Water-absorbing sheet

Abstract

The present invention is characterized by having a hygroscopic resin layer formed of a resin composition in which a chemical desiccant is dispersed in a thermoplastic resin, and by having an indicator function that indicates a decrease in moisture-absorbing capacity by exhibiting transparency due to absorption of moisture.

Description

Water-absorbent sheet

[0001] The present invention relates to a water-absorbent sheet, and more particularly to a water-absorbent sheet provided with a chemical desiccant.

[0002] Conventionally, water-absorbent sheets (also called moisture-absorbing sheets or desiccant sheets) having a moisture-absorbent resin layer in which a desiccant is dispersed have been widely used for absorbing moisture and preventing moisture. Various layer configurations have been proposed for such water-absorbent sheets. Patent Document 1 proposes a moisture-absorbent sheet comprising a desiccant layer, a moisture-permeable sheet provided on one surface of the desiccant layer, and an adhesive layer provided on the other surface of the desiccant layer, and describes that a cellophane sheet, an acetate sheet, a nylon sheet, or the like can be used as the moisture-permeable sheet. Patent Documents 2 and 3 also describe desiccant sheets having a moisture-absorbent resin layer in which a desiccant is dispersed.

[0003] Known desiccants used in the above-mentioned water-absorbent sheets include physical desiccants, such as zeolite and silica gel, which absorb water through physical adsorption, and chemical desiccants, which absorb water through a chemical reaction with water. Among these desiccants, physical desiccants can easily release the adsorbed water by heating or other means even when saturated with water, allowing for repeated use. However, chemical adsorbents generally react with water irreversibly, and therefore do not release the captured water like physical adsorbents. For this reason, water-absorbent sheets using chemical adsorbents require an index of water absorbency.

[0004] For example, Patent Document 4 proposes a resin composition indicator made of a resin composition in which calcium oxide is blended with a polyolefin resin. This indicator shows the degree of depletion of water absorbency by becoming opaque when calcium oxide, a chemical desiccant, reacts with water. That is, this indicator is transparent before absorbing water, but when it absorbs water, the calcium oxide particles become calcium hydroxide and expand, causing cracks to form within the particles, which makes it opaque.

[0005] However, while such resin composition indicators have advantages such as being easy to use as granules and easily removing the parts that have become opaque due to water absorption during the manufacturing process, they have the problem of being difficult to function as indicators when used as a sheet (or film) attached to another substrate. That is, the degree of cracking due to swelling is highly dependent on the particle size distribution, and therefore the indicator does not always become opaque at a constant level of water absorption, resulting in large variations. For this reason, the indicator function is insufficient when attached to another substrate to serve as a guide for the lifespan of water absorption.

[0006] JP 2006-326838 A JP 2019-177645 A JP 2019-179705 A Japanese Patent No. 4998155

[0007] Therefore, the present invention aims to provide a water-absorbent sheet having an indicator function that shows the lifespan of water absorption. Another object of the present invention is to provide a water-absorbent sheet that can be attached to another substrate to effectively demonstrate its drying function.

[0008] As a result of research into the water absorption of chemical desiccants, the inventors discovered that resin films containing chemical desiccants can change from opaque to transparent upon absorption of moisture by selecting conditions such as the type, particle size, and concentration of the desiccant, in stark contrast to conventionally known films containing chemical desiccants (e.g., calcium oxide), and this discovery led to the completion of the present invention.

[0009] That is, according to the present invention, there is provided a water-absorbent sheet which has a moisture-absorbing resin layer formed from a resin composition in which a chemical desiccant is dispersed in the resin, and which has an indicator function that indicates a decrease in moisture absorption capacity by becoming transparent due to moisture absorption.

[0010] The water-absorbent sheet of the present invention can preferably employ the following aspects. (1) The chemical desiccant has a median diameter (D50) in the range of 0.5 to 30 μm and is contained in the resin composition in an amount of 5 to 24% by mass. (2) The chemical desiccant has a refractive index difference of 0.2 or more with respect to the resin in an unabsorbed state and a refractive index difference of less than 0.1 with respect to the resin in an absorbed state. (3) The chemical desiccant is calcium oxide. (4) The resin is a thermoplastic resin. (5) The thermoplastic resin is an olefin-based resin. (6) The water-absorbent sheet has a single-layer structure consisting of only the moisture-absorbent resin layer. (7) The water-absorbent sheet includes the moisture-absorbent resin layer and a surface protective layer provided on one surface of the moisture-absorbent resin layer. (8) The water-absorbent sheet is used by attaching one surface of the moisture-absorbent resin layer to another substrate. (9) The other substrate is a barrier film. (10) The barrier film has an inorganic barrier layer. (11) The other substrate comprises a metal foil. (12) The substrate is used in an electronic device. (13) The electronic device is a solar cell. (14) The electronic device is an organic electroluminescence (EL) device.

[0011] The water-absorbent sheet of the present invention captures (absorbs) moisture using a chemical desiccant. The basic function of a chemical desiccant is to irreversibly react with water to trap moisture. Therefore, it is excellent in that it releases little moisture after absorbing it and can maintain a stable dry state. A major feature of the present invention is that it has the property of improving transparency upon moisture absorption. For example, as shown in FIG. 1, the absorbent sheet produced in Example 1 (see Example 1 for detailed conditions) had a haze of about 90% in the unabsorbed state. However, as moisture was absorbed, the transparency improved. When the absorbed moisture content reached saturation, the haze decreased to 45% or less, indicating a significant improvement in transparency. In other words, the water-absorbent sheet of the present invention exhibits transparency upon moisture absorption as an indicator of a decrease in moisture absorption capacity.

[0012] The indicator function of the water-absorbent sheet of the present invention is consistent and extremely stable, so that the life of the water-absorbent ability (drying ability) of the absorbent sheet can be accurately determined.

[0013] 1 is a diagram showing the relationship between the water absorption amount and haze of the water-absorbent sheet of the present invention (Example 1). FIG. 2 is a diagram showing an example of the layer structure of the water-absorbent sheet of the present invention. FIG. 3 is a diagram showing an example of the usage form of the water-absorbent sheet of the present invention. FIG. 4 is a schematic diagram showing an example of lamination in reference experiment (1).

[0014] <Principle of the Invention> The water-absorbent sheet of the present invention has a moisture-absorbent resin layer in which a chemical desiccant is dispersed in a resin, and utilizes the principle that the difference in refractive index between the reaction product of the chemical desiccant with water and the base resin in which the chemical desiccant is dispersed becomes small, resulting in a change from an opaque state to a transparent state. Therefore, the type, particle size, amount, etc. of the chemical desiccant used are determined depending on the type of resin so that the above change can be clearly seen.

[0015] <Chemical desiccants> Chemical desiccants are those that chemically react with water, and typical examples include those that generate hydroxides upon reaction with water, such as oxides of various metals (e.g., calcium oxide, magnesium oxide, barium oxide, etc.), and those that form hydrates upon reaction with water, such as metal salts (e.g., calcium chloride, copper sulfate, etc.). In particular, those that have a small difference in refractive index between the substance generated by reaction with water and the resin, have moderate water reactivity, and can stably maintain a granular shape are preferably used. For example, diphosphorus pentoxide (P ２ O ５ ) are too reactive with water and are therefore unsuitable as desiccants. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are also unsuitable for use due to their unstable particle size. Furthermore, although aluminum oxide is a metal oxide, it captures water through physical adsorption and is not a chemical desiccant.

[0016] In particular, in the present invention, when polyethylene (refractive index: 1.54) is used as the thermoplastic resin (referred to as binder resin or base resin) in which the chemical desiccant is dispersed, metal oxides such as calcium oxide, magnesium oxide, and barium oxide are suitable because the difference in refractive index between the metal oxide after reaction with water and the binder resin is small, and calcium oxide is particularly suitable because it is relatively inexpensive. Incidentally, the reaction formula between a metal oxide and water is expressed, for example, by the following formula: MO+H ２ O → M(OH) ２ In the formula, M is a metal atom. Here, the refractive index of calcium oxide (CaO) is 1.84, and that of calcium hydroxide (Ca(OH) ２ The refractive index of magnesium oxide (MgO) is 1.74, and that of magnesium hydroxide (Mg(OH) ２ ) has a refractive index of 1.56. Furthermore, the refractive index of barium oxide is 1.98, and the refractive index of barium hydroxide is 1.50. When these metal oxides are used as chemical desiccants and, for example, the above-mentioned polyethylene is used as a resin, the difference in refractive index between the metal oxide and the resin before reaction with water is 0.2 or more, and the difference in refractive index between the metal hydroxide and the resin after reaction with water is less than 0.1, so the indicator function before and after the reaction can be effectively demonstrated. Note that calcium chloride and the like produce hydrates by reaction with water with a refractive index of 1.3 to 1.4, which is a relatively large difference in refractive index from the resin, so the indicator function due to the manifestation of transparency is weaker than that of metal oxides.

[0017] Furthermore, the median diameter (D50) of the chemical desiccant used is in the range of 0.5 to 30 μm, particularly 1 to 15 μm. This median diameter is measured as a particle size converted into volume by, for example, a laser diffraction scattering method, and refers to the particle size at which the cumulative distribution value is 50%. This particle size range is higher than the wavelength of visible light, and as a result, the composition formed by dispersing the particles in the resin, i.e., the hygroscopic resin layer formed from the resin composition, becomes opaque. If the median diameter (D50) is lower than the above range, scattering and absorption of visible light are significantly reduced, resulting in high transparency regardless of the refractive index. In other words, the clarification due to reaction with water no longer serves as an indicator of the degree of water absorption. Furthermore, if the median diameter (D50) is larger than the above range, particle variation increases, making it difficult to uniformly disperse the chemical desiccant. Furthermore, the clarification due to water absorption also varies greatly, impairing its function as an indicator. As will be described later, since the chemical desiccant is mixed with the resin to form the sheet or coating layer, it is desirable to select a desiccant with a particle size smaller than the thickness of the resin layer to be formed. This is because if the particle size is larger than the thickness of the resin layer, defects will be formed in the resin layer or the layer surface will become rough, affecting the film appearance and hindering the indicator function, which is an effect of the present invention, and preventing the layer from fully exhibiting its original moisture absorption function.

[0018] Furthermore, in order to clearly visually recognize the change in transparency, it is preferable that the chemical desiccant be blended in the moisture-absorbing resin layer formed by mixing with the resin in a range of 5 to 24% by mass, particularly 10 to 20% by mass. If this blending amount is small, the change in transparency is difficult to visually recognize, and if more than necessary is blended, the chemical desiccant will be dispersed unevenly, which will also result in an uneven change in transparency, both of which will impair its function as an indicator.

[0019] The above-mentioned chemical desiccants may be surface-treated with a small amount of a surface treatment agent, such as a metal salt of a higher fatty acid, such as zinc stearate or calcium stearate, to enhance their dispersibility in resins, as long as their indicator function is not impaired.

[0020] <Binder Resin> The resin in which the chemical desiccant is dispersed, i.e., the binder resin, can be selected from those suitable for the molding method of the resin layer. For example, in the case of a sheet form, a thermoformable thermoplastic resin is used. Examples of such thermoplastic resins include the following. Olefin resins, for example, low-density polyethylene, high-density polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, or polyolefins such as random or block copolymers of α-olefins such as ethylene, propylene, 1-butene, and 4-methyl-1-pentene, cyclic olefin copolymers, etc.; ethylene-vinyl compound copolymers, for example, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, ethylene-vinyl chloride copolymer, etc.; styrene resins, for example, polystyrene, acrylonitrile-styrene copolymer, ABS, α-methylstyrene-styrene copolymer, etc.; polyvinyl resins, for example, polyvinyl chloride, polyvinylidene chloride, vinyl chloride-vinylidene chloride copolymer, polymethyl acrylate, polymethyl methacrylate, etc.; polyamide resins, for example, nylon 6, nylon 6-6, nylon 6-10, nylon 11, nylon 12, etc.; polyester resins, for example, polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene naphthalate (PEN), etc.; Other resins, for example, polycarbonate, polyphenylene oxide, polyimide resin, polyamideimide resin, polyetherimide resin, fluororesin, allyl resin, polyurethane resin, cellulose resin, polysulfone resin, polyethersulfone resin, ketone resin, amino resin, or biodegradable resin such as polylactic acid; further, blends of the above-mentioned resins, or resins appropriately modified by copolymerization (for example, acid-modified olefin resin) may also be used.

[0021] In addition to the thermoplastic resins described above, resins that can be molded by coating can also be used, such as ultraviolet-curable and / or thermosetting resins, and typical examples include epoxy resins, acrylic resins, etc. One example of a molding method is to disperse the chemical desiccant in the resin, apply the dispersion to any substrate, and mold by ultraviolet curing or thermosetting.

[0022] In the present invention, a chemical desiccant is used that has a refractive index difference of less than 0.1 with respect to the reaction product with water, depending on the type of resin (i.e., the refractive index of the resin). Examples of the refractive index of representative resins are shown below: Polyethylene (PE): 1.54 Polypropylene (PP): 1.49 Polyethylene terephthalate (PET): 1.58 Polystyrene (PS): 1.59 Polyvinyl chloride (PVC): 1.53 Polymethyl methacrylate (PMMA): 1.49. A suitable combination of chemical desiccant and resin is one in which the refractive index difference between the chemical desiccant and the resin before reaction with water is 0.2 or more, and the refractive index difference between the chemical desiccant and the resin after reaction with water is less than 0.1. Specifically, a suitable combination is one selected from calcium oxide, magnesium oxide, and barium oxide with one or more selected from polyethylene, polypropylene, polyethylene terephthalate, polystyrene, polyvinyl chloride, and polymethyl methacrylate.

[0023] In the present invention, in order to fully utilize the performance of the chemical desiccant, it is desirable that the binder resin itself has a low moisture content, and therefore olefin-based resins are particularly suitable.

[0024] <Water-absorbent sheet> In the present invention, the dispersion of the above-mentioned chemical desiccant in the resin is preferably carried out by melt-kneading the above-mentioned amount of chemical desiccant with the above-mentioned resin, and the resin composition prepared by melt-kneading is melt-extruded to form a moisture-absorbent resin layer, which is then used as a moisture-absorbent sheet. In forming the moisture-absorbent resin layer, as in the examples described below, it is possible to form a desiccant masterbatch in advance and then knead it with a binder resin, and the base material of the desiccant masterbatch can be one that can be kneaded with the binder resin of the moisture-absorbent resin layer and molded.

[0025] The thickness of such a moisture-absorbing resin layer is set to a thickness that achieves the desired amount of water absorption depending on the amount of chemical desiccant blended in the resin layer. For example, if it is desired to absorb as much moisture as possible, the thickness is set to be thick. However, if the thickness is made too thick, changes in transparency due to water absorption may become difficult to visually recognize. Therefore, the thickness of this moisture-absorbing resin layer is generally preferably in the range of 5 to 75 μm, particularly 5 to 50 μm. Therefore, a chemical desiccant having a particle size smaller than the thickness of the moisture-absorbing resin layer is preferably used.

[0026] Thus, in the water-absorbent sheet of the present invention, the indicator function is exhibited by optimally selecting the type and particle size of the chemical desiccant and the type of binder resin, but by further appropriately setting the amount of desiccant and the thickness of the moisture-absorbent resin layer, it is possible to exhibit a more clearly visible indicator function.

[0027] Referring to Figure 2, the water-absorbent sheet 1 of the present invention may have a single-layer structure consisting of only the above-mentioned moisture-absorbent resin layer 3 (see Figure 2(a)), or a surface protective layer 5 for preventing moisture permeation may be provided on one surface thereof (see Figure 2(b)) in order to maintain the drying function, which is the original performance of the moisture-absorbent resin layer 3. Furthermore, such a surface protective layer 5 may be provided on both surfaces of the moisture-absorbent resin layer 3, as long as the change in transparency of the moisture-absorbent resin layer 3 due to moisture absorption is visible (see Figure 2(c)). In particular, the embodiment in which the surface protective layer 5 is provided on both surfaces of the moisture-absorbent resin layer 3 has the effect of suppressing deactivation of the moisture-absorbent resin layer 3 before use by forming a laminate with the surface protective layer, for example, by coextrusion. It is desirable that the above-mentioned surface protective layer 5 be transparent, so that the change in transparency can be easily visible.

[0028] It is also possible to impart slight adhesiveness to one or both sides of the moisture-absorbing resin layer, and for example, an adhesive layer can be formed by co-extrusion with an adhesive resin such as ethylene-vinyl acetate copolymer (EVA), soft polyolefin (LLDPE, etc.), metallocene polyolefin elastomer, etc., and the sheet can be used in the form of a laminate. For example, the water-absorbent sheet 1 in Figure 2(d) is a laminate in which a surface protective layer 5 is provided on one side of the moisture-absorbing resin layer 3 and an adhesive layer 7 is provided on the other side.

[0029] The above-mentioned surface protection layer 5 and adhesive layer 7 can be formed to any thickness, but both should be thick enough not to impair transparency, and should be used in a form that allows the improvement in transparency associated with the absorption of water by the absorbent sheet 1 to be visually confirmed, i.e., in a state that has an indicator function.

[0030] In the present invention, as an example of determining whether the indicator function of the water-absorbent sheet 1 is good or bad, the haze value H ０ and the haze value H ｗ The greater the difference (amount of haze change), the clearer the indicator functions. ０ -H ｗWhen the haze change is 10 or more, the image is visually recognizable, and the haze change is desirably 35 or more, more desirably 40 or more, and most desirably 45 or more.

[0031] As mentioned above, the degree of difference in haze value is important in manifesting the indicator function, but considering the application of this water-absorbent sheet to various uses, it is desirable that the total light transmittance be approximately 70% or more as another optical property.

[0032] The water-absorbent sheet 1 of the present invention can be used alone to remove moisture from the atmosphere, or can be attached to another substrate 9 to dry the other substrate 9, or can exhibit a barrier function against moisture, etc. together with the attached other substrate 9. In this case, as shown in Fig. 3, the moisture-absorbent resin layer 3 can be directly attached to the other substrate 9 to exhibit a drying function, or can be attached by co-extrusion with the thermoplastic resin that forms the other substrate 9, or further, it can be attached using a pressure-sensitive adhesive as well as lamination using an adhesive.

[0033] Any suitable material can be used as the other substrate 9, particularly, highly water-absorbent materials such as polyester resins such as polyethylene terephthalate, polyamide resins such as nylon, and acrylic resins. By attaching the water-absorbent sheet 1 of the present invention to the substrate 9, the effects of the water-absorbent sheet 1 can be fully exhibited. Alternatively, the other substrate 9 can be, for example, a barrier film in which an inorganic barrier layer is formed on a polyethylene terephthalate film. In such barrier films, the inorganic barrier layer is an inorganic vapor-deposited film formed by physical vapor deposition such as sputtering, vacuum deposition, or ion plating, or chemical vapor deposition such as plasma CVD, and is made of various metals or metal oxides. For example, silicon oxide films formed by plasma CVD using organosilicon compounds and aluminum oxide films formed by aluminum sputtering are the most typical vapor-deposited layers.

[0034] Furthermore, as other substrates 9, in addition to the barrier film, metal foil (e.g., aluminum foil) provided with polyethylene terephthalate or the like as a protective film can also be used, and the absorbent sheet can be attached to various back sheets as described below.

[0035] The backsheet provided with the metal foil described above is adhesively fixed to one side of a light-opaque device via a sealing resin, for example, and the metal foil completely blocks moisture. However, this metal foil is very thin, for example, 20 μm or less, taking into consideration the ease of workability in adhering it to the device and the overall weight of the sheet. Such thin metal foil is prone to pinholes, which may allow moisture to penetrate into the device. However, by attaching the water-absorbent sheet 1 of the present invention to the metal foil, such moisture intrusion can be effectively prevented. Specific examples of its use include its widespread use as a backsheet component in electronic device products, such as solar cell modules and organic EL products (organic EL lighting, organic EL displays, etc.).

[0036] As described above, in the back sheet (other substrate 9) constructed by attaching the moisture-absorbent resin layer 3 (water-absorbent sheet 1), a thermoplastic resin film, typified by polyester resin such as polyethylene terephthalate, may be provided as a surface protective film on the other side (the side not attached to the back sheet) of this water-absorbent sheet 1. The provision of this surface protective film has the effect of suppressing deactivation of the moisture-absorbent resin layer due to moisture in the atmosphere when the water-absorbent sheet 1 is in use.

[0037] Furthermore, even in the case of the water-absorbent sheet 1 attached to such a back sheet, the transparency of the water-absorbent sheet 1 can be visually confirmed from the edge of the water-absorbent sheet 1. The color of the back sheet may be a transparent color or a colored color; for example, a colored polyester film can be used as the surface protective film, but by configuring a part of the surface of the sheet to be a transparent color, it becomes possible to visually confirm the deactivation of the water-absorbent sheet 1. It is also possible to temporarily peel off the polyester film to check the color of the water-absorbent sheet.

[0038] Furthermore, when using a back sheet provided with a metal foil corresponding to the aforementioned other substrate 9, the performance of the moisture-absorbent resin layer 3 can be fully exhibited by laminating the side where the moisture-absorbent resin layer 3 and the other substrate 9 are not attached so that it faces the side of the target electronic device such as a solar cell, an organic EL, etc. In this case, the same applies even if a thermoplastic resin film, typified by a polyester resin such as polyethylene terephthalate, is provided as a surface protection film on the side of the water-absorbent sheet 1 where the other substrate 9 is not attached.

[0039] In the present invention, by using the water-absorbent sheet 1 with one side attached to the barrier film described above, it is possible to provide both the moisture barrier function of the water-absorbent sheet 1 and the oxygen barrier function of the barrier film. When used with the water-absorbent sheet 1 attached to such a barrier film, it is usually preferable to make the other side of the water-absorbent sheet 1 (the side to which the barrier film is not attached) a moisture-absorbing functional side that exhibits water absorption, thereby preventing the atmosphere on the side facing the water-absorbent sheet from drying out and preventing oxygen from entering the atmosphere.

[0040] When the water-absorbent sheet 1 is attached to a barrier film as described above, if the barrier film is transparent, the transparency of the water-absorbent sheet 1 (the degree of water absorption) can be visually confirmed from the barrier film side. In this case, a printed image that allows the transparency to be visually confirmed can be provided on the surface of the water-absorbent sheet 1 (the surface opposite to the barrier film).

[0041] The adhesive or pressure-sensitive adhesive used to attach the water-absorbent sheet 1 is preferably an epoxy- or urethane-based dry lamination adhesive. When attaching the water-absorbent sheet 1 without using such an adhesive or pressure-sensitive adhesive, an olefin-based resin with high cohesive strength, particularly polyethylene (low, medium, or high-density polyethylene), is preferred as the binder resin for dispersing the chemical desiccant. When providing a resin layer such as the surface protective layer 5 on the surface of the water-absorbent sheet 1, a tacky resin such as ethylene-vinyl acetate copolymer (EVA), soft polyolefin (LLDPE), or metallocene polyolefin elastomer can be co-extruded on the surface opposite the surface protective layer to form the adhesive layer 7, which can then be used in the form of a laminate.

[0042] In the present invention, when the water absorption capacity of the water-absorbent sheet 1 (hygroscopic resin layer 3) reaches saturation, this can be confirmed by the transparency, and the water-absorbent sheet 1 can be replaced as appropriate. In addition, the indicator function of this transparency can be used to perform quality control after production. Furthermore, by visually checking the partial local transparency, it is possible to confirm the location of moisture-permeable areas due to pinholes, etc.

[0043] The excellent effects of the water-absorbent sheet of the present invention will be explained in the following experimental examples.

[0044] <Preparation of calcium oxide-containing masterbatch> Linear low-density polyethylene (LLDPE) pellets were prepared as the base resin of the calcium oxide-containing masterbatch. Calcium oxide powder was prepared as a chemical desiccant, and the powder was adjusted to the desired particle size using a pulverizer. The powder was then mixed with the base resin pellets to prepare calcium oxide-containing masterbatch (A) containing 50% by weight of calcium oxide. A calcium oxide-containing masterbatch prepared in the same manner using polypropylene (PP) as the base resin was designated calcium oxide-containing masterbatch (B), and a calcium oxide-containing masterbatch prepared in the same manner using polyethylene terephthalate (PET) as the base resin was designated calcium oxide-containing masterbatch (C).

[0045] <Preparation of Magnesium Oxide-Containing Masterbatch> Low-density polyethylene (LDPE) pellets were prepared as a base resin for the magnesium oxide-containing masterbatch. Magnesium oxide powder was prepared as a chemical desiccant, and the powder was adjusted to a desired particle size using a pulverizer. The powder was then kneaded with the resin pellets to prepare a magnesium oxide-containing masterbatch (D) having a magnesium oxide content of 50 wt %.

[0046] <Preparation of Zeolite-Containing Masterbatch> Low-density polyethylene (LDPE) pellets were prepared as the base resin of the zeolite-containing masterbatch. Zeolite powder was prepared as a desiccant, adjusted to a desired particle size using a pulverizer, and kneaded with the resin pellets to prepare a zeolite-containing masterbatch (E) with a zeolite content of 50 wt %.

[0047] Example 1: A CaO-containing masterbatch (A) was prepared as a desiccant-containing masterbatch, and low-density polyethylene (LDPE) was prepared as a binder resin. The binder resin (LDPE) was mixed with the CaO-containing masterbatch (A) so that the desiccant component (CaO) was 15 parts by weight (i.e., 15% by weight) relative to 85 parts by weight of the resin component to form a moisture-absorbing resin layer. The resulting resin composition was introduced into an extruder, and a film (30 μm thick) of a calcium oxide-containing LDPE layer (moisture-absorbing resin layer) was molded. The median diameter of the calcium oxide particles contained therein was 2 μm.

[0048] Example 2 An evaluation sample was prepared in the same manner as in Example 1, except that LDPE was mixed so that the desiccant component (CaO) was 20% by weight.

[0049] Example 3 An evaluation sample was prepared in the same manner as in Example 1, except that LDPE was mixed so that the desiccant component (CaO) was 24% by weight.

[0050] Example 4 An evaluation sample was prepared in the same manner as in Example 1, except that LDPE was mixed so that the desiccant component (CaO) was 30% by weight.

[0051] Example 5 An evaluation sample was prepared in the same manner as in Example 1, except that LDPE was mixed so that the desiccant component (CaO) was 10% by weight.

[0052] Example 6 An evaluation sample was prepared in the same manner as in Example 1, except that LDPE was mixed so that the desiccant component (CaO) was 5% by weight.

[0053] Example 7 An evaluation sample was prepared in the same manner as in Example 1, except that LDPE was mixed so that the desiccant component (CaO) was 3.5% by weight.

[0054] Example 8 An evaluation sample was prepared in the same manner as in Example 1, except that a masterbatch prepared by changing the particle size (median size) of calcium oxide in the masterbatch (A) to 50 μm was used.

[0055] Example 9 An evaluation sample was prepared in the same manner as in Example 1, except that a masterbatch prepared by changing the particle size (median size) of calcium oxide in the masterbatch (A) to 30 μm was used.

[0056] Example 10 An evaluation sample was prepared in the same manner as in Example 1, except that a masterbatch prepared by changing the particle size (median size) of calcium oxide in the masterbatch (A) to 15 μm was used.

[0057] Example 11 An evaluation sample was prepared in the same manner as in Example 1, except that a masterbatch prepared by changing the particle size (median size) of calcium oxide in the masterbatch (A) to 1 μm was used.

[0058] Example 12 A CaO-containing masterbatch (B) was prepared as a desiccant-containing masterbatch, and polypropylene (PP) was prepared as a binder resin. The binder resin (PP) was mixed with the CaO-containing masterbatch (B) so that the desiccant component (CaO) was 15 parts by weight (i.e., 15 wt%) relative to 85 parts by weight of the resin component to form a resin composition for forming a moisture-absorbing resin layer. The resin composition was then introduced into an extruder, and a film (thickness 30 μm) of a calcium oxide-containing PP layer (moisture-absorbing resin layer) was formed. The median diameter of the calcium oxide particles contained therein was 2 μm.

[0059] Example 13: A CaO-containing masterbatch (C) was prepared as a desiccant-containing masterbatch, and polyethylene terephthalate (PET) was prepared as a binder resin. This binder resin (PET) was mixed with a CaO-containing masterbatch (A) so that the desiccant component (CaO) was 15 parts by weight (i.e., 15% by weight) relative to 85 parts by weight of the resin component, to form a resin composition for forming a moisture-absorbing resin layer. The resin composition was then fed into an extruder, and a film (thickness 30 μm) of a calcium oxide-containing LDPE layer (moisture-absorbing resin layer) was formed. The median diameter of the calcium oxide particles contained therein was 2 μm.

[0060] Example 14 To form a surface protective layer, LDPE was fed into an extruder for the surface protective layer. A resin composition for forming a moisture-absorbing resin layer, having the same composition as in Example 1, was also fed into the extruder. Furthermore, to form an adhesive layer, a metallocene polyolefin-based elastomer was fed into the extruder for the adhesive resin layer. The above materials were co-extruded to form a film consisting of three layers: an LDPE layer (surface protective layer, 15 μm), a CaO-containing LDPE layer (moisture-absorbing resin layer, 30 μm), and a metallocene polyolefin-based elastomer layer (adhesive layer, 15 μm). The median diameter of the calcium oxide particles contained therein was 2 μm.

[0061] Example 15 An evaluation sample was prepared in the same manner as in Example 14, except that the thickness of the moisture-absorbing resin layer was changed to 10 μm.

[0062] Example 16 An evaluation sample was prepared in the same manner as in Example 14, except that the thickness of the moisture-absorbing resin layer was changed to 15 μm.

[0063] Example 17 An evaluation sample was prepared in the same manner as in Example 14, except that the thickness of the moisture-absorbing resin layer was changed to 50 μm.

[0064] Example 18 To form surface protective layers on both sides of the moisture-absorbing resin layer, LDPE was fed into two extruders for the surface protective layer. The same moisture-absorbing resin layer-forming composition as in Example 1 was also fed into the extruders. The above materials were co-extruded to form a three-layer film: LDPE layer (surface protective layer, 15 μm) / CaO-containing LDPE layer (moisture-absorbing resin layer, 30 μm) / LDPE layer (surface protective layer, 15 μm). The median diameter of the calcium oxide particles contained therein was 2 μm.

[0065] Example 19 An evaluation sample was prepared in the same manner as in Example 1, except that the thickness of the moisture-absorbing resin layer was changed to 75 μm.

[0066] Example 20 An evaluation sample was prepared in the same manner as in Example 1, except that the thickness of the moisture-absorbing resin layer was changed to 100 μm.

[0067] Example 21: An MgO-containing masterbatch (D) was prepared as a desiccant-containing masterbatch, and low-density polyethylene (LDPE) was prepared as a binder resin. The binder resin (LDPE) was mixed with the MgO-containing masterbatch (D) so that the desiccant component (MgO) was 15 parts by weight (i.e., 15% by weight) relative to 85 parts by weight of the resin component, to form a resin composition for forming a moisture-absorbing resin layer. The resin composition was then introduced into an extruder, and a film (30 μm thick) of a magnesium oxide-containing LDPE layer (moisture-absorbing resin layer) was formed. The median diameter of the magnesium oxide particles contained therein was 3 μm.

[0068] Example 22 An evaluation sample was prepared in the same manner as in Example 21, except that a masterbatch prepared by changing the particle size (median size) of magnesium oxide in the masterbatch (D) to 800 nm was used.

[0069] Example 23 An evaluation sample was prepared in the same manner as in Example 21, except that a masterbatch prepared by changing the particle size (median size) of magnesium oxide in the masterbatch (D) to 400 nm was used.

[0070] Example 24 An epoxy-based ultraviolet-curable resin was prepared as the ultraviolet-curable resin. Calcium oxide powder was mixed with 85 parts by weight of the resin at 15 parts by weight, and the mixture was thoroughly stirred to prepare a calcium oxide dispersion. The calcium oxide powder used here had a median diameter of 2 μm adjusted using a grinder. The dispersion was then applied to a polyester film using a bar coater, and the film was cured by ultraviolet irradiation and heat treatment, yielding a 5 μm-thick coating. In this example, the coating was isolated from the polyester film and evaluated to confirm the indicator function.

[0071] Comparative Example 1 An evaluation sample was prepared in the same manner as in Example 1, except that the zeolite-containing masterbatch (E) was used instead of the calcium oxide-containing masterbatch (A). The median diameter of the zeolite powder was 2 μm.

[0072] The samples prepared in Examples 1 to 24 and Comparative Example 1 were evaluated according to the procedures shown below, and the results are shown in Tables 1 and 2.

[0073] <Measurement of refractive index difference> The refractive index difference can be measured using known methods such as the immersion method (Becke method) or an Abbe refractometer. The refractive index of the desiccant before reaction with water and the refractive index of the reaction product with water are measured, and a resin having a refractive index difference within an appropriate range can be selected. The refractive indices in the table were measured using an Abbe refractometer.

[0074] <Confirmation of moisture absorption performance> To confirm the moisture absorption performance of the water-absorbent sheet, each sample prepared in Examples 1 to 24 and Comparative Example 1 was placed on a 100 cm ２ The sample was then placed in a thermostatic chamber at 30°C and 80% humidity for 3 days to absorb moisture, and the weight after absorption was measured. The amount of water absorption [g / m] was calculated from the change in weight before and after absorption.２ ] was calculated.

[0075] <Confirmation of Optical Properties> For each sample prepared in Examples 1 to 24 and Comparative Example 1, the total light transmittance in a non-water-absorbed state and the haze value H ０ Next, each sample was allowed to absorb moisture in a thermostatic chamber at 30°C and 80% humidity for 3 days, and the haze value H ｗ Finally, the change in haze value before and after water absorption, H ０ -H ｗ The total light transmittance and haze value were measured in accordance with JIS K7136, and a haze meter (NDH8000, Nippon Denshoku Industries Co., Ltd.) was used as the measuring instrument. ⊚: Difference in haze value (H ０ -H ｗ ) is 45 or more. ○: Difference in haze value (H ０ -H ｗ ) is 35 or more and less than 45 △: Difference in haze value (H ０ -H ｗ ) is 10 or more and less than 35 ×: difference in haze value (H ０ -H ｗ ) Less than 10

[0076]

[0077]

[0078] <Reference Experiment (1)> To confirm the indicator function, Reference Experiment (1) was carried out according to the following procedure. Preparation of humidity-conditioned PET film: To confirm the drying function of the water-absorbent sheet, a humidity-conditioned PET film that had absorbed water according to the following procedure was prepared. A 100 μm-thick PET film was left standing for 48 hours in an environment of 23° C. and 50% RH to obtain a humidity-conditioned PET film with a moisture content of approximately 3000 ppm.

[0079] Confirmation of drying function of water-absorbent sheet: The moisture-conditioned PET film (moisture content: 3000 ppm) prepared by the above procedure was attached to the water-absorbent sheet prepared in Example 14, and the laminate was formed as shown in Figure 4. Next, for drying treatment, a plurality of the laminates were stacked and sealed in an aluminum bag, degassed, and stored in a thermostatic chamber at 40°C for 1 day. After the treatment, the water-absorbent sheet was peeled off from the moisture-conditioned PET film, and the moisture content of the moisture-conditioned PET film was measured by the Karl Fischer method. As a result, the moisture content was reduced to 10 ppm or less, confirming the drying function of the water-absorbent sheet.

[0080] Confirmation of drying function of the transparentized water-absorbent sheet: The water-absorbent sheet prepared in Example 14 was left to absorb moisture in a thermostatic chamber at 30°C and 80% humidity for 24 hours as a moisture absorption treatment. The haze value before and after moisture absorption was 88% before moisture absorption and 43% after moisture absorption, and the color change was visually confirmed. Next, the moisture-conditioned PET film 13 (moisture content: 3000 ppm) prepared by the above procedure was attached to the moisture-absorbent sheet subjected to the moisture absorption treatment described above, and the laminate was formed as shown in Figure 4. Next, for drying treatment, multiple laminates were stacked and sealed in an aluminum bag, degassed, and stored in a thermostatic chamber at 40°C for 1 day. After the treatment, the water-absorbent sheet was peeled from the moisture-conditioned PET film, and the moisture content of the moisture-conditioned PET film was measured by the Karl Fischer method. The moisture content was 2500 ppm, indicating that sufficient drying function was not exhibited.

[0081] <Reference Experiment (2)> In order to confirm the indicator function in the back sheet configuration, Reference Experiment (2) was carried out according to the following procedure.

[0082] Aluminum foil (20 μm thick) and polyester film (50 μm thick) were laminated in this order on one side of the moisture-absorbing resin layer prepared by the procedure of Example 1 as the other substrate. Next, a black polyester film (50 μm) was dry-laminated to the side of the moisture-absorbing resin layer where the aluminum foil was not laminated. The size of the black polyester film was smaller than that of the moisture-absorbing resin layer. Next, a transparent polyester film (50 μm) cut to the same size as the moisture-absorbing resin layer was dry-laminated to the side of the moisture-absorbing resin layer where the black polyester film was laminated, to prepare a backsheet laminate. It was confirmed that the color of the film was uniform within the plane immediately after preparation of the laminate. The prepared backsheet laminate was then stored for 3 days in a 40°C / 90% humidity environment, and discoloration from the edge of the laminate was visually observed.

[0083] <Test Results> The moisture-absorbing laminated protective film of the present invention has sufficient moisture absorption capacity, and its optical properties change visibly as the remaining capacity decreases. With chemical desiccants, the refractive index of the material changes due to reactions associated with moisture absorption, thereby reducing the refractive index difference with the base resin. However, with desiccants based on physical adsorption, such as zeolite, the change in the refractive index of the desiccant material due to moisture absorption is small, and therefore it is believed that the optical properties of the water-absorbent sheet do not change. Furthermore, by adjusting the film thickness and amount of desiccant to an appropriate range, the indicator function can be used more effectively. Furthermore, as shown in the reference experiment, when an attempt was made to dry a humidity-conditioned PET film using a transparent film (i.e., a film that had lost its moisture absorption properties), no drying properties were observed, confirming that the color change serves as an accurate indicator of moisture absorption properties.

[0084] 1: Water-absorbent sheet 3: Moisture-absorbent resin layer 5: Surface protective layer 7: Adhesive layer 9: Other substrate 13: Humidity-conditioned PET film

Claims

1. A water-absorbent sheet having a moisture-absorbent resin layer formed from a resin composition in which a chemical desiccant is dispersed within the resin, and characterized by having an indicator function that indicates a decrease in moisture absorption capacity by becoming transparent due to moisture absorption.

2. The water-absorbent sheet according to claim 1, wherein the chemical desiccant has a median diameter (D50) in the range of 0.5 to 30 μm and is contained in the resin composition in an amount of 5 to 24% by mass.

3. The water-absorbent sheet according to claim 1 or 2, wherein the chemical desiccant has a refractive index difference from the resin in an unabsorbed state of 0.2 or more and a refractive index difference from the resin in an absorbed state of less than 0.

1.

4. The water-absorbent sheet according to any one of claims 1 to 3, wherein the chemical desiccant is calcium oxide.

5. The water-absorbent sheet according to any one of claims 1 to 4, wherein the resin is a thermoplastic resin.

6. The water-absorbent sheet according to claim 5, wherein the thermoplastic resin is an olefin resin.

7. The water-absorbent sheet according to any one of claims 1 to 6, which has a single-layer structure consisting of the hygroscopic resin layer alone.

8. The water-absorbent sheet according to any one of claims 1 to 6, comprising the moisture-absorbent resin layer and a surface protective layer provided on one surface of the moisture-absorbent resin layer.

9. The water-absorbent sheet according to any one of claims 1 to 8, wherein one surface of the moisture-absorbent resin layer is attached to another substrate.

10. The water-absorbent sheet according to claim 9, wherein the other substrate is a barrier film.

11. The water-absorbent sheet according to claim 10, wherein the barrier film comprises an inorganic barrier layer.

12. The water-absorbent sheet according to claim 10, wherein the other substrate comprises a metal foil.

13. The water-absorbent sheet according to claim 1, which is used in an electronic device.

14. The water-absorbent sheet according to claim 13, wherein the electronic device is a solar cell.

15. The water-absorbent sheet according to claim 13, wherein the electronic device is an organic electroluminescence (EL) display.

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