TECHNICAL FIELD

This application is a national stage of PCT application No. PCT/JP2018/037143 filed Oct. 4, 2018, which in turn claims priority of Japanese Patent Application No. 2017-198716 filed Oct. 12, 2017, the contents of which are incorporated herein by reference.

The present invention relates to a retroreflective sheet.

BACKGROUND ART

The retroreflective sheet has a property of allowing light incident thereon to be reflected toward a light source. Due to such a property, the retroreflective sheet is used for a purpose such as raising visibility of a target object (such as a printed object) irradiated with light during nighttime or in a dark place. The retroreflective sheet is used for, for example, traffic signs, guide signs, vehicle license plates, advertisement signs, traffic lane separators, delineators, and the like.

An example of such a retro reflective sheet is disclosed in Patent Literature 1 described below. The retroreflective sheet includes: a light transmissive layer including retroreflective elements; and a resin back face layer provided with multiple dents smaller than the retroreflective elements, on a surface facing a surface of the light transmissive layer on which the retroreflective elements are formed. In this retroreflective sheet, low refractive index gas is confined in voids formed between the dents formed in the resin back face layer and the retroreflective elements. With a refractive index difference between the low refractive index gas and the retroreflective elements, light being incident from the light transmissive layer and having reached the interface between the retroreflective elements and the voids, can be reflected toward the light transmissive layer.

[Patent Literature 1] Japanese Patent No. 3123693

SUMMARY OF INVENTION

A retroreflective sheet described in Patent Literature 1 described above has no metal vapor-deposited film used therein, and thus can have improved brightness. The retroreflective sheet having excellent brightness can be used for a printed object, such as a vehicle license plate, to enable a printed part to stand out. However, in the retroreflective sheet disclosed in Patent Literature 1 described above, the voids formed between the retroreflective elements and the dents might be crushed due to application of pressure in a thickness direction as a result of bending or embossing. When the voids are thus crushed, the retroreflectivity decreases. Therefore, the retroreflective sheet disclosed in Patent Literature 1 described above might have the retroreflectivity compromised when it is used for a printed object bent or embossed, compared with a case where it is used for a flat object.

In view of this, an object of the present invention is to provide a retroreflective sheet in which a decrease in the retroreflectivity is less likely to be compromised even when the pressure in the thickness direction is applied.

In order to solve the above problems, a retroreflective sheet according to the present invention includes: a retroreflective layer including a plurality of retroreflective elements on one surface; a back face layer provided to face the surface of the retroreflective layer on the side of the retroreflective elements and including a bonding portion bonded to the retroreflective elements; and a plurality of particles disposed between the retroreflective elements and the back face layer. The bonding portion is continuously formed to surround a plurality of capsule portions in plan view. Voids are formed between the retroreflective elements and the back face layer in the plurality of capsule portions.

In the above-described retroreflective sheet, the plurality of particles is fixed by being sandwiched by the retroreflective elements and the back face layer. Furthermore, the voids are formed between the retroreflective elements and the back face layer, and a gas such as air is confined in the voids. Due to the refractive index difference between this gas and the retroreflective elements, the light incident from the retroreflective layer side can be reflected to the retroreflective layer side at an interface between the retroreflective elements and the voids. Furthermore, in the above-described retroreflective sheet, the voids between the retroreflective elements and the back face layer are supported by the plurality of particles and thus can be prevented from being crushed even when pressure in the thickness direction is applied. Furthermore, in the above-described retroreflective sheet, the bonding portion, where the retroreflective elements and the back face layer are bonded to each other, is continuously formed to surround the plurality of capsule portions, and thus the bonding portion supports the voids when the pressure in the thickness direction is applied, whereby the voids in the plurality of capsule portions can be less likely to be crushed. Therefore, in the above-described retroreflective sheet, a decrease in retroreflectivity can be suppressed even when pressure is applied in the thickness direction.

Furthermore, in the above-described retroreflective sheet, the bonding portion where the retroreflective elements and the back face layer are bonded to each other is continuously formed to surround the plurality of capsule portions, and thus can prevent water or foreign matter from entering the plurality of capsule portions when the retroreflective sheet is used in an environment (such as outdoors) exposed to water or foreign matter. Thus, in the capsule portions, the voids can be prevented from being filled with water or foreign matter. For this reason, the above-described retroreflective sheet can have improved reliability when used in an environment (such as outdoors) exposed to water or foreign matter.

Moreover, the plurality of particles is preferably disposed in the bonding portion.

With the plurality of particles also disposed in the bonding portion, the brightness difference between the bonding portion and the capsule portions in plan view of the retroreflective sheet can be reduced.

Alternatively, the plurality of particles is preferably not disposed in the bonding portion.

With no particles disposed in the bonding portion, the bonding area of the retroreflective elements and the back face layer increases, whereby increased bonding strength between the retroreflective elements and the back face layer can be achieved.

Furthermore, in the plurality of capsule portions, a part of the back face layer and a part of the retroreflective elements are preferably bonded to each other.

In plan view, the portions where the retroreflective elements are bonded to the back face layer appear to be brighter than the portions where the retroreflective elements come into contact with the voids. Therefore, the bonding portion looks brighter than the capsule portions where the voids are formed. In view of this, the back face layer and the retroreflective elements are partially bonded to each other also in the capsule portions as described above, whereby the brightness difference between the bonding portion and the capsule portions may be reduced. Thus, the uniformity of the appearance of the retroreflective sheet can be improved. Furthermore, the bonding strength between the retroreflective elements and the back face layer can be increased by increasing bonding portions between the back face layer and the retroreflective elements.

Furthermore, in plan view, a width w1of the bonding portion is preferably 1000 μm or less, and a ratio w1/w2of the width w1of the bonding portion to a width w2of the capsule portions is preferably 0.7 or more and 1.4 or less.

The brightness difference between the bonding portion and the capsule portion can further be reduced by reducing the width w1of the bonding portion and reducing a difference between the width w1of the bonding portion and the width w2of the capsule portion surrounded by the bonding portion to a certain level.

The plurality of particles is preferably colored with a color similar to a color of the back face layer.

With the particles having a color that is similar to the that of the back face layer, the difference between the color of the back face layer and the color of the particles is less likely to be noticeable in plan view, whereby uniformity of the appearance of the retroreflective sheet can be improved.

The refractive index of the plurality of particles is preferably smaller than the refractive index of the retroreflective elements.

When the refractive index of the particles is smaller than the refractive index of the retroreflective elements, reflection of the light incident from the retroreflective elements side toward the retroreflective elements in the interface between the retroreflective elements and the particles is facilitated at the portions where the retroreflective elements and the particles are in contact with each other. Thus, the retroreflectivity of the retroreflective sheet can be further improved.

Furthermore, an average particle diameter da of the plurality of particles may be set to equal to or larger than a height h of the retroreflective elements.

When the average particle diameter da of the plurality of particles is equal to or larger than the height h of the retroreflective elements, formation of the voids between the retroreflective elements and the plurality of particles is facilitated, whereby the retroreflectivity of the retroreflective sheet can be increased.

Furthermore, in the above-described retroreflective sheet, the average particle diameter da of the plurality of particles may be set to smaller than the height h of the retroreflective elements.

When the average particle diameter da of the plurality of particles is smaller than the height h of the retroreflective elements, it becomes easy to increase the area of the bonding portion between the retroreflective elements and the back face layer, whereby the bonding strength between the retroreflective elements and the back face layer can be increased.

Furthermore, in the above-described retroreflective sheet, a ratio da/h of the average particle diameter da of the plurality of particles to the height h of the retroreflective elements is preferably equal to or higher than 0.40 and equal to or lower than 1.25.

In the present invention, the “average particle diameter” means particle diameters measured as follows. First, cross-sectional photographs are taken using a microscope (VHX-1000, manufactured by Keyence Corporation) at any 10 cross sections of the retroreflective sheet. Next, the particle diameters of 100 particles in the 10 cross-sectional photographs are measured, and the average value is calculated by dividing the total length by 100, and this average value is taken as the average particle diameter.

When the ratio da/h of the average particle diameter da of the plurality of particles to the height h of the retroreflective elements is equal to or higher than 0.40 and equal to or lower than 1.25, the voids with an appropriate size are formed between the retroreflective elements and the plurality of particles, whereby the retroreflectivity of the retroreflective sheet can be increased. Furthermore, with the ratio da/h being equal to or larger than 0.40 and equal to or smaller than 1.25, it becomes easy to increase the area of the bonding portion between the retroreflective elements and the back face layer, whereby the bonding strength between the retroreflective elements and the back face layer can be increased.

Furthermore, in the above-described retroreflective sheet, a ratio np/nv of the number np of the plurality of particles to the number nv of valleys formed between the adjacent retroreflective elements is preferably equal to or higher than 0.50 and equal to or lower than 5.00.

When the ratio np/nv of the number np of the plurality of particles to the number nv of valleys formed between the adjacent retroreflective elements is equal to or larger than 0.50 and equal to or smaller than 5.00, the voids with an appropriate size are formed between the retroreflective elements and the plurality of particles, whereby the retroreflectivity of the retroreflective sheet can be increased. Furthermore, with the ratio np/nv being equal to or larger than 0.50 and equal to or smaller than 5.00, it becomes easy to increase the area of the bonding portion between the retroreflective elements and the back face layer, whereby the bonding strength between the retroreflective elements and the back face layer can be increased.

Moreover, in the above-described retroreflective sheet, the plurality of particles is preferably spherical.

When the plurality of particles is spherical, it becomes easy to form the voids with an appropriate size between the retroreflective elements and the plurality of particles, whereby the retroreflectivity of the retroreflective sheet can be increased.

As described above, according to the present invention, a retroreflective sheet in which a decrease in the retroreflectivity is less likely to be compromised even when the pressure in the thickness direction is applied is provided.

