JP7643142B2 - Transparent moisture-absorbing sealant film and transparent moisture-absorbing laminate - Google Patents

Description

The present invention relates to a sealant film which is transparent and has moisture absorbing properties for suppressing moisture absorption by packaged contents.
The transparent moisture-absorbing sealant film of the present invention can be applied to products in various fields, and can be suitably used, for example, as a packaging material for pharmaceutical PTP packages and SP packages, as a packaging material for preventing rust on metal products such as screws, shafts, and metal plates for automobiles, and on electrical components, and as a packaging material for suppressing deterioration of food due to moisture.

Packaging bags are being developed for the purpose of transporting or long-term storing metal products made of metal materials or metal parts, as well as food and medicines. There is a demand for films that have higher and more stable moisture-proof, waterproof and barrier properties and that have a simple layer structure and can be produced through a simple manufacturing process so as to maintain the function and properties of the contained metal products, food and medicines.
Patent Documents 1 to 3 have proposed packaging laminates in which a resin contains a highly volatile rust inhibitor that volatilizes at room temperature to provide a rust-preventing effect, for the purpose of preventing rust from occurring in the contents of metal products. However, these laminates require an exterior seal, and there are concerns that if the vaporized rust inhibitor adheres to the contents, it may have effects other than the rust-preventing effect, such as deterioration of the contents or functional impairment. In addition, removal of the rust inhibitor is time-consuming, and therefore their uses are limited. Therefore, there is a demand for rust-preventive laminates that do not use volatile rust inhibitors.
PTP packaging, SP packaging, and blister packaging are widely used for packaging medicines and foods, but many of the conventional products have insufficient moisture resistance, and to compensate for this, aluminum foil pillow bags are used as outer bags to contain desiccants. However, the moisture resistance is insufficient after the outer bag is opened, and there are many accidents where the desiccants are accidentally ingested.
In order to improve the moisture resistance of PTP packaging materials, SP packaging materials, and blister packaging materials, Patent Document 4 proposes incorporating a desiccant or a barrier layer into the packaging materials. However, the transparency of the packaging materials is insufficient, and the visibility for checking the deterioration state of the contents is poor.

JP 2010-254350 A JP 2007-308726 A JP 2010-052751 A Patent No. 5429948

The present invention aims to solve the above problems and provide a transparent moisture-absorbing sealant film that has excellent manufacturability, a simple layer structure, excellent transparency to allow the contents to be visually confirmed, absorbs moisture in the contents storage section, and prevents the contents from deteriorating due to moisture during transportation and long-term storage, and has high transparency both before and after moisture absorption.

As a result of various investigations, the present inventors have found that a transparent moisture-absorbing sealant film containing at least a specific inorganic moisture absorbent and a binder resin can achieve the above object.
That is, the present invention is characterized in the following points.
1. A transparent moisture-absorbing sealant film containing an inorganic moisture absorbent and a binder resin,
The refractive index of the transparent moisture-absorbing sealant film is 1.3 or more and 2.0 or less,
The binder resin has a refractive index of 1.3 or more and 2.0 or less, and has heat sealability.
The refractive index of the inorganic moisture absorbent is 1.3 or more and 2.0 or less,
The number average particle diameter of the inorganic moisture absorbent is 400 nm or more and 800 nm or less,
A transparent moisture-absorbing sealant film, characterized in that the content of the inorganic moisture absorbent in the layer containing the inorganic moisture absorbent is 0.5% by mass or more and 70% by mass or less.
2. The transparent moisture-absorbing sealant film has a heat seal layer and a moisture-absorbing layer,
the heat seal layer is a layer containing a heat sealable resin and not containing the inorganic moisture absorbent, and is a layer laminated on one surface or both surfaces of the transparent moisture absorbent sealant film,
the moisture absorbing layer is a layer containing the binder resin and the inorganic moisture absorbent,
2. The transparent moisture-absorbing sealant film according to 1 above, wherein the refractive index of the heat-sealable resin is 1.3 or more and 2.0 or less.
3. The total light transmittance is 60% or more and 99% or less;
The haze is 40% or more and 70% or less.
3. A transparent moisture-absorbing sealant film according to 1 or 2 above.
4. The transparent moisture-absorbing sealant film according to any one of the above items 1 to 3, wherein the inorganic moisture absorbent contains a metal oxide.
5. The transparent moisture-absorbing sealant film according to any one of 1 to 3 above, wherein the inorganic moisture absorbent contains one or more selected from the group consisting of hydrotalcite, magnesium oxide, and calcium oxide.
6. The transparent moisture-absorbing sealant film according to any one of the above items 1 to 5, wherein the binder resin and/or the heat-sealable resin contains a polyolefin resin.
7. The transparent moisture-absorbing sealant film according to any one of the above items 3 to 6, wherein the binder resin and/or the heat-sealable resin contains a polyethylene resin.

The transparent moisture-absorbing sealant film of the present invention has excellent manufacturability, a simple layer structure, and excellent transparency that allows the contents to be visually confirmed. It absorbs moisture in the contents storage section and prevents the contents from deteriorating due to moisture during transportation and long-term storage, and has high transparency both before and after moisture absorption.

FIG. 1 is a schematic cross-sectional view showing an example of a layer structure of a transparent moisture-absorbing sealant film of the present invention. FIG. 2 is a schematic cross-sectional view showing an example of another embodiment of the layer structure of the transparent moisture-absorbing sealant film of the present invention. FIG. 2 is a schematic cross-sectional view showing another example of the layer structure of the transparent moisture-absorbing sealant film of the present invention.

In each drawing, the size and ratio of the components may be changed or exaggerated for ease of understanding. Also, for ease of understanding, parts that are not necessary for the explanation or repeated reference numerals may be omitted.
Although not shown in the drawings, an adhesive layer may be provided between each layer.
Furthermore, if necessary, in order to strengthen the adhesive strength (adhesion strength) between each layer, the laminated surfaces of each layer may be previously subjected to a physical surface treatment such as corona discharge treatment, ozone treatment, plasma treatment, glow discharge treatment, sandblasting treatment, etc., or a chemical surface treatment such as an oxidation treatment using chemicals.

The transparent moisture-absorbing sealant film of the present invention will be described in more detail below. Specific examples will be given, but the present invention is not limited thereto.
In the present invention, the term "transparent" means that the contents packaged with the transparent moisture-absorbing sealant film, transparent moisture-absorbing laminate, or transparent moisture-absorbing packaging material can be visually identified, and refers to the ability to easily identify the contents by placing a printed matter underneath the film, for example, and conducting a test to see whether the characters or patterns on the printed matter can be recognized through the printed matter. Alternatively, the term "transparent" may be defined by the total light transmittance or haze.
In the present invention, the terms "film" and "sheet" are used synonymously, and "moisture absorption" refers to the absorption of gaseous and/or liquid water.

<Transparent moisture-absorbing sealant film>
The transparent moisture-absorbing sealant film of the present invention is a transparent resin film containing at least an inorganic moisture absorbent and a binder resin. The binder resin preferably has heat sealability.
The transparent moisture-absorbing sealant film may be composed of a single layer containing an inorganic moisture absorbent and a binder resin, or may be composed of multiple layers including a moisture-absorbing layer containing an inorganic moisture absorbent and a binder resin, and a heat-sealing layer containing a large amount of a heat-sealing resin and having excellent heat-sealing properties.
In addition, if the transparent moisture absorbing sealant film lacks sufficient support, it may contain a reinforcing layer.
It is preferable that one or both surfaces of the transparent moisture-absorbing sealant film have heat-sealability, and for this purpose, it is preferable that a heat-seal layer is laminated on one or both surfaces of the transparent moisture-absorbing sealant film.
If the surface of the transparent moisture-absorbing sealant film has heat-sealing properties, the heat-sealing properties when laminated with other layers and the interfacial adhesive strength can be improved.

The transparent moisture-absorbing sealant film of the present invention has an excellent balance between moisture absorption, excellent transparency (transmitted visibility, total light transmittance, haze) before and after moisture absorption, and film-forming properties, heat-sealing properties, supportability, etc., by controlling the refractive index of the contained inorganic moisture absorbent, the refractive index of the binder resin or heat-sealable resin, the refractive index of the transparent moisture-absorbing sealant film, the number average particle size of the inorganic moisture absorbent, and the content of the inorganic moisture absorbent within specific ranges.
Furthermore, due to its excellent moisture absorption properties, deterioration of the packaged contents due to moisture can be suppressed, and due to its excellent transparency before and after moisture absorption, the contents can be visually confirmed before and after moisture absorption.

The content of the inorganic moisture absorbent in the layer (moisture absorbing layer) containing the inorganic moisture absorbent is preferably 0.5% by mass or more and 70% by mass or less, and more preferably 2% by mass or more and 65% by mass or less. If the content is less than the above range, the moisture absorption is likely to be insufficient, and if the content is more than the above range, the transparent moisture absorbing sealant film is likely to become brittle, the usability as a packaging material is likely to be poor, and the heat sealability and interfacial adhesive strength with other layers are likely to be insufficient.

There is no particular limit to the thickness of the transparent moisture-absorbing sealant film, but it is preferably 20 μm or more and 100 μm or less. If it is thinner than the above range, it is difficult to fully exert the moisture absorption effect and the resulting moisture absorption effect, and if it is thicker than the above range, the moisture absorption effect and heat sealability are not significantly improved, and the rigidity of the transparent moisture-absorbing sealant film becomes too strong, which makes it difficult to use as a packaging material and tends to reduce transparency.

The total light transmittance of the transparent moisture-absorbing sealant film of the present invention is preferably 60% or more and 99% or less, more preferably 65% or more and 95% or less. If it is lower than the above range, the transparency tends to be poor, and the through visibility of the contents is likely to be difficult. It is difficult to increase it above the above range, and the production process and quality control become complicated, which tends to reduce productivity.
The total light transmittance herein means the light transmittance measured by a color information measuring device or the like according to the method specified in JIS K 7361 (Testing method for optical properties of plastics).

The haze of the transparent moisture-absorbing sealant film of the present invention is preferably 40% or more and 70% or less, more preferably 50% or more and 65% or less. It is difficult to make it lower than the above range, and the production process and quality control become complicated, and the productivity is likely to be deteriorated. If it is higher than the above range, the transparency tends to be deteriorated, and the visibility of the contents is likely to be difficult.
Here, haze refers to the rate at which light is diffused, as measured using a color information measuring device or the like, according to the method specified in JIS K 7361 (Testing method for optical properties of plastics), and represents the degree of cloudiness.

The transparent moisture-absorbing sealant film of the present invention can maintain its transparency (visibility of contents, total light transmittance, haze) even after it is allowed to stand for 11 days in an environment of a temperature of 25° C. and a humidity of 50% RH, or a temperature of 40° C. and a humidity of 90% RH to absorb moisture, or even after it has absorbed moisture of about 0.1 to 3.5% by mass.
The total light transmittance after moisture absorption under the above conditions is preferably 60% or more and 99% or less, more preferably 65% or more and 95% or less. If it is lower than the above range, the transparency tends to be poor, and it is easy to make it difficult to visually confirm the contents. It is difficult to increase it above the above range, and the production process and quality control become complicated, and the productivity is easy to deteriorate.
The haze after moisture absorption under the above conditions is preferably 40% or more and 70% or less, more preferably 50% or more and 65% or less. It is difficult to make it lower than the above range, and the production process and quality control become complicated, and the productivity tends to be poor. If it is higher than the above range, the transparency tends to be poor, and the visibility of the contents tends to be difficult.

Specifically, the total light transmittance of each layer is preferably 50% or more and 99% or less, and more preferably 60% or more and 95% or less. If it is lower than the above range, the transparency of the transparent moisture-absorbing sealant film as a whole tends to be poor, and it is easy to make it difficult to see the contents. It is difficult to increase it above the above range, and the production process and quality control become complicated, which tends to reduce productivity.

The haze of each layer is preferably 30% or more and 70% or less, and more preferably 35% or more and 70% or less. It is difficult to make it lower than the above range, and the production process and quality control become complicated, which tends to reduce productivity. If it is higher than the above range, the transparency of the entire transparent moisture-absorbing sealant film tends to be poor, and it is easy to make it difficult to see the contents.

In order to obtain the above-mentioned transparency, it is preferable that the raw materials contained in the transparent moisture-absorbing sealant film and the transparent moisture-absorbing sealant film have a refractive index within a specific range.
In the present invention, the refractive index is a value measured in accordance with JIS K 7142:2008 using a film formed thereon and an Abbe refractometer with light having a wavelength of 589 nm.

Each raw material and each layer has a refractive index within a specific range, so that the transparent moisture absorbing sealant film can have excellent transparency.
In order to balance transparency with rigidity, heat sealability, etc., the refractive index of the transparent moisture-absorbing sealant film is preferably 1.3 to 2.0, more preferably 1.5 to 1.8. If the refractive index is lower than the above range, the rigidity tends to be poor and it is difficult to use the film as a sealant film, whereas if the refractive index is higher than the above range, the rigidity tends to be too high and the heat sealability tends to be reduced.
The transparent moisture-absorbing sealant film of the present invention can maintain its refractive index even after being left to stand for 11 days in an environment of a temperature of 25° C. and a humidity of 50% RH, or a temperature of 40° C. and a humidity of 90% RH to absorb moisture, or even after absorbing approximately 0.1 to 3.5% by mass of moisture.
The refractive index after moisture absorption under the above conditions is preferably 1.3 or more and 2.0 or less, and more preferably 1.5 or more and 1.8 or less.

The refractive index of the binder resin and/or heat sealable resin is preferably 1.3 or more and 2.0 or less, and more preferably 1.5 or more and 1.7 or less. If the refractive index is higher or lower than the above range, it is difficult to achieve a refractive index difference with the inorganic moisture absorbent within the above range, and it is likely to be difficult to obtain a transparent moisture-absorbing sealant film.

The refractive index of the inorganic moisture absorbent is preferably 1.3 or more and 2.0 or less, and more preferably 1.5 or more and 1.85 or less. If the refractive index of the inorganic moisture absorbent is higher or lower than the above range, it is likely to be difficult to keep the refractive index difference with the thermoplastic resin within the above range.

Next, the absolute value of the difference between the refractive index of the inorganic moisture absorbent and that of the binder resin and/or heat sealable resin is preferably 0 to 0.7, more preferably 0 to 0.5, and even more preferably 0 to 0.3. If the absolute value is greater than the above range, reflection increases at the interface between the inorganic moisture absorbent and the binder resin and/or heat sealable resin, making it difficult to obtain a transparent moisture absorbing layer or transparent moisture absorbing sealant film.

In addition, the absolute value of the difference in refractive index between adjacent layers, for example, between the moisture absorbing layer and the heat seal layer, is preferably 0 to 0.7, more preferably 0 to 0.5, and even more preferably 0 to 0.3. If the absolute value is larger than the above range, the interfacial reflection between the layers becomes too much, and the transparency of the entire transparent moisture absorbing sealant film tends to be poor, making it difficult to visually recognize the contents.
For this purpose, the refractive index of the heat sealable resin contained in the heat seal layer is preferably 1.3 to 2.0, more preferably 1.5 to 1.7, similar to the binder resin and/or heat sealable resin contained in the moisture absorbing layer. If the refractive index is larger or smaller than the above range, it is difficult to achieve a refractive index difference between the moisture absorbing layer and the heat seal layer of 0 to 0.7, and it is likely to be difficult to obtain a transparent moisture absorbing sealant film.

When the transparent moisture-absorbing sealant film has a multi-layer structure, it is preferable that the total light transmittance, haze, and refractive index of each layer are approximately the same. When each layer is transparent, the laminated film as a whole can be transparent, and when the absolute value of the refractive index difference between adjacent layers is small, the interfacial reflection between the layers is suppressed, and the transparency can be increased.
When the transparent moisture-absorbing sealant film has a multi-layer structure, each layer may be laminated via an adhesive layer, but in order to suppress interfacial reflection between the layers and increase transparency, it is preferable that the absolute value of the refractive index difference between the adhesive layer and the adjacent layer is similarly small.
Therefore, the refractive index of the adhesive layer is preferably 1.3 or more and 2.0 or less, and more preferably 1.5 or more and 1.7 or less.

The transparent moisture-absorbing sealant film may contain various plastic compounding agents and additives for the purpose of improving or modifying, for example, processability, heat resistance, weather resistance, mechanical properties, dimensional stability, oxidation resistance, slipperiness, releasability, flame retardancy, mildew resistance, electrical properties, strength, etc. The content of these may range from a trace amount to several tens of percent, and may be arbitrarily contained within a range that does not significantly reduce transparency and moisture absorption, depending on the purpose.
Examples of compounding agents and additives include antiblocking agents, lubricants, crosslinking agents, antioxidants, ultraviolet absorbers, light stabilizers, fillers, reinforcing agents, antistatic agents, pigments, modifying resins, and the like.

[Moisture absorption layer]
The moisture absorbing layer is a layer containing an inorganic moisture absorbing agent and a binder resin for dispersing the inorganic moisture absorbing agent. The moisture absorbing layer preferably contains a heat sealable resin, and the binder resin may be a heat sealable resin.
If the moisture-absorbing layer has sufficient heat-sealing properties, it can also function as a heat-sealing layer, and the transparent moisture-absorbing sealant film can be constructed from a single moisture-absorbing layer.

There is no particular limit to the thickness of the moisture-absorbing layer, but it is preferably 10 μm or more and 100 μm or less. If it is thinner than the above range, it is difficult to fully exert the moisture-absorbing effect, and if it is thicker than the above range, the moisture-absorbing effect and heat sealability are not significantly improved, and the transparent moisture-absorbing sealant film becomes too rigid, which tends to make it difficult to use as a packaging material.

The inorganic moisture absorbent contained in the moisture absorbing layer is preferably contained via a master batch in which the inorganic moisture absorbent is melt-blended with a thermoplastic resin.
Specifically, it is preferable to prepare a master batch by melt blending an inorganic moisture absorbent into a thermoplastic resin at a relatively high concentration, and then dry blending the master batch with other components to achieve the desired concentration in the moisture absorption layer.
The thermoplastic resin to be melt-blended may be one kind or two or more kinds, and may be a binder resin and/or a heat-sealable resin.
The content of the inorganic moisture absorbent in the master batch is preferably 20% by mass or more and 90% by mass or less, and more preferably 30% by mass or more and 70% by mass or less. Within the above range, it is easy to include a necessary and sufficient amount of the inorganic moisture absorbent in the moisture absorption layer in a dispersed state.

(Inorganic moisture absorbent)
Inorganic moisture absorbents are inorganic compounds that absorb moisture from gases or liquids.

The inorganic moisture absorbent preferably contains a metal oxide, more preferably contains one or more selected from the group consisting of hydrotalcite (refractive index 1.54), magnesium oxide (refractive index 1.73), and calcium oxide (refractive index 1.83), further preferably contains hydrotalcite, and particularly preferably contains hydrotalcite and magnesium oxide. Other moisture absorbents can also be used in combination.

The inorganic moisture absorbent is preferably dispersed in the binder resin in the form of primary particles or a complex of primary particles and secondary particles.
The number average particle size of the inorganic moisture absorbent is preferably 400 nm or more and 800 nm or less, more preferably 500 nm or more and 700 nm or less. It is difficult to stably obtain particles smaller than the above range, and if it is larger than the above range, the transparency (transmitted visibility, high total light transmittance, low haze) is likely to be insufficient.

The specific surface area of the inorganic moisture absorbent is preferably 10 ^m2 /g to 300 ^m2 /g, more preferably 20 ^m2 /g to 200 ^m2 /g. If it is smaller than the above range, the transparency (high total light transmittance, low haze) and moisture absorbency are likely to be insufficient, and it is difficult to stably obtain one larger than the above range.
By having the refractive index and/or number average particle size of the inorganic moisture absorbent within the above range, and preferably further by having the composition and/or specific surface area within the above range, the transparent moisture absorbing sealant film can have high transparency both before and after moisture absorption.

(Binder resin)
The binder resin contained in the moisture absorbing layer is not particularly limited as long as it has excellent dispersibility of the inorganic moisture absorbent and can withstand the use of the sealant film, but it is preferable that the binder resin has heat sealability.

Specific examples of binder resins include polyolefin resins, unsaturated carboxylic acid modified polyolefin resins, polycyclic olefin resins, cyclic olefin copolymers, ionomer resins, polyethylene terephthalate (PET), polyacrylonitrile (PAN), etc.

Specific examples of polyolefin resins include polyethylene resins and polypropylene resins, with polyethylene resins being preferred.
Specific examples of polyethylene-based resins include low-density polyethylene (LDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), metallocene polyethylene, ethylene-based plastomers, general-purpose polyethylene, various polyethylene-based copolymers, and mixtures of these resins.

Various polyethylene copolymers include ethylene-vinyl acetate copolymer, ethylene-(meth)acrylic acid ester copolymer, ethylene-(meth)acrylic acid copolymer, ethylene-propylene copolymer, ethylene-(meth)acrylic acid ester-unsaturated carboxylic acid copolymer, and mixtures of these resins.

Unsaturated carboxylic acid modified polyolefin resins are resins obtained by modifying the above polyolefin resins with unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, and other unsaturated carboxylic acids.

Among the above, the binder resin preferably contains a polyolefin-based resin, and among the polyolefin-based resins, a polyethylene-based resin is preferable, and among the polyethylene-based resins, LDPE, LLDPE, and ethylene-based plastomers are more preferable, and LLDPE and ethylene-based plastomers are particularly preferable.

The refractive index of various polyethylene resins is 1.5 to 1.6, with many being around 1.54, the refractive index of various (meth)acrylic resins is around 1.5, and the refractive index of various PET resins is around 1.5 to around 1.7.
By using the binder resin with the above-mentioned composition, refractive index, and dispersibility of the inorganic moisture absorbent, the transparent moisture absorbing sealant film can have high transparency both before and after moisture absorption.

[Heat seal layer]
The heat seal layer is a layer containing a heat sealable resin and having sufficient heat sealability. It may or may not contain an inorganic moisture absorbent, but it is preferable not to contain an inorganic moisture absorbent because it is easier to obtain high heat sealability.
Furthermore, the resin may contain thermoplastic resins other than the heat-sealable resin and various additives within the range that does not significantly impair the heat-sealability.

(Heat sealable resin)
There are no particular limitations on the heat-sealable resin as long as it is a thermoplastic resin that can be melted and fused under normal heat-sealing conditions (150 to 200°C, 1 to 5 kgf/ ^cm2 , 0.5 to 3 seconds), but it is preferable that the refractive index is 1.3 or more and 2.0 or less, and that it can be used as a binder resin.
Furthermore, the binder resin contained in the moisture absorbing layer and the heat sealable resin contained in the heat seal layer may be the same or different.
The heat-sealable resin is preferably one having an MFR of 1 to 10 g/10 min at 150 to 200° C., which is easy to handle.
As a heat-sealable resin that satisfies the above requirements, a polyethylene-based resin is preferably used.
Among the polyethylene-based resins, LDPE, LLDPE and ethylene-based plastomers are more preferred, and LLDPE and ethylene-based plastomers are even more preferred.
The heat-sealable resin may be mixed with other thermoplastic resins within the range that does not significantly deteriorate the transparency or film-forming properties.
When the composition and refractive index of the heat-sealable resin are within the above ranges, the transparent moisture-absorbing sealant film can have high transparency both before and after moisture absorption.

<Reinforcing layer>
The reinforcing layer is a layer made of a reinforcing film, and is preferably included when the transparent moisture-absorbing sealant film lacks rigidity and support.
The reinforcing layer has a total light transmittance, haze, and refractive index comparable to those of the heat seal layer and the moisture absorbing layer, and has excellent mechanical, physical, chemical, and other properties, and is particularly strong and tough, and preferably has heat resistance.
Because the reinforcing layer is transparent, the entire transparent moisture-absorbing sealant film can also be transparent, and because the refractive indices of adjacent layers are similar, interfacial reflection between the layers is suppressed, thereby increasing transparency.

Specifically, the total light transmittance of the reinforcing layer is preferably 60% or more and 99% or less, and more preferably 65% or more and 95% or less. If it is lower than the above range, the transparency of the entire transparent moisture-absorbing sealant film tends to be poor, and it is easy to make it difficult to see the contents. It is difficult to increase it above the above range, and the production process and quality control become complicated, which tends to reduce productivity.

The haze of the reinforcing layer is preferably 30% or more and 70% or less, more preferably 35% or more and 70% or less. It is difficult to lower it below the above range, and the production process and quality control become complicated, which tends to reduce productivity. If it is higher than the above range, the transparency of the transparent moisture-absorbing sealant film as a whole tends to be poor, and it tends to be difficult to visually check the contents.
In addition, the absolute value of the difference in refractive index between adjacent layers is preferably 0 to 1.0, more preferably 0 to 0.9. If it is greater than the above range, the interfacial reflection between the layers becomes too much, and the transparency of the entire transparent moisture-absorbing sealant film tends to be poor, making it difficult to visually recognize the contents.

Specific examples of the resin contained in the reinforcing layer include resin films made of tough thermoplastic resins such as polyester-based resins such as polyethylene terephthalate (PET) and polyethylene naphthalate, polyolefin-based resins such as polypropylene, polyamide-based resins such as nylon, polyaramid-based resins, polycarbonate-based resins, polyacetal-based resins, fluorine-based resins, and others.
The reinforcing film may be either an unstretched film or a uniaxially or biaxially stretched film.
Among the above, resin films containing polyester-based resins and/or polyamide-based resins are preferred, and biaxially oriented PET films and biaxially oriented nylon films are more preferably used.

<How to make transparent moisture-absorbing sealant film>
A method for producing a transparent moisture-absorbing sealant film using the above-mentioned materials will be described below. The production method described below is an example and does not limit the present invention.
The layers constituting the transparent moisture-absorbing sealant film can be laminated by any method used in the production of ordinary packaging materials, such as wet lamination, dry lamination, solventless dry lamination, extrusion lamination, T-die coextrusion molding, coextrusion lamination, inflation molding, and the like.

When carrying out the above lamination, if necessary, the surface of each layer can be pretreated, for example, by corona treatment or ozone treatment. In addition, anchor coating agents such as isocyanate-based (urethane-based), polyethyleneimine-based, polybutadiene-based, and organic titanium-based, or anchor coating agents such as polyurethane-based, polyacrylic-based, polyester-based, epoxy-based, polyvinyl acetate-based, cellulose-based, and other laminating adhesives can be used as desired.

For example, an example of the production of a transparent moisture-absorbing sealant film having a layer structure of heat seal layer 1/moisture-absorbing layer/heat seal layer 2 will be described.
The transparent moisture-absorbing sealant film having the above-mentioned layer structure can be produced by forming and laminating a resin for the heat-sealing layer 1, a resin composition for the moisture-absorbing layer, and a resin for the heat-sealing layer 2 by inflation film formation.

The transparent moisture-absorbing sealant film can also be subjected to secondary processing for the purpose of imparting surface functions such as chemical functions, electrical functions, magnetic functions, mechanical functions, friction/wear/lubrication functions, optical functions, thermal functions, and biocompatibility, within the scope of maintaining transparency and moisture absorption properties.
Examples of secondary processing include embossing, painting, adhesive, printing, metallizing (plating, etc.), machining, surface treatment (antistatic treatment, corona discharge treatment, plasma treatment, photochromic treatment, physical vapor deposition, chemical vapor deposition, coating, etc.), etc. The transparent moisture absorbing sealant film can also be subjected to lamination (dry lamination or extrusion lamination), bag making, and other post-processing.

The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

Details of the raw materials used in the examples are as follows.
[Inorganic moisture absorbent]
Inorganic moisture absorbent 1: Kyowamag MF30, a metal oxide moisture absorbent manufactured by Kyowa Chemical Industry Co., Ltd. Magnesium oxide. Number average particle size: 800 nm, specific surface area: 27 m ² /g, refractive index: 1.73.
Inorganic moisture absorbent 2: Kyowamag MF150, a metal oxide moisture absorbent manufactured by Kyowa Chemical Industry Co., Ltd. Magnesium oxide. Number average particle size: 800 nm, specific surface area: 121 m ² /g, refractive index: 1.73.
Inorganic moisture absorbent 3: KW2200, a metal oxide moisture absorbent manufactured by Kyowa Chemical Industry Co., Ltd. Hydrotalcite (magnesium-aluminum solid solution) having a number average particle size of 400 nm, a specific surface area of 125 m ² /g, and a refractive index of 1.57.
Inorganic moisture absorbent 4: Kyowamag MA150, a metal oxide moisture absorbent manufactured by Kyowa Chemical Industry Co., Ltd., magnesium oxide. Number average particle size: 3.16 μm, specific surface area: 145 m ² /g, refractive index: 1.73.

[Binder resin, heat sealable resin]
LLDPE1: Evolue SP2020, an LLDPE manufactured by Prime Polymer Co., Ltd. Density: 0.916 g/cm ³ , MFR: 2.0 g/10 min, refractive index: 1.54.
LDPE1: LDPE, LC600A, manufactured by Japan Polyethylene Co., Ltd. Density 0.918 g/cm ³ , MFR 7 g/10 min, refractive index 1.54.

<Preparation of Masterbatch>
The masterbatch was prepared as follows.
[Preparation of Masterbatch 1]
LDPE 1 and inorganic moisture absorbent 1 were melt-blended in the following ratio to obtain master batch 1 (MB1).
LDPE 1 50 parts by weight Inorganic moisture absorbent 1 50 parts by weight

[Preparation of Masterbatches 2 to 5]
According to the formulations in Table 1, melt blending was carried out in the same manner as for Masterbatch 1 to obtain Masterbatches 2 to 5 (MB2 to 5).

Example 1
[Preparation of Resin Composition 1 for Moisture Absorption Layer]
MB1 and LLDPE1 were dry blended in the following ratio to obtain a resin composition 1 for a moisture absorbing layer.
MB1 80 parts by mass LLDPE1 20 parts by mass

[Preparation of transparent moisture-absorbing sealant film]
A transparent moisture-absorbing sealant film having the following layer structure was prepared by laminating LLDPE1 for the heat seal layer 1, the resin composition for the moisture-absorbing layer 1 for the moisture-absorbing layer, and LLDPE1 for the heat seal layer 2 by inflation film formation at 170° C. Then, various evaluations were performed.
Layer structure: heat seal layer 1 (15 μm thick) / moisture absorption layer (30 μm thick) / heat seal layer 2 (15 μm thick)

<Examples 2 to 6, 8, Comparative Examples 1, 2>
According to the formulations in Table 2, the same procedure as in Example 1 was carried out to prepare a resin composition for a moisture absorbing layer and a transparent moisture absorbing sealant film, which were then similarly evaluated.

Example 7
[Preparation of Resin Composition 1 for Moisture Absorption Layer]
MB1 and LLDPE1 were dry blended in the following ratio to obtain a resin composition 7 for a moisture absorbing layer.
MB1 10 parts by weight LLDPE1 90 parts by weight [Preparation of transparent moisture-absorbing sealant film]
Using the moisture absorbing layer resin composition 7, a transparent moisture absorbing sealant film consisting only of a moisture absorbing layer (thickness: 60 μm) was obtained by inflation film formation at 170° C. Then, various evaluations were carried out in the same manner as in Example 1.

<Comparative Example 3>
According to the formulations in Table 2, the same procedure as in Example 7 was carried out to prepare a resin composition for a moisture absorbing layer and a transparent moisture absorbing sealant film, which were then similarly evaluated.

<Evaluation method>
[Film-forming ability]
The appearance of the transparent moisture-absorbing sealant film was observed with the naked eye, and the presence or absence of defects was evaluated according to the following evaluation criteria.
◯: The transparent moisture-absorbing sealant film had no wrinkles, bumps, or peeling.
×: The transparent moisture-absorbing sealant film had wrinkles, bumps, or peeling.

[Heat sealability]
The transparent moisture-absorbing sealant film was cut into a piece of 100 mm x 100 mm, and the heat seal layer surface or the heat sealable surface was placed face-to-face and overlapped. Then, using a heat seal tester (TP-701-A manufactured by Tester Sangyo Co., Ltd.), an area of 1 cm x 10 cm was heat sealed so that the ends were not heat sealed and were split into two, and the piece was then cut into strips 15 mm wide to prepare test pieces for measuring heat seal strength.
Each bifurcated end of this test piece was attached to a tensile tester, and the heat-sealed portion was pulled so as to peel off, and the heat-seal strength (N/15 mm width) was measured and the pass/fail judgment was made according to the pass/fail criteria below.

(Heat sealing conditions)
Temperature: 160°C
Pressure: 1 kgf/ ^cm2
Time: 1 second (tensile strength test conditions)
Test speed: 300 mm/min Load range: 50 N
(Pass/Fail criteria)
○: 30N/15mm or more, passed.
×: Less than 30N/15mm, not acceptable.

[Transparent visibility]
Two transparent moisture-absorbing sealant films were cut to dimensions of 100 mm length x 100 mm width, stacked with the heat seal layer surfaces or heat sealable surfaces facing each other, and heat sealed on three sides. A piece of paper with printed letters was inserted, and one of the remaining unheat-sealed sides was heat-sealed to enclose 75 cc of air.
Then, it was judged whether the characters on the inner piece of paper could be visually recognized from the outside.
(Pass/Fail criteria)
○: Visible, passed.
×: Difficult to see, failed.

[Moisture absorption rate]
The transparent moisture-absorbing sealant film was cut into a size of 100 mm length x 100 mm width and heated at 25
The transparent moisture-absorbing sealant film was allowed to stand for 11 days in an environment of temperature 40° C. and humidity 50% RH, or temperature 40° C. and humidity 90% RH, and the moisture absorption rate was calculated from the change in weight of the film.

[Total Light Transmittance and Haze]
The transparent moisture-absorbing sealant film was cut to a size of 50 mm length x 50 mm width, and the total light transmittance and haze were measured using a color information measuring device haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.) in accordance with JIS K 7361 (Testing method for optical properties of plastics).

<Summary of results>
The transparent moisture-absorbing sealant films of all the examples of the present invention exhibited good film-forming properties, heat sealability, low moisture absorption, transmission visibility, high total light transmittance, and low haze, and even after absorbing moisture, they exhibited good transmission visibility, high total light transmittance, and low haze.
On the other hand, the sealant film containing an inorganic moisture absorbent with an excessively large number average particle size in Comparative Example 1 showed poor transparency (transmission visibility, total light transmittance, haze), and the refractive index could not be measured accurately. The sealant film not containing an inorganic moisture absorbent in Comparative Example 2 showed insufficient moisture absorption. The film containing an excessive amount of an inorganic moisture absorbent in Comparative Example 3 showed insufficient film-forming properties and heat sealability, so the subsequent evaluation was discontinued.

1 Transparent moisture absorbing sealant film 2 Heat seal layer 1
3 Moisture absorption layer 4 Heat seal layer 2

Claims (5)

A transparent moisture-absorbing sealant film containing an inorganic moisture absorbent and a binder resin,
The refractive index of the transparent moisture-absorbing sealant film is 1.3 or more and 2.0 or less,
The binder resin has a refractive index of 1.3 or more and 2.0 or less, and has heat sealability.
The refractive index of the inorganic moisture absorbent is 1.3 or more and 2.0 or less,
The number average particle diameter of the inorganic moisture absorbent is 400 nm or more and 800 nm or less,
the content of the inorganic moisture absorbent in the layer containing the inorganic moisture absorbent is 0.5% by mass or more and 70% by mass or less;
The transparent moisture-absorbing sealant film is characterized in that the inorganic moisture absorbent contains one or more kinds selected from the group consisting of hydrotalcite, magnesium oxide, and calcium oxide. The transparent moisture-absorbing sealant film has a heat seal layer and a moisture-absorbing layer,
the heat seal layer is a layer containing a heat sealable resin and not containing the inorganic moisture absorbent, and is a layer laminated on one surface or both surfaces of the transparent moisture absorbent sealant film,
the moisture absorbing layer is a layer containing the binder resin and the inorganic moisture absorbent,
2. The transparent moisture-absorbing sealant film according to claim 1, wherein the refractive index of the heat-sealable resin is 1.3 or more and 2.0 or less. The total light transmittance is 60% or more and 99% or less,
The haze is 40% or more and 70% or less.
The transparent moisture-absorbing sealant film according to claim 1 or 2. The transparent moisture-absorbing sealant film according to any one of claims 1 to 3, characterized in that the binder resin contains a polyolefin resin. 3. The transparent moisture-absorbing sealant film according to claim 2 , wherein the binder resin and/or the heat-sealable resin contains a polyethylene resin.

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