[0032] (1) Measurement of the softening point of PVA film A square sample measuring 3 cm in width and 3 cm in length was cut from the center of the PVA film to be measured, and the softening point temperature of the sample was measured using the EX-820 automatic softening point measuring device manufactured by Daiichi Rika Co., Ltd. Specifically, the sample was sandwiched between a 3 cm square stainless steel plate with a 1 cm diameter circular hole in the center and a 3 cm square stainless steel plate with a 1 cm x 2 cm rectangular hole in the center. The stainless steel plate with the circular hole was placed on a stand with the side facing upwards, and a steel ball specified in JIS B 1501:2009 (nominal size: 3 / 8 (diameter 9.525 mm), grade: G60, mass: 3.5 g ± 0.05 g) was placed on the film located in the center of the circular hole. Subsequently, 750 mL of distilled water at 25°C was added and the temperature was increased at 5°C per minute. The temperature at which the sample descended to a position 25 mm from the stand was defined as the softening point temperature of the film.

[0033] (2) Measurement of tensile strength In the following manufacturing example, the stretch tension during the stretching process was measured by the tension between adjacent rolls using a tension roll installed between them. When three or more rolls were used, the maximum stretch tension among them was adopted.

[0034] (3) Heat cycle test of polarizing film A heat cycle test was conducted using the manufactured polarizing film. The polarizing film manufactured according to the manufacturing example below was conditioned at 23°C / 50%RH for 16 hours, and then cut from the center in the width direction (perpendicular to the stretching direction) to a size of 140 mm in the stretching direction and 80 mm in the width direction. Subsequently, the polarizing film was laminated to one side surface of a glass plate (EAST JAPAN GLASS Co., Ltd.) with a long side of 160 mm, a short side of 90 mm, and a thickness of 1 mm, using an acrylic adhesive (MPD62, 25 μm thick, manufactured by Bikan Imaging Co., Ltd.) and a laminating machine (Lamyman IKO-360EII, manufactured by Yubon Co., Ltd.) so that the stretching direction of the polarizing film was parallel to the long side direction of the glass plate. A glass plate laminated with a polarizing film was placed in a heat cycle test machine (ETAC WINTECH NT530A, manufactured by Kusumoto Kasei Co., Ltd.) and subjected to six cycles of thermal shock testing from -35°C / 30 minutes to 80°C / 30 minutes. After the six cycles, the glass plate laminated with the polarizing film was removed, and the number of cracks (cracks parallel to the direction of stretching) that had formed in the polarizing film was counted. A smaller number of cracks indicated better heat cycle resistance.

[0035] Method for manufacturing polarizing film [Manufacturing Example 1] A PVA film "VF-PE#4500 (softening point temperature 68.1℃)" manufactured by Kuraray Co., Ltd. was slit to a width of 650 cm. In the swelling process, the slit PVA film was uniaxially stretched in the longitudinal direction (MD) to twice its original length while immersed in water at 25℃ for 90 seconds (first stage stretching). Subsequently, in the dyeing process, it was uniaxially stretched in the longitudinal direction (MD) to 2.4 times its original length while immersed in an aqueous solution at 32℃ containing 0.093 mass% iodine and 2.14 mass% potassium iodide for 163 seconds (second stage stretching). Subsequently, in the crosslinking process, it was uniaxially stretched in the longitudinal direction (MD) to three times its original length while immersed in an aqueous solution at 32℃ containing 2.6 mass% boric acid for 135 seconds (third stage stretching). In the subsequent stretching process, the film was uniaxially stretched in the longitudinal direction (MD) to 6.0 times its original length (4th stage stretching) while immersed in an aqueous solution containing 2.8% by mass of boric acid and 5% by mass of potassium iodide at a temperature of 59.3°C. The maximum stretching tension during the stretching process was 294 N. In the subsequent washing process, the film was washed by immersing it for 10 seconds in an aqueous solution containing 1.5% by mass of boric acid and 5.4% by mass of potassium iodide at a temperature of 22°C. In the subsequent drying process, a polarizing film with a thickness of 19.0 μm was produced by drying in a dryer at 80°C for 90 seconds. The thickness of the polarizing film was measured using Ono Sokki's "Digital Gauge Counter DG-5100", Ono Sokki's "Linear Gauge Sensor GS-3813", and Ono Sokki's "Gauge Stand ST-0230".

[0036] [Manufacturing Example 2] A polarizing film was manufactured using the same method as in Manufacturing Example 2, except that a PVA film "VF-PE#4500 (softening point temperature 67.4℃)" manufactured by Kuraray Co., Ltd. was used. The maximum tensile strength during manufacturing was 237N, and the thickness of the manufactured polarizing film was 18.0μm.

[0037] [Manufacturing Example 3] A polarizing film was manufactured using the same method as in Manufacturing Example 2, except that a PVA film "VF-PE#4500 (softening point temperature 67.1℃)" manufactured by Kuraray Co., Ltd. was used. The maximum tensile strength during manufacturing was 197N, and the thickness of the manufactured polarizing film was 17.8μm.

[0038] [Manufacturing Example 4] In the swelling process, a PVA film "VF-PS#6000 (softening point temperature 68.5℃)" manufactured by Kuraray Co., Ltd. was uniaxially stretched in the longitudinal direction (MD) to twice its original length (1st stage stretching) while immersed in water at 25℃ for 90 seconds. Subsequently, in the dyeing process, it was uniaxially stretched in the longitudinal direction (MD) to 2.4 times its original length (2nd stage stretching) while immersed in an aqueous solution at 32℃ containing 0.093 mass% iodine and 2.14 mass% potassium iodide for 163 seconds. Subsequently, in the crosslinking process, it was uniaxially stretched in the longitudinal direction (MD) to three times its original length (3rd stage stretching) while immersed in an aqueous solution at 32℃ containing 2.6 mass% boric acid for 135 seconds. In the subsequent stretching process, the film was uniaxially stretched in the longitudinal direction (MD) to 6.0 times its original length (4th stage stretching) while immersed in an aqueous solution containing 2.8% by mass of boric acid and 5% by mass of potassium iodide at a temperature of 55.4°C. The maximum stretching tension during the stretching process was 571 N. In the subsequent washing process, the film was washed by immersing it for 10 seconds in an aqueous solution containing 1.5% by mass of boric acid and 5.4% by mass of potassium iodide at a temperature of 22°C. In the subsequent drying process, a polarizing film with a thickness of 25.0 μm was produced by drying in a dryer at 80°C for 90 seconds.

[0039] [Example 1] For the 19.0 μm thick polarizing film produced in Manufacturing Example 1, a piece measuring 50 mm in the stretching direction and 30 mm in the width direction was cut from the center of the width direction. Next, a 5 mm cut in the stretching direction was made in the center of the cut polarizing film. The test piece thus prepared was conditioned at 23°C / 53% RH for 16 hours. Next, under a 23°C / 53% RH environment, the conditioned test piece was mounted on a universal material testing machine 5942 (manufactured by Instron Co., Ltd.) with the width direction parallel to the tensile direction, using a chuck (chuck distance 10 mm). The test piece was then pulled at a speed of 1 mm / min until it broke, and the tension at which the test piece broke was measured. A load cell with a capacity of 500 N, manufactured by Instron Co., Ltd., was used. The breaking tension was measured with N=5, and the average value (breaking tension) and coefficient of variation were calculated. Furthermore, the stress at fracture (N / mm²) is calculated by dividing the tension at fracture by the cross-sectional area of ​​the test specimen. 2 The stress at which the material breaks was also calculated, along with its average value (breaking stress) and coefficient of variation. The results are shown in Table 1. The relationship between tensile strength and breaking strength is shown in Figure 1.

[0040] [Example 2] The procedure is the same as in Example 1, except that the polarizing film of Manufacturing Example 2 is used instead of the polarizing film of Manufacturing Example 1.

[0041] [Example 3] The procedure is the same as in Example 1, except that the polarizing film of Manufacturing Example 3 is used instead of the polarizing film of Manufacturing Example 1.

[0042] [Example 4] The procedure is the same as in Example 1, except that the polarizing film of Manufacturing Example 4 is used instead of the polarizing film of Manufacturing Example 1.

[0043] [Example 5] This example is the same as Example 1, except that the polarizing film of Manufacturing Example 4 was used instead of the polarizing film of Manufacturing Example 1, and the tensile speed was changed from 1 mm / min to 5 mm / min.

[0044] [Comparative Example 1] This example is the same as Example 1, except that the polarizing film of Manufacturing Example 4 was used instead of the polarizing film of Manufacturing Example 1, and the tensile speed was changed from 1 mm / min to 20 mm / min.

[0045] [Comparative Example 2] This example is the same as Example 1, except that the polarizing film of Manufacturing Example 4 was used instead of the polarizing film of Manufacturing Example 1, and the tensile speed was changed from 1 mm / min to 100 mm / min.

[0046] [Comparative Example 3] This example is the same as Example 1, except that the polarizing film of Manufacturing Example 4 was used instead of the polarizing film of Manufacturing Example 1, and the 5 mm cut was not made.

[0047] [Comparative Example 4] Except for using the polarizing film from Manufacturing Example 4 instead of the polarizing film from Manufacturing Example 1, and increasing the size of the cutout from 5 mm to 25 mm, this is the same as Example 1.

[0048] [Table 1]

[0049] Table 1 shows that the greater the tensile strength during polarizing film manufacturing, the lower the breaking tension and breaking stress in the tensile test (i.e., the breaking tension and breaking stress perpendicular to the stretching direction). Furthermore, as the breaking tension and breaking stress in the tensile test decrease, the number of cracks occurring in the heat cycle test increases. In other words, films that are prone to cracking when pulled perpendicular to the stretching direction are also prone to cracking in the heat cycle test. These results indicate a correlation between the tensile strength during polarizing film manufacturing, the breaking tension and breaking stress in the tensile test, and the number of cracks occurring in the heat cycle test. Furthermore, when a polarizing film with a specific cut is subjected to a tensile test at a specific test speed, it can be seen that the coefficient of variation of the breaking tension and breaking stress is small, meaning that the numerical variation is small (Examples 1-5). From the above, by using the method of the present invention, the heat cycle resistance of polarizing plates and polarizing films can be evaluated with high reliability in a simple operation without the need to manufacture polarizing plates.

Claims

1. A method for evaluating the heat cycle resistance of a polarizing plate having a polarizing film made of polyvinyl alcohol-based film, comprising performing the following operations A to C in order. Operation A: Obtain a test piece from the stretched film with dimensions of 5 cm in the stretching direction and 3 cm in the perpendicular direction (this will be called test piece 1). Operation B: Make a cut of 1 to 20 mm in the stretching direction in the center of the test piece 1 (this will be called test piece 2). Operation C: Set the chuck distance of the tensile testing machine to 10 mm, and pull the test piece 2 perpendicular to the stretching direction of the stretched film at a tensile speed of 1 to 15 mm / min. However, the stretched film is the polarizing film.

2. The evaluation method according to claim 1, wherein the thickness of the stretched film is 10 to 40 μm.

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