Interlayer adhesive, image display device, and touch panel

Abstract

This interlayer adhesive contains a polyvinyl acetal resin which comprises a specific structural unit (a), a specific structural unit (b), and a specific structural unit (c), wherein the integrated value of a peak attributed to a methylene C atom at a specific position in the structural unit (a), the integrated value of a peak attributed to a methylene C atom at a specific position in the structural unit (b), and the integrated value of a peak attributed to a methylene C atom at a specific position in the structural unit (c) in the 13C-NMR spectrum satisfy a specific relationship. With respect to this interlayer adhesive, the content of alkali metal and alkaline earth metal ions is more than 5 ppm but not more than 60 ppm.　The present invention makes it possible to provide an interlayer adhesive that contains a polyvinyl acetal resin and that is capable of suppressing whitening of a sensor glass even in a state in which the humidity and the temperature are high.

Description

Interlayer adhesive, image display device, and touch panel

[0001] The present invention relates to an interlayer adhesive, an image display device, and a touch panel.

[0002] In recent years, touch panels that allow information to be input by touching with a finger have been installed in mobile information terminals such as smartphones and tablets, in-vehicle displays, and the like. Touch panels have a multilayer structure formed from components such as a cover glass, a sensor glass, and a liquid crystal panel. The spaces between the layers of each component are filled with a filler material to improve the transparency, brightness, contrast, and other aspects of the display screen, thereby enhancing visibility. For example, Patent Document 1 discloses an invention relating to an interlayer filler material for touch panels, which contains a polyvinyl acetal resin, a reactive diluent, and a photopolymerization initiator, and describes that a touch panel laminate can be obtained that has excellent conformability to uneven surfaces and is resistant to cracking and breakage.

[0003] Regarding polyvinyl acetal resins, the relationship between the arrangement of structural units constituting the molecular chain and their physical properties has been investigated. For example, it has been reported that the fluidity of polyvinyl acetal resins can be increased by adjusting the OH block degree while keeping the degree of acetalization within a specific range (Patent Document 2), or that the fluidity of polyvinyl acetal resins can be reduced by adjusting the molar ratio of the amount of three-membered syndiotactic continuous arrangement of vinyl alcohol groups while keeping the degree of acetalization within a specific range (Patent Document 3).

[0004] International Publication No. 2016 / 186029 Japanese Patent Application Laid-Open No. 2011-127117 Special Publication No. 2019-513865

[0005] As described in Patent Document 1, using polyvinyl acetal resin as a material for bonding between layers of a touch panel increases impact absorption and reduces cracking and breakage. However, the inventors' research has revealed that using polyvinyl acetal resin as an interlayer adhesive in a capacitive touch panel equipped with a sensor glass causes the sensor glass to whiten under the operating conditions of the touch panel (when a voltage is applied), particularly under high humidity and temperature conditions. Whitening reduces the visibility of the touch panel, and therefore improvement is required.

[0006] Therefore, an object of the present invention is to provide an interlayer adhesive containing a polyvinyl acetal resin, which is capable of suppressing whitening of the sensor glass even under conditions of high humidity and temperature, and an image display device and a touch panel that include the interlayer adhesive.

[0007] As a result of extensive investigation, the present inventors have found that １３ The inventors have found that the above-mentioned problems can be solved by using an interlayer adhesive containing a polyvinyl acetal resin having a C-NMR spectrum and having a specific range of alkali metal and alkaline earth metal ion content, and have completed the present invention.

[0008] [1] A structural unit (a) represented by the following formula (a), a structural unit (b) represented by the following formula (b), and a structural unit (c) represented by the following formula (c), １３ Obtained by C-NMR (nuclear magnetic resonance) measurement １３ The interlayer adhesive contains a polyvinyl acetal resin that satisfies the following formula (1), where I(a)' is the integral of the peak assigned to the methylene C atom at position (a)' of the structural unit (a), I(b)' is the integral of the peak assigned to the methylene C atom at position (b)' of the structural unit (b), and I(c)' is the integral of the peak assigned to the methylene C atom at position (c)' of the structural unit (c) in a C-NMR spectrum, and the content of alkali metal and alkaline earth metal ions is more than 5 ppm and not more than 60 ppm. I(a)' / (I(a)'+I(b)'+I(c)')>0.22 (1) R in formula (b) and formula (c) １ , R ２ and R ３ are each hydrogen or a hydrocarbon group having 1 to 9 carbon atoms. [2] The interlayer adhesive according to the above item [1], wherein the content of Na ions is more than 5 ppm and not more than 50 ppm. [3] The interlayer adhesive according to the above item [1] or [2], wherein the weight average molecular weight (Mw) of the polyvinyl acetal resin is 220,000 or more. [4] A polyvinyl acetal resin comprising a structural unit (d) represented by the following formula (d) and a structural unit (e) represented by the following formula (e), １３ Obtained by C-NMR (nuclear magnetic resonance) measurement １３ The interlayer adhesive according to any one of the above [1] to [3], wherein the following formula (2) is satisfied when I(d)' is the integral value of the peak assigned to the methine C atom at position (d)' of the structural unit (d) in a C-NMR spectrum, and I(e)' is the integral value of the peak assigned to the methine C atom at position (e)' of the structural unit (e): I(d)' / (I(d)'+I(e)')<0.9 (2) R in the formula (d) and the formula (e) ４ and R ５and each represent hydrogen or a hydrocarbon group having from 1 to 9 carbon atoms. [5] The interlayer adhesive according to any one of [1] to [4] above, wherein a measurement sample is prepared using the interlayer adhesive, in which a cover glass, a resin layer made of the interlayer adhesive, and a sensor glass having an ITO pattern formed thereon are laminated in this order, and a current-applied humidity test is performed for 1,000 hours at a voltage of 5 V, a temperature of 60°C, and a relative humidity of 90%. After that, the distribution of sodium in the depth direction from the surface of the resin layer in contact with the sensor glass at the end of the measurement sample is measured by glow discharge optical emission spectroscopy (GD-OES). The ratio (I1 / I2) of the maximum sodium emission intensity (I1) in a depth range of 0.5 to 5 μm from the surface to the maximum sodium emission intensity (I2) at a depth of 10 μm from the surface is 2.5 or less. [6] The interlayer adhesive according to any one of [1] to [5] above, wherein the polyvinyl acetal resin has a hydroxyl group content of from 25 mol % to 35 mol %. [7] The interlayer adhesive according to any one of [1] to [6] above, wherein the polyvinyl acetal resin has a degree of acetylation of 1.5 mol% or less and a degree of acetalization of 65 mol% to 75 mol%. [8] An interlayer adhesive for a touch panel, comprising the interlayer adhesive according to any one of [1] to [7] above. [9] An image display device having the interlayer adhesive according to any one of [1] to [7] above.

[10] A touch panel having the interlayer adhesive according to any one of [1] to [7] above.

[0009] According to the present invention, it is possible to provide an interlayer adhesive containing a polyvinyl acetal resin, which is capable of suppressing whitening even under conditions of high humidity and high temperature, and an image display device and a touch panel having the interlayer adhesive.

[0010] 1 is a schematic cross-sectional view showing one embodiment of a laminate comprising the interlayer adhesive of the present invention; 2 is a schematic cross-sectional view showing one embodiment of a laminate comprising the interlayer adhesive of the present invention; 3 is a schematic cross-sectional view showing one embodiment of a laminate comprising the interlayer adhesive of the present invention;

[0011] <Interlayer adhesive> The interlayer adhesive of the present invention is an interlayer adhesive that contains a polyvinyl acetal resin that satisfies formula (1) and has an alkali metal and alkaline earth metal ion content of more than 5 ppm and not more than 60 ppm. According to the present invention, an interlayer adhesive that can suppress whitening even under high humidity and high temperature conditions can be provided. Note that high humidity and high temperature conditions refer to, for example, the conditions of the moist heat resistance test described below. Therefore, the interlayer adhesive of the present invention can suppress whitening in the moist heat resistance test described below.

[0012] (Polyvinyl acetal resin) The polyvinyl acetal resin contained in the interlayer adhesive of the present invention contains a structural unit (a) represented by formula (a), a structural unit (b) represented by formula (b), and a structural unit (c) represented by formula (c).

[0013] The polyvinyl acetal resin in the present invention is １３ Obtained by C-NMR (nuclear magnetic resonance) measurement １３ The integral values ​​I(a)', I(b)', and I(c)' in the C-NMR spectrum satisfy the following formula (1). Here, the integral value I(a)' is the integral value of the peak to which the methylene C atom at the position (a)' of the structural unit (a) belongs, the integral value I(b)' is the integral value of the peak to which the methylene C atom at the position (b)' of the structural unit (b) belongs, and the integral value I(c)' is the integral value of the peak to which the methylene C atom at the position (c)' of the structural unit (c). I(a)' / (I(a)'+I(b)'+I(c)')>0.22 (1) R in formulas (b) and (c) １ , R ２ and R ３ are each hydrogen or a hydrocarbon group having 1 to 9 carbon atoms.

[0014] ( １３C-NMR spectrum) As described above, the polyvinyl acetal resin of the present invention satisfies the above formula (1). If the polyvinyl acetal resin does not satisfy the above formula (1), it becomes difficult to suppress whitening in a moist heat resistance test using an interlayer adhesive containing the polyvinyl acetal resin. If the polyvinyl acetal resin satisfies formula (1), that is, if I(a)' / (I(a)'+I(b)'+I(c)') is greater than 0.22, whitening in the moist heat resistance test is easily suppressed. The reason for this is unclear, but is thought to be as follows: Due to the reactivity of vinyl acetate in the polymerization of polyvinyl acetate, the probability of head-to-head bonds and tail-to-tail bonds occurring in polyvinyl acetate is approximately 1 to 3%. Therefore, it is believed that the blocking of hydroxyl groups in polyvinyl acetal resins made from polyvinyl acetate as a raw material can be discussed in terms of the ratio of the above structural units (a) to (c) based on head-to-tail bonds. When the value of I(a)' / (I(a)'+I(b)'+I(c)') of a polyvinyl acetal resin is large, it means that the polyvinyl acetal contains a large number of structural units (a) represented by formula (a), in which structural units having hydroxyl groups are present consecutively (the blocking property of hydroxyl groups is high). Whitening is presumed to occur due to the presence of alkali metal and alkaline earth metal ions and water, and a polyvinyl acetal resin with a high blocking property of hydroxyl groups is highly effective in making water less mobile, which makes it easier to suppress aggregation of alkali metal and alkaline earth metal ions with water, and as a result, it is thought that whitening can be suppressed.

[0015] From the viewpoint of easily suppressing whitening in a moist heat resistance test using an interlayer adhesive containing a polyvinyl acetal resin, the polyvinyl acetal resin preferably satisfies the following formula (1-a), more preferably satisfies the following formula (1-b), even more preferably satisfies the following formula (1-c), and even more preferably satisfies the following formula (1-d): I(a)' / (I(a)'+I(b)'+I(c)')≧0.24 (1-a) I(a)' / (I(a)'+I(b)'+I(c)')≧0.26 (1-b) I(a)' / (I(a)'+I(b)'+I(c)')≧0.28 (1-c) I(a)' / (I(a)'+I(b)'+I(c)')≧0.29 (1-d)

[0016] The upper limit of the value of I(a)' / (I(a)'+I(b)'+I(c)') in the polyvinyl acetal resin of the present invention is not particularly limited, but is, for example, 1, preferably 0.4, and more preferably 0.35.

[0017] I(a)' is the integral of the peak assigned to the methylene C atom at position (a)' of the structural unit (a), for example, the integral in the interval from 44.6 to 46 ppm. I(b)' is the integral of the peak assigned to the methylene C atom at position (b)' of the structural unit (b), for example, the integral in the interval from 43 to 44.6 ppm. I(c)' is the integral of the peak assigned to the methylene C atom at position (c)' of the structural unit (c), for example, the integral in the interval from 43 to 44.6 ppm. Since the peak assigned to the methylene C atom at the position (b)' of the structural unit (b) and the peak assigned to the methylene C atom at the position (c)' of the structural unit (c) both exist in the range of 43 to 44.6 ppm, (I(b)' + I(c)') is the integral value of the peaks existing in the range of 43 to 44.6 ppm.

[0018] The value of I(a)' / (I(a)'+I(b)'+I(c)') in a polyvinyl acetal resin can be adjusted by the production conditions when producing the polyvinyl acetal resin. For example, as described below, this value can be adjusted by the aging temperature in the aging step performed when producing the polyvinyl acetal resin. For example, it is thought that setting the aging temperature to a relatively high temperature promotes the transition of the polyvinyl acetal resin from a racemo structure to a meso structure, and as a result, the hydroxyl groups in the polyvinyl acetal resin become block-like, thereby increasing the value of I(a)' / (I(a)'+I(b)'+I(c)').

[0019] Polyvinyl acetal resin １３ The C-NMR spectrum can be measured, for example, by the method described in the Examples.

[0020] The polyvinyl acetal resin in the present invention preferably contains a structural unit (d) represented by the following formula (d) and a structural unit (e) represented by the following formula (e). The polyvinyl acetal resin preferably satisfies the following formula (2). In the formula (2), I(d)' is １３ I(e)' in formula (2) is the integral value of the peak assigned to the methine C atom at position (d)' of the structural unit (d) in the C-NMR spectrum. １３ This is the integral value of the peak assigned to the methine C atom at position (e)' of the structural unit (e) in the C-NMR spectrum.

[0021]

[0022]

[0023] In addition, R in formula (d) and formula (e) ４ and R ５ are each hydrogen or a hydrocarbon group having 1 to 9 carbon atoms.

[0024] I(d)' / (I(d)'+I(e)')<0.9 (2)

[0025] When the polyvinyl acetal resin satisfies the above formula (2), the polyvinyl acetal resin is likely to have good fluidity, flexibility, etc. at low temperatures, and the formability of the interlayer adhesive can be improved. From this viewpoint, the polyvinyl acetal resin of the present invention more preferably satisfies the following formula (2-a), even more preferably satisfies the following formula (2-b), and particularly preferably satisfies the following formula (2-c): I(d)' / (I(d)'+I(e)')≦0.85 (2-a) I(d)' / (I(d)'+I(e)')≦0.82 (2-b) I(d)' / (I(d)'+I(e)')≦0.80 (2-c)

[0026] The lower limit of the range of the value of I(d)' / (I(d)'+I(e)') in the polyvinyl acetal resin of the present invention is not particularly limited, but is, for example, 0.5, preferably 0.6, and more preferably 0.65.

[0027] I(d)' is the integral of the peak assigned to the methine C atom at position (d)' of the structural unit (d), for example, the integral in the range of 98.5 to 101.4 ppm. I(e)' is the integral of the peak assigned to the methine C atom at position (e)' of the structural unit (e), for example, the integral in the range of 92.5 to 94.5 ppm.

[0028] The value of I(d)' / (I(d)'+I(e)') in a polyvinyl acetal resin represents the proportion of meso structure in the polyvinyl acetal resin. It is believed that a decrease in the proportion of meso structure in a polyvinyl acetal resin improves the flexibility of the polyvinyl acetal resin.

[0029] The value of I(d)' / (I(d)'+I(e)') in the polyvinyl acetal resin of the present invention can be adjusted by, for example, the production conditions when producing the polyvinyl acetal resin. Specifically, as will be described later, the value can be adjusted by appropriately adjusting the aging temperature in the aging step performed when producing the polyvinyl acetal resin so that it does not become too high.

[0030] The R in the above formulas (b) to (e) １ ~R５ Examples of the alkyl group include an n-propyl group, an isopropyl group, an n-butyl group, a 1-ethylpropyl group, an n-pentyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, hydrogen, a methyl group, and an aryl group. Among the above, an alkyl group having about 1 to 6 carbon atoms, such as an n-propyl group, an n-butyl group, or an n-pentyl group, is preferred, and an n-propyl group is more preferred. Therefore, the polyvinyl acetal resin is preferably a polyvinyl butyral resin.

[0031] (Weight Average Molecular Weight) The weight average molecular weight (Mw) of the polyvinyl acetal resin in the present invention is preferably 220,000 or more. When the weight average molecular weight (Mw) of the polyvinyl acetal resin is 220,000 or more, the impact resistance of the polyvinyl acetal resin can be improved. From this viewpoint, the weight average molecular weight (Mw) of the polyvinyl acetal resin is more preferably 230,000 or more, and even more preferably 240,000 or more. Furthermore, the weight average molecular weight (Mw) of the polyvinyl acetal resin in the present invention is preferably 310,000 or less. When the weight average molecular weight (Mw) of the polyvinyl acetal resin is 310,000 or less, the interlayer adhesive has sufficient flexibility, and as a result, the impact energy absorption of the interlayer adhesive can be further improved. From this viewpoint, the weight average molecular weight (Mw) of the polyvinyl acetal resin is more preferably 305,000 or less, and even more preferably 300,000 or less. The weight average molecular weight (Mw) of the polyvinyl acetal resin is measured by gel permeation chromatography.

[0032] (Hydroxyl Group Amount) The hydroxyl group amount of the polyvinyl acetal resin in the present invention is preferably 25 mol% or more, and preferably 35 mol% or less. By setting the hydroxyl group amount to 25 mol% or more, it becomes easier to adjust the value of the above-mentioned formula (1) to the desired range, and it becomes easier to suppress whitening in a moist heat resistance test using an interlayer adhesive. Furthermore, by setting the hydroxyl group amount to 35 mol% or less, it becomes easier to ensure flexibility and improve moldability. The hydroxyl group amount is more preferably 27 mol% or more, and even more preferably 30 mol% or more. Furthermore, the hydroxyl group amount is more preferably 34% or less, and even more preferably 32 mol% or less. When a polyvinyl butyral resin is used as the polyvinyl acetal resin, from the same viewpoint, the hydroxyl group amount is 25 mol% or more, and preferably 35 mol% or less, more preferably 27 mol% or more, even more preferably 30 mol% or more, more preferably 34% or less, and even more preferably 32 mol% or less. The amount of hydroxyl groups in the polyvinyl acetal resin is the molar fraction calculated by dividing the amount of ethylene groups having hydroxyl groups by the total amount of ethylene groups in the main chain, and is expressed as a percentage. The amount of ethylene groups having hydroxyl groups can be measured by the procedure described in the Examples.

[0033] (Degree of Acetylation) The degree of acetylation of the polyvinyl acetal resin of the present invention is preferably 1.5 mol% or less, more preferably 1 mol% or less, and even more preferably 0.8 mol% or less. When the degree of acetylation is below the upper limit, the moisture resistance of the interlayer adhesive is improved. Furthermore, the degree of acetylation is not particularly limited, but is preferably 0.01 mol% or more, more preferably 0.1 mol% or more. The degree of acetylation is a molar fraction calculated by dividing the amount of ethylene groups bonded to acetyl groups by the total amount of ethylene groups in the main chain, expressed as a percentage. The amount of ethylene groups bonded to acetyl groups can be measured by the procedure described in the Examples.

[0034] (Degree of Acetalization) The degree of acetalization of the polyvinyl acetal resin in the present invention is 65 mol% or more and 75 mol% or less. When the degree of acetalization of the polyvinyl acetal resin is 65 mol% or more, the flexibility of the polyvinyl acetal resin is increased and the moldability is improved. Furthermore, when the degree of acetalization of the polyvinyl acetal resin is 75 mol% or less, the amount of hydroxyl groups in the polyvinyl acetal resin increases, making it easier to adjust the value of the above-mentioned formula (1) within the desired range and making it easier to suppress whitening in a moist heat resistance test using an interlayer adhesive. From this perspective, the degree of acetalization of the polyvinyl acetal resin in the present invention is preferably 66 mol% or more and 74 mol% or less, more preferably 67 mol% or more and 73 mol% or less. Note that the degree of acetalization means the degree of butyralization when the acetal group is a butyral group and the polyvinyl acetal resin is a polyvinyl butyral resin.

[0035] The degree of acetalization is a molar fraction calculated by subtracting the amount of ethylene groups having hydroxyl groups and the amount of ethylene groups having acetyl groups from the total amount of ethylene groups in the main chain, and dividing the result by the total amount of ethylene groups in the main chain. The degree of acetalization (degree of butyralization) can be calculated based on the amount of ethylene groups having hydroxyl groups and the amount of ethylene groups having acetyl groups, which are calculated by the procedures described in the Examples.

[0036] (Aldehyde) The polyvinyl acetal resin in the present invention is preferably a polyvinyl acetal resin obtained by acetalizing polyvinyl alcohol (PVA) with an aldehyde. The aldehyde is not particularly limited, but generally, an aldehyde having 1 to 10 carbon atoms is suitably used. The aldehyde having 1 to 10 carbon atoms is not particularly limited, and examples thereof include n-butyl aldehyde, isobutyraldehyde, n-valeraldehyde, 2-ethylbutyraldehyde, n-hexyl aldehyde, n-octyl aldehyde, n-nonyl aldehyde, n-decyl aldehyde, formaldehyde, acetaldehyde, and benzaldehyde. These aldehydes may be used alone or in combination of two or more. Among the above, n-butylaldehyde, n-hexylaldehyde, and n-valeraldehyde are preferred, and n-butylaldehyde is more preferred. Therefore, the polyvinyl acetal resin is preferably a polyvinyl butyral resin.

[0037] (Polyvinyl Alcohol (PVA)) Polyvinyl alcohol (PVA) can be obtained, for example, by saponifying a polyvinyl ester such as polyvinyl acetate. The degree of saponification of polyvinyl alcohol is generally 70 to 99.9 mol %. The average degree of polymerization of PVA is preferably 200 or more, more preferably 500 or more, even more preferably 1000 or more, and even more preferably 1500 or more. The average degree of polymerization of PVA is preferably 5000 or less, more preferably 4000 or less, even more preferably 3500 or less, and even more preferably 2500 or less. The average degree of polymerization of polyvinyl alcohol is determined by a method in accordance with JIS K6726 "Testing Methods for Polyvinyl Alcohol." When two or more types of polyvinyl alcohol are used as raw materials, the average degree of polymerization of the polyvinyl alcohol can be estimated by calculation from the average degrees of polymerization of each polyvinyl alcohol.

[0038] Two or more polyvinyl alcohols having different average degrees of polymerization may be used as the polyvinyl alcohol raw material for the polyvinyl acetal resin. When two or more polyvinyl alcohols are used, for example, it is preferable to use a first polyvinyl alcohol having an average degree of polymerization of 1500 or more and a second polyvinyl alcohol having an average degree of polymerization of 1000 or less. The average degree of polymerization of the first polyvinyl alcohol is preferably 1500 or more and 3500 or less, more preferably 1600 or more and 2500 or less, and even more preferably 1600 or more and 2000 or less. The average degree of polymerization of the second polyvinyl alcohol is preferably 200 or more and 1000 or less, more preferably 300 or more and 900 or less, and even more preferably 400 or more and 700 or less. When the first and second polyvinyl alcohols are used, the blending ratio of the first polyvinyl alcohol to the second polyvinyl alcohol is not particularly limited, but the blending amount of the second polyvinyl alcohol relative to the total amount of the first and second polyvinyl alcohols is preferably 1% by mass or more and 50% by mass or less, more preferably 2% by mass or more and 40% by mass or less, even more preferably 3% by mass or more and 30% by mass or less, and still more preferably 5% by mass or more and 15% by mass or less.

[0039] (Modified Polyvinyl Acetal Resin) The polyvinyl acetal resin in the present invention may be an unmodified polyvinyl acetal resin, but may also be a modified polyvinyl acetal resin. The modified polyvinyl acetal resin has a structure (modifying group) other than an acetal group, a hydroxyl group, and an acetyl group, and preferably has a modifying group on the side chain. Examples of the modifying group include those having a polyalkylene oxide structure on the side chain, and those having an alkyl group (e.g., having about 2 to 30 carbon atoms) other than an acetal group or an acetyl group on the side chain. The modification amount is not particularly limited, but is, for example, about 0.1 mol % to 10 mol %. The modification amount represents the ratio of functional groups to all vinyl monomer units constituting the polyvinyl acetal resin.

[0040] (Method for Producing Polyvinyl Acetal Resin) The polyvinyl acetal resin of the present invention is preferably produced by a production method including a mixing step of mixing the polyvinyl alcohol and the aldehyde, and an aging step of aging the mixture obtained in the mixing step.

[0041] In the mixing step, polyvinyl alcohol and aldehyde may be mixed according to a conventional method. In addition to polyvinyl alcohol and aldehyde, a catalyst such as an acid catalyst may be added to promote the acetalization reaction. For example, the aldehyde may be added to a mixture of polyvinyl alcohol and an acid catalyst at a low temperature of about 0 to 40°C. A solvent such as water is usually added. When two or more polyvinyl alcohols are used in combination (for example, when two or more polyvinyl alcohols with different molecular weights are used), the two or more polyvinyl alcohols may be mixed with the aldehyde.

[0042] The aging step is not particularly limited, but may involve, for example, adding a catalyst such as an acid catalyst to the mixture (reaction mixture) obtained by the mixing step, heating to the aging temperature, and maintaining the mixture at the aging temperature for a certain period of time. In this production method, acetalization of polyvinyl alcohol proceeds in the mixing step and the aging step to obtain a polyvinyl acetal resin. After maintaining the reaction mixture at the aging temperature for a certain period of time, the reaction mixture may be appropriately cooled and neutralized, and then washed with water, dried, or the like, as necessary.

[0043] Examples of the acid catalyst added in the mixing step and the aging step include inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid, and boric acid. The concentration of the acid catalyst in the aging step is adjusted to, for example, about 0.5% by mass to about 5% by mass, and preferably about 1% by mass to about 2.5% by mass. The aging step is preferably carried out at a relatively high temperature, for example, 50°C to about 80°C, preferably 60°C to about 78°C, and more preferably 65°C to about 75°C. The time (aging time) maintained at the aging temperature is, for example, 75 minutes to about 180 minutes, preferably 90 minutes to about 150 minutes, and more preferably 100 minutes to about 140 minutes.

[0044] When the aging temperature and aging time are within the above-described desired ranges, the polyvinyl acetal resin is more likely to satisfy the requirements of the above-described formula (1), and whitening in a moist heat resistance test using an interlayer adhesive containing the polyvinyl acetal resin is more likely to be suppressed.

[0045] If the acid catalyst added in the mixing and aging steps remains in the polyvinyl acetal resin, it can cause discoloration such as yellowing, and therefore is preferably neutralized with a neutralizing agent. Examples of neutralizing agents include sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, potassium carbonate, and potassium bicarbonate. Of these, sodium bicarbonate or potassium bicarbonate is preferred. As described above, a compound containing an alkali metal is generally used as the neutralizing agent. Therefore, in order to maintain the content of alkali metal and alkaline earth metal ions in the interlayer adhesive below a certain level, as described below, it is preferable to minimize the amount of neutralizing agent used. The amount of neutralizing agent used relative to the polyvinyl acetal resin is preferably 0.1 to 5% by mass, more preferably 0.3 to 3% by mass, and even more preferably 0.5 to 1.5% by mass, based on 100% by mass of the polyvinyl acetal resin.

[0046] After the aging step, a water-washing step is preferably performed to wash the polyvinyl acetal resin with water. By performing the water-washing step, the content of alkali metal and alkaline earth metal ions, such as Na ions, is reduced, which makes it easier to suppress whitening in a moist heat resistance test using an interlayer adhesive containing the polyvinyl acetal resin. The water-washing step is preferably performed with an excess amount of water relative to the polyvinyl alcohol resin. The water-washing step may be performed once or twice or more times. Repeated water-washing steps make it easier to further reduce the content of alkali metal and alkaline earth metal ions. However, repeated water-washing steps reduce production efficiency, so it is preferable to perform the production step few times, and it is preferable to perform the water-washing step once. In this regard, the polyvinyl acetal resin of the present invention has a specific structure that satisfies the above-mentioned formula (1), and therefore whitening in a moist heat resistance test is easily suppressed. Therefore, even if the number of water-washing steps is small, whitening in a moist heat resistance test can be suppressed.

[0047] (Alkali Metal and Alkaline Earth Metal Ion Content) The interlayer adhesive of the present invention has an alkali metal and alkaline earth metal ion content (mass) of more than 5 ppm and not more than 60 ppm. When the alkali metal and alkaline earth metal ion content is 60 ppm or less, whitening is easily suppressed in a moist heat resistance test using an interlayer adhesive containing a polyvinyl acetal resin. Furthermore, considering the suitability of using a neutralizing agent during the production of polyvinyl acetal resin as described above and the production efficiency depending on the number of cleaning steps, the alkali metal and alkaline earth metal ion content in the interlayer adhesive is more than 5 ppm. From this perspective, the alkali metal and alkaline earth metal ion content in the interlayer adhesive is more preferably 6 ppm or more and 50 ppm or less, even more preferably 8 ppm or more and 40 ppm or less, and even more preferably 9.5 ppm or more and 30 ppm or less. The alkali metal and alkaline earth metal ion content refers to the total content of alkali metal ions and alkaline earth metal ions.

[0048] Alkali metals refer to Li (lithium), Na (sodium), K (potassium), Rb (rubidium), and Cs (cesium) among the Group 1 elements of the periodic table, and alkaline earth metals refer to Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), and Ba (barium) among the Group 2 elements. The content of alkali metals and alkaline earth metal ions refers to the total content of alkali metals and alkaline earth metal ions in the interlayer adhesive. In addition, the content unit "ppm" in this specification is based on mass and means "mass ppm." The content of alkali metals and alkaline earth metal ions can be adjusted to a desired value by adjusting the manufacturing method of the polyvinyl acetal resin and the type and amount of additives blended into the interlayer adhesive.

[0049] The alkali metal and alkaline earth metal contained in the interlayer adhesive of the present invention are at least one metal selected from the group consisting of Na, K, and Mg, and the amount of the metal ions is preferably more than 5 ppm and not more than 60 ppm, more preferably 5 ppm to 50 ppm, even more preferably 8 ppm to 40 ppm, and even more preferably 10 ppm to 30 ppm. The metal amount refers to the total amount of Na, K, and Mg. By adjusting the amount of these specific alkali metal and alkaline earth metal ions to a certain level or less, whitening of the sensor glass can be effectively suppressed in a moist heat resistance test using the interlayer adhesive. The amount of alkali metal and alkaline earth metal ions in the interlayer adhesive can be measured by ICP atomic emission spectrometry.

[0050] (Na ion content) The Na ion content (mass) in the interlayer adhesive of the present invention is preferably 5 ppm or more and 60 ppm or less, more preferably more than 5 ppm and 50 ppm or less, even more preferably 6 ppm or more and 50 ppm or less, even more preferably 6.5 ppm or more and 40 ppm or less, even more preferably 10 ppm or more and 30 ppm or less, even more preferably 25 ppm or less, and even more preferably 15 ppm or less.

[0051] (Distribution of Na in the Depth Direction) When a measurement sample is prepared using the interlayer adhesive of the present invention, in which a cover glass, a resin layer made of the interlayer adhesive, and a sensor glass on which an ITO pattern has been formed are laminated in this order, and an electric current humidity resistance test is performed, it is preferable that the distribution of Na in the depth direction from the surface of the resin layer in contact with the sensor glass at the end of the measurement sample is as follows: The electric current humidity resistance test is performed for 1000 hours while maintaining a voltage of 5 V, a temperature of 60°C, and a relative humidity of 90%.

[0052] When measuring the distribution of Na in the depth direction from the surface using glow discharge optical emission spectroscopy (GD-OES), the ratio (I1 / I2) of the maximum Na emission intensity (I1) in the depth direction from the surface to the Na emission intensity (I2) at a position 10 μm from the surface in the depth direction is preferably 2.5 or less. When the ratio (I1 / I2) is 2.5 or less, the amount of Na ions at the surface of the resin layer is lower than that in the interior, making it easier to suppress whitening of the sensor glass. From this perspective, the ratio (I1 / I2) is preferably 2 or less, more preferably 1.5 or less, and usually 1 or more. Note that the maximum Na emission intensity (I1) in the depth direction from the surface to 0.5 μm is the maximum value of the measured values ​​when Na emission intensity is measured at equal intervals from 0.5 to 5 μm. The measurement sample, in which a cover glass, a resin layer made of an interlayer adhesive, and a sensor glass on which an ITO pattern is formed are laminated in this order, is prepared by bringing the surface of the sensor glass on which the ITO (indium tin oxide) pattern is formed into contact with the resin layer made of the interlayer adhesive. The details of the measurement of the luminescence intensity ratio (I1 / I2) are as described in the Examples.

[0053] (Other Resins) The interlayer adhesive may contain a resin other than polyvinyl acetal resin, as long as the effects of the present invention are not impaired. Examples of resins other than polyvinyl acetal resin include ethylene-vinyl acetate copolymer resin, ionomer resin, polyurethane resin, thermoplastic elastomer, and acrylic resin. Based on the total amount of resins contained in the interlayer adhesive, the content of polyvinyl acetal resin is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 100% by mass.

[0054] (Plasticizer) The interlayer adhesive preferably contains a plasticizer. By including a plasticizer in the interlayer adhesive together with the polyvinyl acetal resin, the impact absorption properties can be further improved. Furthermore, by including a plasticizer in the interlayer adhesive, the interlayer adhesive becomes more flexible and can be easily adjusted to a predetermined shape.

[0055] Examples of the plasticizer include organic ester plasticizers, organic phosphorus-based plasticizers such as organic phosphate ester plasticizers and organic phosphite ester plasticizers, organic ether-based plasticizers such as polyalkylene glycol-based plasticizers, and alcohol-based plasticizers. One type of plasticizer may be used alone, or two or more types may be used in combination. Among the above, organic ester plasticizers and organic ether-based plasticizers are preferred.

[0056] Preferred organic ester plasticizers include monobasic organic acid esters and polybasic organic acid esters. Examples of monobasic organic acid esters include esters of glycols and monobasic organic acids. Examples of glycols include polyalkylene glycols in which each alkylene unit has 2 to 4 carbon atoms, preferably 2 or 3 carbon atoms, and the number of repeating alkylene units is 2 to 10, preferably 2 to 4. Examples of glycols include monoalkylene glycols having 2 to 4 carbon atoms, preferably 2 or 3 carbon atoms (i.e., one repeating unit). Specific examples of glycols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, and butylene glycol. Examples of the monobasic organic acid include organic acids having 3 to 10 carbon atoms, and specific examples thereof include butyric acid, isobutyric acid, caproic acid, 2-ethylbutyric acid, heptyl acid, n-octylic acid, 2-ethylhexyl acid, n-nonylic acid, and decylic acid.

[0057] Specific examples of the monobasic organic acid ester include triethylene glycol di-2-ethylbutyrate, triethylene glycol di-2-ethylhexanoate, triethylene glycol dicaprylate, triethylene glycol di-n-octanoate, triethylene glycol di-n-heptanoate, tetraethylene glycol di-n-heptanoate, tetraethylene glycol di-2-ethylhexanoate, diethylene glycol di-2-ethylbutyrate, diethylene glycol di-2-ethylhexanoate, dipropylene glycol di-2-ethylbutyrate, and triethylene glycol di-n-heptanoate. Examples of suitable alkyl acrylate copolymers include ethylene glycol di-2-ethylpentanoate, tetraethylene glycol di-2-ethylbutyrate, diethylene glycol dicaprylate, triethylene glycol di-n-heptanoate, tetraethylene glycol di-n-heptanoate, triethylene glycol di-2-ethylbutyrate, ethylene glycol di-2-ethylbutyrate, 1,2-propylene glycol di-2-ethylbutyrate, 1,3-propylene glycol di-2-ethylbutyrate, 1,4-butylene glycol di-2-ethylbutyrate, and 1,2-butylene glycol di-2-ethylbutyrate.

[0058] Examples of polybasic organic acid esters include ester compounds of dibasic organic acids having 4 to 12 carbon atoms, such as adipic acid, sebacic acid, and azelaic acid, with alcohols having 4 to 10 carbon atoms. The alcohols having 4 to 10 carbon atoms may be linear, branched, or cyclic. Specific examples include dibutyl sebacate, dioctyl azelaate, dihexyl adipate, dioctyl adipate, hexylcyclohexyl adipate, diisononyl adipate, heptylnonyl adipate, dibutyl carbitol adipate, and mixed adipates. Oil-modified alkyd sebacate is also suitable. Examples of mixed adipates include adipates prepared from two or more alcohols selected from alkyl alcohols having 4 to 9 carbon atoms and cyclic alcohols having 4 to 9 carbon atoms.

[0059] The organic ester plasticizer is not limited to the complete esters of the above-mentioned esters, but may also be a partial ester. For example, it may be a partial ester of a glycol and a monobasic organic acid, or a partial ester of a dibasic organic acid and an alcohol. Specific examples include triethylene glycol-mono-2-ethylhexanoate. Furthermore, it may be a partial ester of a monobasic organic acid with a trihydric or higher alcohol, such as glycerin. Examples of monobasic organic acids include monobasic organic acids having 3 to 24 carbon atoms, preferably 6 to 18 carbon atoms. Specific examples of partial esters of a trihydric or higher alcohol and a monobasic organic acid include a mono- or diester of glycerin and stearic acid, and a mono- or diester of glycerin and 2-ethylhexyl acid. Among the organic ester plasticizers listed above, triethylene glycol-di-2-ethylhexanoate (3GO) is particularly preferred.

[0060] Examples of the organic phosphorus plasticizer include phosphoric acid esters such as tributoxyethyl phosphate, isodecylphenyl phosphate, and triisopropyl phosphate.

[0061] Examples of polyalkylene glycol plasticizers include polyoxyalkylene compounds having a polyoxyalkylene structure. Specific examples include polyhydric alcohol compounds such as glycol, ester compounds of glycol with monobasic organic acids or polybasic organic acids, and ether compounds of monohydric or polyhydric alcohols and polyoxyalkylenes. Examples of glycols include polyoxyalkylene glycols and their derivatives. Examples of polyoxyalkylenes include polyoxyethylene, polyoxypropylene, polyoxybutylene, and random copolymers or block copolymers thereof. The polyoxyalkylene compounds may be polyhydric alcohol compounds, ester compounds, ether compounds, or other compounds, as described above. Examples of polyoxyalkylene compounds include polyoxyalkylenes or their derivatives. More specific examples include polyoxyalkylene glycols composed of the above polyoxyalkylenes, and ether compounds of polyoxyalkylenes and polyhydric alcohols. These may all have hydroxyl groups at their terminals, or may be derivatives in which some or all of the hydrogen atoms of the terminal hydroxyl groups have been substituted with alkyl groups or acyl groups. The number of carbon atoms in the alkyl group and acyl group is not particularly limited, but may be about 1 to 8, preferably 1 to 4. Examples of polyoxyalkylene glycols include polyoxyethylene polyoxypropylene glycols such as polyethylene glycol (polyoxyethylene glycol), polypropylene glycol (polyoxypropylene glycol), poly(ethylene oxide / propylene oxide) block copolymers and poly(ethylene oxide / propylene oxide) random copolymers, and polyoxybutylene glycols such as polytetramethylene glycol.Examples of ether compounds of polyoxyalkylenes and polyhydric alcohols include ether compounds of polyoxyalkylenes and polyhydric alcohols such as glycerol, diglycerol, trimethylolpropane, erythritol, pentaerythritol, and bisphenol A, specifically polyoxyethylene glyceryl ether, polyoxypropylene glyceryl ether, polyoxyethylene diglyceryl ether, polyoxypropylene diglyceryl ether, polyoxyalkylene pentaerythritol ether, etc. Examples of derivatives in which some or all of the hydrogen atoms of terminal hydroxyl groups have been substituted with alkyl groups or acyl groups include the above-mentioned polyoxyalkylene glycols and derivatives in which some or all of the hydrogen atoms of terminal hydroxyl groups of ether compounds have been substituted with alkyl groups or acyl groups. Specific examples include polyoxyethylene glycol monomethyl ether, polyoxyethylene glycol dimethyl ether, polyoxypropylene glycol monomethyl ether, polyoxypropylene glycol dimethyl ether, polyoxyethylene polyoxypropylene glycol monomethyl ether, polyoxyethylene polyoxypropylene glycol dimethyl ether, polyoxyethylene glycol monobutyl ether, polyoxypropylene glycol monobutyl ether, and polyoxyethylene polyoxypropylene monobutyl ether. Among the above-mentioned polyoxyalkylene compounds, those having a polyoxyethylene, polyoxypropylene, or polyoxyethylene polyoxypropylene structure are preferred, and those having a polyoxypropylene or polyoxyethylene polyoxypropylene structure are more preferred. Specific examples include polyoxyethylene polyoxypropylene glycol, polyoxypropylene glyceryl ether, polyoxypropylene diglyceryl ether, or derivatives thereof in which some of the hydrogen atoms of the terminal hydroxyl groups are substituted with alkyl groups. Examples of alcohol-based plasticizers include various polyhydric alcohols such as butanediol, hexanediol, trimethylolpropane, and pentaerythritol. Of these, trimethylolpropane is preferred.

[0062] The above plasticizers can be used alone or in combination of two or more. Among the above plasticizers, triethylene glycol-di-2-ethylhexanoate (3GO), polyoxyethylene polyoxypropylene glycol, polyoxypropylene glyceryl ether, polyoxypropylene diglyceryl ether, or derivatives thereof in which some of the hydrogen atoms of the terminal hydroxyl groups have been substituted with alkyl groups are preferred, and triethylene glycol-di-2-ethylhexanoate (3GO) is more preferred.

[0063] The content of the plasticizer in the interlayer adhesive is preferably 5 parts by mass or more, more preferably 15 parts by mass or more, and even more preferably 30 parts by mass or more, per 100 parts by mass of the polyacetal resin, and is preferably 100 parts by mass or less, more preferably 60 parts by mass or less, and even more preferably 50 parts by mass or less.

[0064] [Adhesion modifier] The interlayer adhesive of the present invention may contain an adhesion modifier from the viewpoint of improving adhesive strength. However, since compounds containing alkali metals or alkaline earth metals are generally used as adhesion modifiers, it is preferable to use as little adhesion modifier as possible from the viewpoint of keeping the content of alkali metal and alkaline earth metal ions contained in the interlayer adhesive below a certain level. When an adhesion modifier is used, its content is preferably an amount such that the content of alkali metal and alkaline earth metal ions in the interlayer adhesive containing polyvinyl acetal is 60 ppm or less, preferably 50 ppm or less, more preferably 40 ppm or less, and even more preferably 30 ppm or less. In the present invention, it is preferable that the interlayer adhesive does not contain an adhesion modifier.

[0065] Examples of the adhesion modifier include metal salts of organic acids such as carboxylic acids. Examples of the organic acid include carboxylic acids having 2 to 10 carbon atoms, and examples of the metal salts include potassium salts, sodium salts, and magnesium salts. Specific examples of the adhesion modifier include magnesium acetate, magnesium propionate, magnesium 2-ethylbutanoate, and magnesium 2-ethylhexanoate.

[0066] (Other Additives) The interlayer adhesive of the present invention may contain known additives such as tackifying resins, emulsifiers, softeners, fine particles, fillers, pigments, dyes, silane coupling agents, antioxidants, surfactants, and waxes, as needed.

[0067] (Shape) The interlayer adhesive of the present invention is preferably in the form of a film from the viewpoint of ease of application. The interlayer adhesive of the present invention may have a single-layer structure such as a single-layer film, or a multilayer structure such as a multilayer film. The interlayer adhesive of a single-layer structure may be composed of the polyvinyl acetal resin described above, or may be composed of a resin composition containing a polyvinyl acetal resin and, if necessary, a plasticizer and other additives. The composition and alkali metal and alkaline earth metal ion content of the resin composition in the interlayer adhesive of a single-layer structure may be adjusted as described above. Furthermore, when the interlayer adhesive has a multilayer structure, each layer may be composed of a polyvinyl acetal resin, or may be composed of a resin composition containing a polyvinyl acetal resin and, if necessary, a plasticizer and other additives. In the case of a multilayer structure, the interlayer adhesive may have the composition and alkali metal and alkaline earth metal ion content of the entire multilayer structure as described above, but it is also sufficient that the composition and alkali metal and alkaline earth metal ion content of each layer be as described above. When the interlayer adhesive has a multilayer structure, each layer may be formed by extrusion molding, press molding, or the like, and then laminated. For example, a method of co-extrusion using two or more extruders and attaching multilayer feed blocks to the tips of the extruders is preferred. Furthermore, when multiple layers are provided and two or more layers have the same composition, two or more layers having the same composition may be extruded from a single extruder.

[0068] (Thickness) The thickness of the interlayer adhesive of the present invention is not particularly limited and may be adjusted appropriately depending on the application. The thickness of the interlayer adhesive is preferably 0.05 mm or more, more preferably 0.1 mm or more, even more preferably 0.15 mm or more, and preferably 5 mm or less, more preferably 3 mm or less, even more preferably 1 mm or less, still more preferably 800 μm or less, and even more preferably 500 μm or less.

[0069] (Uses) The interlayer adhesive of the present invention is preferably used as an interlayer adhesive for touch panels comprising the interlayer adhesive. A touch panel comprising the interlayer adhesive for touch panels can suppress whitening of the sensor glass of the touch panel even in environments with large changes in humidity and temperature. The interlayer adhesive for touch panels may be provided in only one layer on the touch panel, or in two or more layers. The interlayer adhesive of the present invention can be used as a laminate for touch panels, image display devices, and the like in this way. An embodiment in which the interlayer adhesive of the present invention is used as part of a laminate will be described below.

[0070] (Laminate) The laminate of the present invention comprises the interlayer adhesive of the present invention and at least one transparent substrate. Examples of the transparent substrate include an organic material substrate and an inorganic material substrate.

[0071] Examples of organic material substrates include organic resin plates and resin films. Organic resin plates are also called organic glass plates. Examples of organic resin plates include, but are not limited to, polycarbonate plates, (meth)acrylic plates such as polymethyl methacrylate plates, polyester plates such as acrylonitrile-styrene copolymer plates, acrylonitrile-butadiene-styrene copolymer plates, and polyethylene terephthalate plates, and various organic glass plates such as fluorine-based resin plates, polyvinyl chloride plates, chlorinated polyvinyl chloride plates, polypropylene plates, polystyrene plates, polysulfone plates, epoxy resin plates, phenolic resin plates, unsaturated polyester resin plates, and polyimide resin plates. The organic resin plates may be subjected to appropriate surface treatments. Among the above, polycarbonate plates are preferred because of their excellent transparency and impact resistance, and (meth)acrylic plates are preferred because of their high transparency, weather resistance, and mechanical strength, with polycarbonate plates being more preferred. The thickness of the organic resin plate is not particularly limited, but is preferably 0.1 mm or more, more preferably 0.4 mm or more, and is preferably 5.0 mm or less, more preferably 3.0 mm or less.

[0072] The resin film is not particularly limited, but examples thereof include polyester resin films such as (meth)acrylic resin film, polycarbonate film, polyethylene terephthalate (PET) film, and polyethylene naphthalate (PEN) film; polyolefin resin films such as polyethylene film and polypropylene film; cyclic polyolefin (COP) film, triacetyl cellulose (TAC) film, polyethersulfone (PES) resin film, and polyimide resin film. Furthermore, a surface layer such as a hard coat layer made of a (meth)acrylic resin may be provided on the surface of the resin film. The thickness of the resin film is not particularly limited, but is preferably 30 μm or more, more preferably 50 μm or more, and also preferably 500 μm or less, and more preferably 450 μm or less. While a relatively thick, low-flexibility, and generally unbendable material is referred to as an organic resin plate, a relatively thin, generally bendable material is generally referred to as a resin film, but these are not clearly distinguishable.

[0073] Examples of inorganic material substrates include inorganic glass plates. The inorganic glass plates are not particularly limited, but include various glass plates such as float glass, tempered glass, colored glass, polished glass, patterned glass, wired glass, lined glass, ultraviolet absorbing glass, infrared reflecting glass, infrared absorbing glass, and green glass. The inorganic glass may be subjected to surface treatment. The thickness of the inorganic glass is not particularly limited, but is preferably 0.1 mm or more, more preferably 1.0 mm or more, and is preferably 5.0 mm or less, more preferably 3.0 mm or less.

[0074] The organic material substrate or inorganic material substrate may be appropriately provided with an electrode, a sensor, or the like. The electrode is composed of a conductive layer laminated on each of the above substrates. A touch sensor may be laminated on each of the above substrates as a sensor to form a substrate with a touch sensor. The touch sensor is a sensor that detects touch input when a finger, a touch pen, or other object approaches or contacts the substrate, and is composed of a conductive layer laminated on the above substrate. When a finger, a touch pen, or other object approaches or contacts the substrate, an electrical change such as capacitance, current, or voltage occurs in the conductive layer, and the touch sensor detects the touch input based on this electrical change. The conductive layer is not particularly limited, and any conventionally known electrode material having transparency can be used without particular limitation, and examples thereof include an indium tin oxide (ITO) conductive film, a tin oxide conductive film, a zinc oxide conductive film, and a polymer conductive film.

[0075] Among the above, the inorganic material substrate is preferably selected from the group consisting of an inorganic glass plate and an inorganic glass plate to which at least one of an electrode or a sensor is attached. The organic material substrate is preferably at least one selected from the group consisting of a polycarbonate plate, a (meth)acrylic plate, a PET film, a COP film, a polycarbonate film, and a film to which at least one of an electrode or a sensor is attached. Furthermore, an organic material substrate (particularly a film) on which a conductive layer such as an electrode or a sensor is laminated may have the above-mentioned hard coat layer formed on the surface opposite to the surface on which the conductive layer is provided.

[0076] The laminate is not particularly limited, but preferably has a multilayer structure of three or more layers, including a pair of transparent substrates selected from inorganic material substrates and organic material substrates, and an interlayer adhesive disposed between the pair of transparent substrates. In such a multilayer structure, the interlayer adhesive may be bonded to both of the pair of transparent substrates, thereby bonding the pair of transparent substrates via the interlayer adhesive.

[0077] The laminate may also have a structure in which another intermediate member is disposed between the pair of transparent substrates. In such a structure, an adhesive film may be disposed between each transparent substrate and the intermediate member, resulting in a multilayer structure of five or more layers. Here, the adhesive film may be adhered to each transparent substrate and the intermediate member, thereby bonding the transparent substrate and the intermediate member via the adhesive film. In the multilayer structure of five or more layers described above, the adhesive film between the transparent substrate and the intermediate member is a resin film, and at least one of them may be the interlayer adhesive of the present invention, but it is preferable that both be the interlayer adhesive of the present invention. The intermediate member may have at least one of the inorganic material substrate and the organic material substrate described above, and at least one of the inorganic material substrate and the organic material substrate may be disposed at a position where the interlayer adhesive of the present invention will adhere. Furthermore, a multilayer structure of five or more layers may have three or more transparent substrates, for example, arranged in the order of transparent substrate, transparent substrate, intermediate member, and transparent substrate, with an adhesive film disposed between each of these members. In this case, it is sufficient that any one of the adhesive films is the interlayer adhesive of the present invention. Furthermore, the intermediate member may be a substrate with a touch sensor, as will be described later, but is not limited to this.

[0078] The laminate described above may constitute an image display device, a touch panel, or the like. In this case, the image display device has the interlayer adhesive of the present invention. Furthermore, the image display device may further have a cover glass and an image display panel in addition to the interlayer adhesive of the present invention. Furthermore, the touch panel has the interlayer adhesive of the present invention. Furthermore, the touch panel may further have a touch sensor, a cover glass, and an image display device. However, the laminate described above is not limited to these.

[0079] The image display device is preferably an in-vehicle display device, and is preferably provided in the front section in front of the driver's seat. In particular, the in-vehicle display device is preferably disposed below the windshield of the automobile, in front of either the driver's seat or the passenger seat. That is, the image display device is preferably disposed in a position where a conventional instrument panel would be disposed. The image display device preferably includes an image display panel, and may also include a touch sensor, if necessary, and may be an image display device with a touch sensor.

[0080] The laminate of the present invention can be produced, for example, by preparing the interlayer adhesive of the present invention and pressing each member together via the prepared interlayer adhesive. For example, it can be produced by stacking a transparent substrate, the interlayer adhesive of the present invention, and a transparent substrate in this order and pressing them together. Furthermore, when an intermediate member is provided, it can be produced, for example, by stacking a transparent substrate, an interlayer adhesive, an intermediate member, an interlayer adhesive, and a transparent substrate in this order and pressing them together.

[0081] Next, specific examples of the laminate will be described with reference to the drawings. Fig. 1 shows an example in which the laminate is a touch panel 50. The touch panel 50 is preferably an in-vehicle touch panel. The touch panel is preferably a capacitance type. Note that the drawings show only one example of a touch panel, and the touch panel of the present invention is not limited to the drawings.

[0082] The touch panel 50 includes, in this order, a first substrate 20a, a first interlayer adhesive 10a, a second substrate 20c, a second interlayer adhesive 10b, and a touch sensor 20b. Either one or both of the first interlayer adhesive 10a and the second interlayer adhesive 10b is the interlayer adhesive of the present invention. From the viewpoint of suppressing whitening of the touch sensor 20b, which is made of a sensor glass or the like, a preferred embodiment is one in which the interlayer adhesive of the present invention is laminated on the sensor-containing surface of the touch sensor 20b. That is, in the touch panel 50, the second interlayer adhesive 10b is preferably the interlayer adhesive of the present invention, and both the first interlayer adhesive 10a and the second interlayer adhesive 10b are preferably the interlayer adhesive of the present invention. The first substrate 20a and the second substrate 20c are cover glass, and are preferably either an organic resin plate or an inorganic glass plate, but preferably an inorganic glass plate. The surface of the first substrate 20a serves as the operation surface that comes into contact with a user's fingers or the like during touch operations. In this way, the structure having two cover glasses and two interlayer adhesives alternately can provide excellent shock absorption.

[0083] The touch sensor 20b is preferably a substrate with a touch sensor (such as sensor glass), and the substrate is preferably any of inorganic glass, an organic resin plate, or a resin film, and inorganic glass or a resin film is preferred. The substrate may also be a multilayer structure in which two or more of inorganic glass, an organic resin plate, or a resin film are laminated.

[0084] As shown in FIG. 2 , the touch panel 50 may include an image display panel 30 on the surface of the touch sensor 20b opposite to the surface on which the second interlayer adhesive 10b is laminated, and an optically transparent resin (OCR) 40 may be provided between the touch sensor 20b and the image display panel 30. The touch panel of FIG. 2 is also an image display device including the image display panel 30, and is an image display device with a touch sensor. The optically transparent resin (OCR) 40 may be the interlayer adhesive of the present invention. Also, in FIG. 2 , instead of the touch sensor 20b, the optically transparent resin (OCR) 40, and the image display panel 30, an image display device with a touch sensor directly patterned thereon may be used, i.e., an on-cell or in-cell touch panel may be used.

[0085] Examples of the image display panel 30 include an organic EL display element and a liquid crystal display element. The image display panel 30 preferably has a polarizing plate (polarizing film) on its outermost surface on the front side. The outermost surface on the front side refers to the outermost surface on the cover glass side, and the opposite side is also referred to as the back side. A polarizing plate (polarizing film) generally has a configuration in which protective films are provided on both sides of a polarizer such as a polyvinyl alcohol resin film. The protective film is made of the above-mentioned resin film, preferably a PET film, a COP film, or a TAC film. Therefore, an organic material substrate is generally disposed on the outermost surface on the front side of the image display panel 30. Even if the image display panel 30 does not have a polarizing plate (polarizing film) on its front side, a protective film may be provided on the outermost surface on the front side. Therefore, even in such cases, the outermost surface on the front side of the image display panel 30 is composed of an organic material substrate.

[0086] 1 and 2, the touch panel of the present invention may not use a second substrate, and may instead have a configuration including a first substrate 20a, a first interlayer adhesive 10a, a touch sensor 20b, a second interlayer adhesive 10b, and an image display panel 30 in this order, as shown in Fig. 3. The touch panel 60 of Fig. 3 is also an image display device including the image display panel 30, and is an image display device with a touch sensor. The first substrate 20a, the first interlayer adhesive 10a, the touch sensor 20b, the second interlayer adhesive 10b, and the image display panel 30 are as described above.

[0087] Furthermore, in the present invention, the image display device also includes a micro LED display in which a micro LED is sandwiched between large-sized glass panels. The micro LED display is a transparent image display device that can display advertisements and videos. The micro LED display can be applied to the walls of buildings, indoor glass, and in the future, the windows of minibuses.

[0088] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. The methods for measuring and evaluating the various physical properties in the present invention are as follows.

[0089] < １３ Measurement of C-NMR spectrum> A nuclear magnetic resonance (NMR) apparatus (manufactured by Bruker, trade name "AVANCE III HD") was used, and a 400 MHz spectrometer was used. １３ C-NMR measurements were carried out. ６ The polyvinyl acetal resin was dissolved in DMSO-d to a concentration of 12% by mass. NMR measurements were performed using the inverse gate decoupling method at a temperature of 80°C. The horizontal axis represents the chemical shift in ppm, and the chemical shift is measured using DMSO-d ６ was set to 39.5 ppm. １３ Table 1 shows the range of integral values ​​of the peaks assigned to the methylene C atoms or methine C atoms at the above positions of the above structural units in the C-NMR spectrum.

[0090]

[0091] <Degree of Acetalization, Hydroxyl Value, and Degree of Acetylation> Measurement of the Content (% by mass) of Hydroxyl-Bound Ethylene Groups: 0.4 g of sample was weighed into a 200 mL Erlenmeyer flask with a stopper. 10.0 mL of pyridine-acetic anhydride mixture was added to the sample, and the sample was dissolved in the pyridine-acetic anhydride mixture by heating and ultrasonic irradiation in a water bath at 90°C. A reflux condenser was attached to the Erlenmeyer flask, and the mixture was heated to reflux in the water bath for 120 minutes. After the reaction, the condenser was rinsed with 25 mL of pyridine, and the post-reaction sample solution was cooled to room temperature. 20 mL of 1,2-dichloroethane was added to the cooled sample solution and shaken. 50 mL of water was then added, shaken, and allowed to stand at room temperature for 30 minutes. The sample solution was then subjected to potentiometric titration with 0.5 mol / L (0.5 N) sodium hydroxide solution. A blank test was carried out in the same manner except that no sample was used, and the content (mass %) of ethylene groups to which hydroxyl groups were bonded in the sample was calculated based on the following formula. In the formula, W OH is the content of ethylene groups with hydroxyl groups (mass%), V BL is the volume of sodium hydroxide solution used in the blank test (mL), V sp is the amount of sodium hydroxide solution used in the sample titration (mL), f NaOH is the factor of the 0.5 mol / L sodium hydroxide solution actually used in the potentiometric titration, and m is the sample mass (g).

[0092] Measurement of the acetyl group-bound ethylene group content (mass%): 0.4 g of sample was weighed into a 200 mL Erlenmeyer flask with a stopper. 100.0 mL of ethanol was added to the sample, and the sample was dissolved in ethanol by heating and ultrasonic irradiation in a water bath at 90 °C. While shaking the Erlenmeyer flask, 10.0 mL of 0.2 mol / L (0.2 N) sodium hydroxide was added. A reflux condenser was attached to the Erlenmeyer flask, and the mixture was heated and refluxed in a water bath for 60 minutes. After the reaction, the condenser was rinsed with 25 mL of ethanol, and the reacted sample solution was cooled to room temperature. 10.0 mL of 0.2 mol / L (0.2 N) hydrochloric acid was added to the cooled sample solution, shaken well, and allowed to stand at room temperature for 30 minutes. The sample solution was then subjected to potentiometric titration with 0.1 mol / L (0.1 N) sodium hydroxide solution. A blank test was carried out in the same manner except that no sample was used, and the content (mass %) of ethylene groups to which acetyl groups were bonded in the sample was calculated based on the following formula. In the formula, W Ac is the content of ethylene groups bonded to acetyl groups (mass%), V BL is the volume of sodium hydroxide solution used in the blank test (mL), V sp is the amount of sodium hydroxide solution used in the sample titration (mL), f NaOH is the factor of the 0.1 mol / L sodium hydroxide solution actually used in the potentiometric titration, and m is the sample mass (g).

[0093] Measurement of the content (% by mass) of ethylene groups having butyral groups bonded thereto From the content (% by mass) of ethylene groups having hydroxyl groups bonded thereto and the content (% by mass) of ethylene groups having acetyl groups bonded thereto obtained by the above-mentioned method, the content (% by mass) of ethylene groups having butyral groups bonded thereto was calculated based on the following formula. In the formula, W Bu is the content of ethylene groups to which butyral groups are bonded (mass%), W OH is the content of ethylene groups with hydroxyl groups (mass%), W Ac is the content (mass %) of ethylene groups to which acetyl groups are bonded.

[0094] Amount of Hydroxyl Groups, Degree of Acetylation, Degree of Acetalization (Degree of Butyralization) Using the content of ethylene groups to which hydroxyl groups are bonded, the content of ethylene groups to which acetyl groups are bonded, and the content of ethylene groups to which butyral groups are bonded, which were determined by the methods described above, the amount of hydroxyl groups (mol %), the degree of acetylation (mol %), and the degree of acetalization (mol %) were determined based on the following formulas.

[0095] <Weight-Average Molecular Weight> The polyvinyl acetal resin used in each Example and Comparative Example was dissolved at a concentration of 0.05% by mass in an N-methyl-2-pyrrolidone solution to which lithium bromide had been added to make the concentration 10 mM. The solution was filtered using a syringe filter (Millex-LH 0.45 μm, manufactured by Merck & Co.), and the molecular weight was measured using gel permeation chromatography (e2690, manufactured by Waters). The number-average molecular weight (Mn) and weight-average molecular weight (Mw) were calculated using a molecular weight calibration curve prepared using monodisperse polystyrene standard samples, and the molecular weight distribution (Mw / Mn) was also determined. A Shodex GPC KF-806L (manufactured by Showa Denko K.K.) column was used, and an N-methyl-2-pyrrolidone solution to which lithium bromide had been added to make the concentration 10 mM was used as the eluent.

[0096] <Content of Alkali Metal and Alkaline Earth Metal Ions> The amount of Na ions contained in the polyvinyl acetal resin and the interlayer adhesive, and the total content of alkali metal and alkaline earth metal ions were quantified by ICP emission elemental analysis. The ICP emission elemental analysis was performed by thermally decomposing a sample with sulfuric acid and nitric acid, adjusting the volume of the decomposed product to a constant volume with ultrapure water, and then using an ICP-AES emission elemental analyzer (ICAP-575 model, manufactured by Nippon Jarrell Ash Co., Ltd.).

[0097] <Preparation of Cover Glass Laminate> A cover glass (soda glass manufactured by Nippon Sheet Glass Co., Ltd.) and a sensor glass (manufactured by Technoprint Co., Ltd., ITO thickness 150 nm, resistance value approximately 10 Ω / □) on which an ITO pattern was formed were prepared. The interlayer adhesive of each Example and Comparative Example was sandwiched between the sensor glass and the cover glass to obtain a laminate. This laminate was placed in a rubber bag and degassed at a vacuum of 1.0 MPa for 10 minutes. After that, the degassed bag was transferred to an oven heated to 100°C and temporarily pressed until the temperature reached 85°C. The temporarily pressed laminate was pressed in an autoclave at 140°C and a pressure of 1.3 MPa for 20 minutes to obtain a cover glass laminate in which the cover glass, interlayer adhesive (resin layer), and sensor glass were laminated in this order. The amount of Na ions in the interlayer adhesive and the total content of alkali metal and alkaline earth metal ions in the cover glass laminate were evaluated as described above. The cover glass laminate was also measured as follows.

[0098] <Electrical Moisture and Heat Resistance Test: Presence or Absence of Haze and Whitening> A thermal humidity and heat resistance test was conducted on the cover glass laminate prepared as described above. A voltage (5 V) was applied from a function generator while maintaining a temperature of 60°C and a relative humidity of 90% for 500 hours. After that, water droplets on the surface were wiped off, and the glass was left to stand for 24 hours at 25°C and 50% RH. The haze of the surface edge of the sensor glass on which the ITO pattern was formed was measured using a HAZE METER "HM-150N" manufactured by Murakami Color Co., Ltd. in accordance with JIS K7136:2000. The presence or absence of whitening on the surface edge of the sensor glass on which the ITO pattern was formed was visually confirmed. Those without whitening were rated "A," those with slight whitening were rated "B," and those with whitened areas were rated "C." When the whitened portion is small, the increase in the value is small when measured with a haze meter, but when a minute amount of white turbidity is present in the visual evaluation, it is judged as white turbidity present and x.

[0099] <Emission Intensity Ratio (I1 / I2)> For Examples 3 and 5, the cover glass laminate prepared as described above was subjected to a voltage (5 V) applied by a function generator while maintaining a temperature of 60°C and a relative humidity of 90% for 1,000 hours. The interlayer adhesive (resin layer) in contact with the sensor glass at the edge of the measurement sample was subjected to glow discharge optical emission spectroscopy (GD-OES) under the following measurement conditions using a Horiba, Ltd. Marcus-type high-frequency glow discharge optical emission surface analyzer "GDProfiler2." The ratio (I1 / I2) of the maximum Na emission intensity (I1) in the depth range of 0.5 to 5 μm from the surface to the Na emission intensity (I2) at 10 μm was measured. <Measurement Conditions> Sputtering Method: Normal sputtering Measurement Range (Anode Diameter): φ 4 mm Gas Type: Ar Measurement Method: The sample was embedded in In, and GD-OES measurement was performed until a depth of approximately 10 μm was reached from the sample surface.

[0100] Example 1 Production of Polyvinyl Butyral Resin 1,800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1,700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added to a concentration of 10 mol% while stirring. Subsequently, n-butylaldehyde was added to a concentration of 58 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 62-67°C, and the mixture was aged at this aging temperature for 2 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.5% by mass relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed once with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded using a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0101] Example 2 Production of Polyvinyl Butyral Resin 1,800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1,700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added to a concentration of 10 mol% while stirring. Subsequently, n-butylaldehyde was added to a concentration of 58 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 62-67°C, and the mixture was aged at this aging temperature for 2 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.53 mass % relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed once with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded using a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0102] Example 3 Production of Polyvinyl Butyral Resin 1,800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1,700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added to a concentration of 10 mol% while stirring. Subsequently, n-butylaldehyde was added to a concentration of 62 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 62-67°C, and the mixture was aged at this aging temperature for 2 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.5% by mass relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed twice with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded in a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0103] Example 4 Production of Polyvinyl Butyral Resin 1800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 60% nitric acid was added as a catalyst to this solution to a nitric acid concentration of 0.4% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added to a concentration of 10 mol% while stirring. Subsequently, n-butylaldehyde was added to a concentration of 62 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 60% nitric acid was added to a nitric acid concentration of 1.6% by mass. The temperature was raised to 62-67°C, and the mixture was aged at this aging temperature for 2 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.5% by mass relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed once with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded using a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0104] Example 5 Production of Polyvinyl Butyral Resin 1800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added to a concentration of 10 mol% while stirring. Subsequently, n-butylaldehyde was added to a concentration of 62 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 62-67°C, and the mixture was aged at this aging temperature for 2 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.5% by mass relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed once with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded using a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0105] Comparative Example 1 Production of Polyvinyl Butyral Resin 1,800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1,700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added with stirring to a concentration of 15 mol%. Subsequently, n-butylaldehyde was added to a concentration of 55 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 49-51°C, and the mixture was aged at this aging temperature for 3 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.5% by mass relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed once with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded using a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0106] Comparative Example 2 Production of Polyvinyl Butyral Resin 1,800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1,700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added to 10 mol% while stirring. Subsequently, n-butylaldehyde was added to 58 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 62-67°C, and the mixture was aged at this aging temperature for 2 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a general-purpose neutralizing agent, were added in an amount of 0.5 mass % relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, and then dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded using a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0107] Comparative Example 3 [Production of Polyvinyl Butyral Resin] 1,800 mL of ion-exchanged water and 200 g of polyvinyl alcohol (average degree of polymerization: 1,700, degree of saponification: 98 mol%) were placed in a reactor equipped with a stirrer and heated to dissolve while stirring, yielding a polyvinyl alcohol solution. Next, 30% hydrochloric acid was added as a catalyst to this solution to a hydrochloric acid concentration of 0.2% by mass. The temperature was then adjusted to 15°C, and n-butylaldehyde was added with stirring to a concentration of 15 mol%. Subsequently, n-butylaldehyde was added to a concentration of 55 mol%, resulting in the precipitation of white particulate polyvinyl butyral resin. Ten minutes after precipitation, 30% hydrochloric acid was added to a hydrochloric acid concentration of 1.8% by mass. The temperature was raised to 49-51°C, and the mixture was aged at this aging temperature for 3 hours. The solution was then cooled and washed with excess water to remove unreacted butyraldehyde, yielding a polyvinyl butyral resin. Next, the polyvinyl butyral resin obtained above and ion-exchanged water were added to a reactor equipped with a stirrer so that the slurry concentration was 11 wt %. Furthermore, a hydrochloric acid catalyst and sodium bicarbonate, a common neutralizing agent, were added in an amount of 0.5% by mass relative to the polyvinyl butyral resin. The mixture was heated to 75-80°C for neutralization, cooled, washed twice with excess water, and dried to obtain a polyvinyl butyral resin. The structure of the obtained polyvinyl butyral resin is shown in Table 2. [Production of Interlayer Adhesive] 100 parts by mass of the polyvinyl butyral resin and 40 parts by mass of 3GO (triethylene glycol-di-2-ethylhexanoate) as a plasticizer were mixed, thoroughly melt-kneaded using a mixing roll, and then press-molded in a press molding machine at 150°C for 10 minutes to obtain an interlayer adhesive with a thickness of 0.76 mm.

[0108]

[0109] As shown in each example, a predetermined value satisfying the formula (1) is １３In an interlayer adhesive containing a polyvinyl acetal resin having a C-NMR spectrum and having an alkali metal and alkaline earth metal ion content of more than 5 ppm and not more than 60 ppm, no whitening was observed in the moist heat resistance test. In contrast, as shown in the comparative examples, when the polyvinyl acetal resin did not satisfy formula (1) or when the alkali metal and alkaline earth metal ion content in the interlayer adhesive exceeded 60 ppm, whitening was observed in the moist heat resistance test.

[0110] 10a First interlayer adhesive 10b Second interlayer adhesive 20a First base material 20b Touch sensor 20c Second base material 30 Image display panel 40 Optically transparent resin (OCR)

Claims

1. A polymer comprising a structural unit (a) represented by the following formula (a), a structural unit (b) represented by the following formula (b), and a structural unit (c) represented by the following formula (c): １３ Obtained by C-NMR (nuclear magnetic resonance) measurement １３ An interlayer adhesive comprising a polyvinyl acetal resin that satisfies the following formula (1), where I(a)' is the integral of the peak assigned to the methylene C atom at position (a)' of the structural unit (a), I(b)' is the integral of the peak assigned to the methylene C atom at position (b)' of the structural unit (b), and I(c)' is the integral of the peak assigned to the methylene C atom at position (c)' of the structural unit (c), in a C-NMR spectrum; and the content of alkali metal and alkaline earth metal ions is more than 5 ppm but not more than 60 ppm. I(a)' / (I(a)'+I(b)'+I(c)')>0.22 (1) R in formula (b) and formula (c) １ , R ２ and R ３ are each hydrogen or a hydrocarbon group having 1 to 9 carbon atoms.

2. The interlayer adhesive according to claim 1, wherein the content of Na ions is more than 5 ppm and not more than 50 ppm.

3. The interlayer adhesive according to claim 1 or 2, wherein the weight average molecular weight (Mw) of the polyvinyl acetal resin is 220,000 or more.

4. Contains a structural unit (d) represented by the following formula (d) and a structural unit (e) represented by the following formula (e): １３ Obtained by C-NMR (nuclear magnetic resonance) measurement １３ The interlayer adhesive according to claim 1, wherein the following formula (2) is satisfied when I(d)' is the integral value of the peak assigned to the methine C atom at position (d)' of the structural unit (d) in the C-NMR spectrum, and I(e)' is the integral value of the peak assigned to the methine C atom at position (e)' of the structural unit (e). I(d)' / (I(d)'+I(e)')<0.9 (2) R in the formula (d) and the formula (e) ４ and R ５ are each hydrogen or a hydrocarbon group having 1 to 9 carbon atoms.

5. The interlayer adhesive according to claim 1 or 2, wherein a measurement sample is prepared using the interlayer adhesive, in which a cover glass, a resin layer made of the interlayer adhesive, and a sensor glass on which an ITO pattern has been formed are laminated in this order, and a current-carrying humidity test is carried out for 1,000 hours at a voltage of 5 V, a temperature of 60°C, and a relative humidity of 90%. After that, the distribution of sodium in the depth direction from the surface of the resin layer in contact with the sensor glass at the end of the measurement sample is measured by glow discharge optical emission spectroscopy (GD-OES), and the ratio (I1 / I2) of the maximum sodium emission intensity (I1) in the depth direction from the surface to the maximum sodium emission intensity (I2) at a position 10 μm deep is 2.5 or less.

6. An interlayer adhesive according to claim 1 or 2, wherein the amount of hydroxyl groups in the polyvinyl acetal resin is 25 mol % or more and 35 mol % or less.

7. An interlayer adhesive according to claim 1 or 2, wherein the polyvinyl acetal resin has a degree of acetylation of 1.5 mol % or less and a degree of acetalization of 65 mol % or more and 75 mol % or less.

8. An interlayer adhesive for touch panels, comprising the interlayer adhesive according to claim 1 or 2.

9. An image display device comprising the interlayer adhesive according to claim 1 or 2.

10. A touch panel having the interlayer adhesive according to claim 1 or 2.

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