Microsphere-based retroreflective sheet

Abstract

To provide a microsphere-based retroreflective sheet with superior alkali resistance.SOLUTION: A microsphere-based retroreflective sheet 1 comprises a microsphere layer 30 having a plurality of light-transmissive microspheres 31 arranged in a plane, a specular reflective layer 50 facing the microspheres 31, and a light-transmissive focusing layer 40 filling the gap between the microspheres 31 and the specular reflective layer 50. The focusing layer 40 contains an acrylic resin as a base material containing acrylic acid and a melamine-based crosslinking agent, where an acrylic acid content of the acrylic resin is 7.00 wt.% or less, and a crosslinking agent content of the acrylic resin is 8.3 wt.% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a microsphere type retroreflective sheeting. [Background technology]

[0002] Retroreflective sheets have excellent visibility at night and are therefore used for traffic signs, vehicle license plates, authentication labels, decorative sheets, etc. Known examples of such retroreflective sheets include cube-corner retroreflective sheets and microsphere retroreflective sheets. Examples of these retroreflective sheets are described in Patent Document 1 listed below.

[0003] In microsphere-type retroreflective sheets, a focusing layer is provided between the microspheres and the specular reflective layer, and the focusing layer adjusts the distance between the microspheres and the specular reflective layer so that light that has passed through the microspheres is focused on the specular reflective layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-34659 Summary of the Invention [Problem to be solved by the invention]

[0005] Retroreflective sheets are sometimes used on vehicles, such as for license plates and decorative sheets. In such cases, the retroreflective sheet may be exposed to alkali during car washes, etc. If the focusing layer is eroded by alkali, the distance between the microspheres and the specular reflective layer may change from the designed value, resulting in a decrease in retroreflective performance or a weakening of the focusing layer, which may cause part of the retroreflective sheet to be destroyed.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a microsphere-type retroreflective sheeting that is highly resistant to alkalis. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention relates to a first microsphere-type retroreflective sheeting comprising a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane, a specular reflective layer facing the microspheres, and a light-transmitting focal point forming layer filling the gap between the microspheres and the specular reflective layer, wherein the focal point forming layer contains an acrylic resin as a main component containing acrylic acid and a melamine-based crosslinking agent, and has the following characteristics:

[0008] That is, the present invention is characterized in that, in the first microsphere type retroreflective sheet, the content of the acrylic acid in the acrylic resin is 7.00 wt% or less, and the content of the crosslinking agent relative to the acrylic resin is 8.3 wt% or more.

[0009] Alternatively, the present invention is characterized in that in the first microsphere type retroreflective sheeting, the content of the acrylic acid in the acrylic resin is 6.00 wt% or less, and the content of the crosslinking agent relative to the acrylic resin is 6.2 wt% or more.

[0010] Alternatively, the present invention is characterized in that in the first microsphere type retroreflective sheeting, the content of the acrylic acid in the acrylic resin is 4.50 wt% or less, and the content of the crosslinking agent relative to the acrylic resin is 4.2 wt% or more.

[0011] Alternatively, the present invention is characterized in that in the first microsphere type retroreflective sheeting, the content of the acrylic acid in the acrylic resin is 1.50 wt% or less, and the content of the crosslinking agent relative to the acrylic resin is 2.1 wt% or more.

[0012] Furthermore, in order to solve the above-mentioned problems, the present invention relates to a second microsphere-type retroreflective sheeting comprising a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane, a specular reflective layer facing the microspheres, and a light-transmitting focal point forming layer filling the gap between the microspheres and the specular reflective layer, wherein the focal point forming layer contains an acrylic resin as a main component containing acrylic acid and 2-hydroxyethyl methacrylate, and a melamine-based crosslinking agent, and has the following characteristics:

[0013] That is, the present invention is characterized in that, in the second microsphere-type retroreflective sheet, the content of 2-hydroxyethyl methacrylate in the acrylic resin is 14.00 wt% or more and 28.00 wt% or less, the content of acrylic acid in the acrylic resin is 6.00 wt% or less, and the content of the crosslinking agent in the acrylic resin is 4.2 wt% or more.

[0014] Alternatively, the present invention is characterized in that in the second microsphere type retroreflective sheeting, the content of 2-hydroxyethyl methacrylate in the acrylic resin is 14.00 wt% or more and 28.00 wt% or less, the content of acrylic acid in the acrylic resin is 3.00 wt% or less, and the content of the crosslinking agent in the acrylic resin is 2.1 wt% or more.

[0015] The present inventors have found that a focal point forming layer having a composition that satisfies any of the above conditions has excellent alkali resistance, and therefore, a microsphere-type retroreflective sheeting having such a focal point forming layer can have excellent alkali resistance.

[0016] In either the first or second microsphere type retroreflective sheeting having the above characteristics, the acrylic resin preferably contains ethyl acrylate and methyl methacrylate. [Effects of the Invention]

[0017] As described above, according to the present invention, a microsphere type retroreflective sheeting having excellent alkali resistance can be provided. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a diagram showing a microsphere retroreflective sheeting of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Preferred embodiments of the microspherical retroreflective article according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the scope of the present invention. The present invention can be modified and improved within the scope of the claims. Note that in the drawings referred to below, the dimensions of each component may be changed to facilitate understanding. Also, in the drawings, for ease of viewing, only some reference symbols are assigned to similar components, and some reference symbols may be omitted.

[0020] FIG. 1 is a diagram showing a microsphere-type retroreflective sheeting of the present invention. Hereinafter, the microsphere-type retroreflective sheeting may be referred to as a retroreflective sheeting. As shown in FIG. 1 , the retroreflective sheeting 1 of this embodiment includes a surface protective layer 10 having one exposed surface serving as the light incidence surface; a support layer 20 disposed on the other surface of the surface protective layer 10; a microsphere layer 30 consisting of a plurality of microspheres 31 held by the support layer 20 on the side of the support layer 20 opposite the surface protective layer 10; a specular reflection layer 50 disposed on the side of the microsphere layer 30 opposite the support layer 20; a focus forming layer 40 disposed between the microspheres 31 and the specular reflection layer 50; an adhesive layer 60 disposed on the surface of the specular reflection layer 50 opposite the microsphere layer 30; and a protective paper 70 that protects the adhesive layer 60. If necessary, a printing layer for printing a predetermined pattern may be provided between the surface protective layer 10 and the support layer 20, for example.

[0021] The surface protective layer 10 is made of a light-transmitting, flat film-like resin. The total light transmittance of the surface protective layer 10 is preferably 80% or more. In the example of FIG. 1, the surface protective layer 10 is made of one layer, but the surface protective layer 10 may be made of multiple layers.

[0022] The holder layer 20 is light-transmitting and holds the microspheres 31. The total light transmittance of the holder layer 20 is preferably 80% or more.

[0023] The microspheres 31 in the microsphere layer 30 are optically transparent and function as lenses, and are therefore sometimes called microlenses. Approximately half of the surface of each microsphere 31 is covered with the support layer 20. The portions of each microsphere 31 that are not covered with the support layer 20 are covered with the focus forming layer 40. Therefore, the support layer 20 and the focus forming layer 40 are in contact with each other between the microspheres 31. The average particle size of the microspheres 31 is, for example, 70 μm to 100 μm, and the particle size distribution of the particle sizes of the microspheres 31 is, for example, 75% or more within a range of ±10 μm from the average particle size. The average particle size and particle size distribution of the microspheres 31 are measured, for example, as follows. First, 10 g of the microspheres 31 that serve as a measurement sample are placed in a dry glass container. Next, about 230 ml of an electrolyte manufactured by BECKMAN (trade name: Coulter Isoton III Diluent) is poured into the container and stirred with a glass rod until uniformly dispersed, creating a dispersion of microspheres 31. Next, this dispersion is placed in a Coulter counter (Multisizer 2) manufactured by BECKMAN and measured.

[0024] The focus forming layer 40 is a light-transmitting member, and has a thickness that allows light incident from the surface protective layer 10 to be focused on the specular reflection layer 50 when it enters the microspheres 31 through the holder layer 20 and exits from the microspheres 31. The total light transmittance of the focus forming layer 40 is preferably 80% or more. The thickness of the focus forming layer 40 is determined based on the refractive index of the microspheres 31 and the focus forming layer 40, etc., so that the incident light is focused on the specular reflection layer 50, and is, for example, 10 μm to 60 μm.

[0025] The specular reflection layer 50 is a layer for reflecting light, and is provided on the surface of the focus forming layer 40 opposite to the microspheres 31 side. The surface of the specular reflection layer 50 facing the microspheres 31 faces the microspheres 31 at a certain distance and serves as a reflective area. The thickness of the specular reflection layer 50 is, for example, 0.05 μm to 0.2 μm.

[0026] The adhesive layer 60 is provided on the side of the specular reflection layer 50 opposite the focus forming layer 40. The adhesive layer 60 is a layer for attaching the retroreflective sheet 1 to another object (not shown) and has adhesive properties. The adhesive layer 60 may be adhered to the other object to such an extent that it cannot be peeled off after being attached thereto, or may be adhered to the other object to such an extent that it can be peeled off after being attached thereto. Note that adhesive properties include stickiness.

[0027] The protective paper 70 is a member that protects the adhesive layer 60 until the retroreflective sheet 1 is attached to another object, and can be peeled off from the adhesive layer 60 .

[0028] Next, the materials of the members that make up the retroreflective sheet 1 will be described.

[0029] The surface protective layer 10 is not particularly limited as long as it is a resin having optical transparency as described above. Examples of such resins include acrylic resins, alkyd resins, fluororesins, vinyl chloride resins, polyester resins, urethane resins, polycarbonate resins, ethylene-vinyl acetate copolymers, and other resins, as well as combinations of these resins. From the viewpoints of weather resistance and processability, acrylic resins, polyester resins, and vinyl chloride resins are preferred, and from the viewpoints of coating suitability and dispersibility of colorants when coloring, acrylic resins are more preferred.

[0030] Additives such as ultraviolet absorbers, stabilizers, plasticizers, crosslinkers, and colorants may be added to the surface protective layer 10 within the range that does not significantly impair light transmittance. The content of the colorant is, for example, 1 to 30 parts by weight per 100 parts by weight of the solid content of the base resin.

[0031] The support layer 20 is not particularly limited as long as it is a resin capable of supporting the microspheres 31 and having optical transparency. Examples of such resins include acrylic resin, alkyd resin, fluororesin, vinyl chloride resin, polyester resin, urethane resin, polycarbonate resin, and the like, or a combination of these resins. From the viewpoints of weather resistance and processability, acrylic resin is preferred, while acrylic resin is more preferred in terms of coating suitability and dispersibility of colorants during coloring. The acrylic resin preferably contains a plurality of resins selected from the group consisting of methyl methacrylate, ethyl acrylate, 2-hydroxyethyl methacrylate, and acrylic acid.

[0032] When a colorant is incorporated into the support layer 20, the colorant is incorporated in the same manner as when a colorant is incorporated into the surface protective layer 10. Additives such as an ultraviolet absorber, a stabilizer, a plasticizer, and a curing agent may be added to the support layer 20. The support layer 20 may also contain a curing agent such as an alkylated amino resin, an alkylated urea resin, or an isocyanate-based crosslinking agent, a release agent such as a silicone-based release agent or a cellulose-based release agent, or a surface conditioner such as polyester-modified polydimethylsiloxane, a silicone-based surfactant, or an acrylic polymer.

[0033] The microspheres 31 are made of, for example, glass, and have a refractive index of, for example, 1.5 to 2.0. When the microspheres 31 are made of glass, the refractive index may be adjusted by adding a dopant such as germanium or fluorine to the glass. The microspheres 31 may also be made of resin.

[0034] The focus-forming layer 40 is made of a resin containing an acrylic copolymer as a base containing acrylic acid and a melamine-based crosslinking agent. In this embodiment, the acrylic copolymer is made of ethyl acrylate (EA), methyl methacrylate (MMA), and acrylic acid (AA). The content of the acrylic copolymer in the focus-forming layer 40 is preferably 76.9 wt% or more. The acrylic acid content of the acrylic copolymer improves the adhesion strength between the focus-forming layer 40 and the specular reflection layer 50. The amount of acrylic acid in the acrylic copolymer is preferably 0.4 wt% or more, more preferably 1.5 wt% or more. The acrylic copolymer of the focus-forming layer 40 may also contain acrylic acid and 2-hydroxyethyl methacrylate (2HEMA). In this case, the acrylic copolymer is made of, for example, ethyl acrylate, methyl methacrylate, acrylic acid, and acrylic acid and 2-hydroxyethyl methacrylate. As long as the acrylic copolymer of the focus forming layer 40 contains acrylic acid, it may contain other acrylic resins such as butyl acrylate (BA), and may not contain ethyl acrylate or methyl methacrylate.

[0035] The specular reflection layer 50 is made of a metal such as aluminum, chromium, nickel, magnesium, tin, etc. The specular reflection layer 50 is formed using these metals by a film formation method such as vacuum deposition or sputtering.

[0036] Since the adhesive layer 60 is an adhesive layer, there are no particular limitations on the resin as long as it has adhesive properties. Examples of such resins include acrylic resins, silicone resins, rubber resins, and phenolic resins. From the viewpoints of excellent weather resistance and good adhesive properties, acrylic resins and silicone resins are preferred.

[0037] There are no particular limitations on the material of the protective paper 70, as long as it protects the adhesive layer 60 and can be peeled off from the adhesive layer 60. An example of the protective paper 70 is a sheet-like member whose surface on the adhesive layer side is coated with a fluororesin. [Example]

[0038] Next, the material of the focus forming layer 40 will be described in more detail with reference to the results of measurements using samples.

[0039] <Sample 1> The retroreflective sheet 1 of this sample was produced by the following procedure: The surface protective layer 10 was made up of two layers, a first surface protective layer and a second surface protective layer.

[0040] First, a first surface protective layer was prepared, which was the outermost layer of the surface protective layer 10. The first surface protective layer was made of a resin containing polyvinyl chloride resin as a main component and further containing ethylene-acetic acid copolymer resin, acrylic resin, etc. The thickness of the first surface protective layer was 30 μm.

[0041] Next, a second surface protective layer was formed on the surface opposite to the exposed surface of the first surface protective layer. The second surface protective layer was made of a resin containing a urethane resin as a main component. The thickness of the second surface protective layer was 20 μm.

[0042] Next, a support layer 20 was provided on the surface of the second surface protective layer opposite the first surface protective layer side. The support layer 20 was produced by applying a monomer containing an acrylic resin with a molecular weight of 150,000 to 200,000 and a solids content of 42.5%, which contained 21.01 parts by mass of methyl methacrylate, 64.95 parts by mass of ethyl acrylate, 14.00 parts by mass of 2-hydroxyethyl methacrylate, and 0.04 parts by mass of acrylic acid as the main ingredients, and 37.65 dry parts of a hexamethylene diisocyanate (HMDI) crosslinking agent (manufactured by Asahi Kasei Corporation, product name "Duranate E405-80T", solids content 80%) onto the surface protective layer 2 and semi-curing it.

[0043] The dry parts refer to the mass of the crosslinking agent, etc. when the mass of the acrylic resin, which is the main component in the monomer, is taken as 100. Therefore, the mass parts of the crosslinking agent, etc. per 100 mass parts of the main component in the cured resin is equal to the dry parts of the crosslinking agent, etc. per 100 mass parts of the main component in the monomer.

[0044] Next, before the support layer 20 was completely hardened, the microspheres 31 were held by the support layer 20 so that approximately half of the microspheres 31 were exposed from the support layer 20. The microspheres 31 were made of glass and had a diameter of approximately 60 μm.

[0045] Next, a focus-forming layer 40 was formed on the microspheres 31 and the support layer 20. The focus-forming layer 40 was fabricated by applying a monomer containing an acrylic resin with a molecular weight of 201,000, containing 50.06 parts by weight of ethyl acrylate, 34.95 parts by weight of methyl methacrylate, and 14.99 parts by weight of acrylic acid as the main component, and 2.1 dry parts of a melamine-based crosslinker (manufactured by Allnex Japan, product name "Mycoat 715") relative to the main component, onto the microspheres 31 and the support layer 20 and then curing the monomer. The thickness of the focus-forming layer 40 was 20 μm. Therefore, in the cured focus-forming layer 40, the content of the crosslinker per 100 parts by weight of the main component was 2.1 parts by weight.

[0046] Next, a specular reflection layer 50 was provided on the side of the focus forming layer 40 opposite to the support layer 20. The specular reflection layer 50 was formed by vapor deposition of aluminum. The thickness of the specular reflection layer 50 was 0.13 μm.

[0047] Next, an adhesive layer 60 was formed on the side of the specular reflection layer 50 opposite the focus forming layer 40. The adhesive layer 60 was made using a resin containing butyl acrylate as a main component and further containing 2-ethylhexyl acrylate, acrylic acid, etc., and was hardened to a degree that did not lose its adhesive strength.

[0048] In addition, a single film of the focus forming layer 40 having the same composition and thickness as the focus forming layer 40 of Sample 1 was prepared.

[0049] <Sample 2-42> Retroreflective sheet 1 was prepared in the same manner as sample 1, except that the focus forming layer 40 was formed using an acrylic resin as the main component containing ethyl acrylate, methyl methacrylate, and acrylic acid in the amounts shown in Table 1, and having the molecular weight shown in Table 1, and a monomer containing the same melamine-based crosslinking agent as sample 1 in the dry parts relative to the main component shown in Table 1.

[0050] In addition, a single film of the focus forming layer 40 having the same composition and thickness as the focus forming layer 40 of sample 2-42 was prepared.

[0051] TIFF2025153134000002.tif230170

[0052] Next, sample 1-42 was measured.

[0053] <Alkali resistance test> A single film of the focus forming layer 40 measuring 50 mm x 50 mm was immersed in a 0.1 mol / l NaOH aqueous solution for 8 hours, and after drying, the appearance was observed and the dimensional change rate before and after immersion was measured. A dimensional change rate of less than 1% was considered to be normal.

[0054] <Gel fraction> Two samples were prepared by wrapping a 75 mm × 75 mm single film of the focus forming layer 40 in a 250 mesh, 100 mm × 100 mm wire mesh so that it would not leak when immersed in a solvent, and one sample was immersed in 80 g of a 0.1 mol / L NaOH aqueous solution and one in an ethyl acetate (EAc) solution at 25° C. for three days. After immersion, the wire mesh was dried, and the gel fraction (undissolved portion) was calculated from the change in weight before and after immersion.

[0055] <Alkali resistance test of retroreflective sheeting 1> Each 30mm x 25mm retroreflective sheeting 1 sample was attached to a degreased white-painted plate and aged for two days at a constant temperature and humidity of 23°C and 50% RH. It was then immersed in a 0.1 mol / L NaOH aqueous solution for eight hours. The distance (mm) from the outer periphery of the focus-forming layer 40 of the retroreflective sheeting 1 to which the NaOH aqueous solution had eroded was then measured using a microscope.

[0056] The results of the above measurements are shown in Table 2. TIFF2025153134000003.tif234170

[0057] As shown in Table 2, when the acrylic acid content in the acrylic copolymer is 7.00 wt% or less and the dry parts of the crosslinking agent are 8.3 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is less than 0.4 mm, specifically 0.36 mm or less.

[0058] When the acrylic acid content in the acrylic copolymer is 6.00 wt% or less and the dry parts of the crosslinking agent are 6.2 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.36 mm or less; when the dry parts of the crosslinking agent are 16.5 or more, the erosion distance of the focus forming layer 40 is 0.28 mm or less.

[0059] When the acrylic acid content in the acrylic copolymer is 4.50 wt% or less and the dry parts of the crosslinking agent are 4.2 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.31 mm or less; when the dry parts of the crosslinking agent are 6.2 or more, the erosion distance of the focus forming layer 40 is 0.28 mm or less.

[0060] When the acrylic acid content in the acrylic copolymer is 3.00 wt% or less and the dry parts of the crosslinking agent are 4.2 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.31 mm or less; when the dry parts of the crosslinking agent are 6.2 or more, the erosion distance of the focus forming layer 40 is 0.27 mm or less.

[0061] When the acrylic acid content in the acrylic copolymer was 1.5 wt% and the dry parts of the crosslinking agent were 2.1 or more, the focus forming layer 40 had excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 was 0.26 mm or less.

[0062] In Table 2, the lower limit of the acrylic acid content in the acrylic copolymer is 1.5 wt %, but as long as the alkali copolymer contains an alkaline acid, it can be understood that even if the content is less than 1.50 wt %, the alkali resistance and erosion distance will not be affected.

[0063] As described above, the dry parts refer to the mass of the crosslinking agent etc. when the mass of the main agent in the monomer is 100. Therefore, for example, if the dry parts of the crosslinking agent are 6.2 or more, this means that the focus forming layer 40 contains 6.2 wt% or more of the crosslinking agent relative to the acrylic copolymer which is the main agent, or 6.2 parts by mass or more of the crosslinking agent relative to 100 parts by mass of the acrylic copolymer which is the main agent.

[0064] <Sample 43-84> Retroreflective sheeting 1 was produced in the same manner as sample 1, except that the focus forming layer 40 was formed using an acrylic resin as the main component containing ethyl acrylate, methyl methacrylate, acrylic acid, and 2-hydroxyethyl methacrylate in the amounts shown in Table 3 and having the molecular weight shown in Table 3, and a monomer containing the same melamine-based crosslinking agent as sample 1 in the number of dry parts relative to the main component shown in Table 1. In this sample, the content of 2-hydroxyethyl methacrylate in the acrylic copolymer was 14.00 wt%.

[0065] In addition, a single film of the focus forming layer 40 having the same composition and thickness as the focus forming layer 40 of Sample 43-84 was prepared.

[0066] TIFF2025153134000004.tif230170

[0067] Next, samples 43-84 were measured in the same manner as samples 1-42.

[0068] The measurement results are shown in Table 4. TIFF2025153134000005.tif234170

[0069] As shown in Table 4, when the content of 2-hydroxyethyl methacrylate in the acrylic copolymer is 14.00 wt%, when the content of acrylic acid in the acrylic copolymer is 6.00 wt% or less and the dry parts of the crosslinking agent is 4.2 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is less than 0.4 mm and is 0.36 mm or less.

[0070] Furthermore, when the acrylic acid content in the acrylic copolymer is 4.50 wt% or less and the dry parts of the crosslinking agent are 4.2 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.32 mm or less.

[0071] Furthermore, when the acrylic acid content in the acrylic copolymer is 3.00 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.38 mm or less, and when the dry parts of the crosslinking agent are 4.2 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.27 mm or less.

[0072] Furthermore, when the acrylic acid content in the acrylic copolymer is 1.50 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.36 mm or less, and when the dry parts of the crosslinking agent are 4.2 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.24 mm or less.

[0073] Furthermore, when the acrylic acid content in the acrylic copolymer was 0.04 wt% and the dry parts of the crosslinking agent were 2.1 or more, the focus forming layer 40 had excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 was 0.26 mm or less.

[0074] In Table 4, the lower limit of the acrylic acid content in the acrylic copolymer is 0.04 wt %, but as long as the alkali copolymer contains an alkaline acid, it can be understood that even if the content is less than 0.04 wt %, it will not affect the alkali resistance or erosion distance.

[0075] <Sample 85-126> Next, retroreflective sheeting 1 was produced in the same manner as sample 1, except that the focus forming layer 40 was formed using an acrylic resin as the main component containing ethyl acrylate, methyl methacrylate, acrylic acid, and 2-hydroxyethyl methacrylate in the amounts shown in Table 5 and having the molecular weight shown in Table 5, and a monomer containing the same melamine-based crosslinking agent as sample 1 in the number of dry parts relative to the main component shown in Table 1. In this sample, the content of 2-hydroxyethyl methacrylate in the acrylic copolymer was 28.00 wt%.

[0076] In addition, a single film of the focus forming layer 40 having the same composition and thickness as the focus forming layer 40 of Sample 43-84 was prepared.

[0077] TIFF2025153134000006.tif230170

[0078] Next, samples 85-126 were measured in the same manner as samples 1-42.

[0079] The measurement results are shown in Table 6. TIFF2025153134000007.tif234170

[0080] As shown in Table 6, when the content of 2-hydroxyethyl methacrylate in the acrylic copolymer is 28.00 wt%, when the content of acrylic acid in the acrylic copolymer is 6.00 wt% or less and the dry parts of the crosslinking agent is 2.1 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.34 mm or less, and when the dry parts of the crosslinking agent is 4.2 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.33 mm or less.

[0081] Furthermore, when the acrylic acid content in the acrylic copolymer is 4.50 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.23 mm or less.

[0082] Furthermore, when the acrylic acid content in the acrylic copolymer is 3.00 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is less than 0.2 mm and 0.19 mm or less, and when the dry parts of the crosslinking agent are 8.3 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.16 mm or less.

[0083] Furthermore, when the acrylic acid content in the acrylic copolymer is 1.50 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.16 mm or less, and when the dry parts of the crosslinking agent are 4.2 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.15 mm or less.

[0084] Furthermore, when the acrylic acid content in the acrylic copolymer was 0.04 wt% and the dry parts of the crosslinking agent were 2.1 or more, the focus forming layer 40 had excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 was 0.14 mm or less.

[0085] In Table 6, the lower limit of the acrylic acid content in the acrylic copolymer is 0.04 wt %, but as long as the alkali copolymer contains an alkaline acid, it can be understood that even if the content is less than 0.04 wt %, it will not affect the alkali resistance or erosion distance.

[0086] From the results in Tables 4 and 6, when the 2-hydroxyethyl methacrylate content in the acrylic copolymer is 14.00 wt% or more and 28.00 wt% or less, the following can be said: That is, when the acrylic acid content in the acrylic copolymer is 6.00 wt% or less and the dry parts of the crosslinker are 4.2 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheeting 1 is 0.36 mm or less.

[0087] Furthermore, when the acrylic acid content in the acrylic copolymer is 4.50 wt% or less and the dry parts of the crosslinking agent are 4.2 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.32 mm or less.

[0088] Furthermore, when the acrylic acid content in the acrylic copolymer is 3.00 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.38 mm or less, and when the dry parts of the crosslinking agent are 4.2 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.27 mm or less.

[0089] Furthermore, when the acrylic acid content in the acrylic copolymer is 1.50 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.36 mm or less, and when the dry parts of the crosslinking agent are 4.2 or more, the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.24 mm or less.

[0090] Furthermore, when the acrylic acid content in the acrylic copolymer is 0.04 wt% or less and the dry parts of the crosslinking agent are 2.1 or more, the focus forming layer 40 has excellent alkali resistance, and the erosion distance of the focus forming layer 40 in the retroreflective sheet 1 is 0.26 mm or less.

[0091] From the above explanation, in a microsphere-type retroreflective sheeting 1 comprising a microsphere layer 30 in which a plurality of light-transmitting microspheres 31 are arranged in a plane, a specular reflective layer 50 facing the microspheres 31, and a light-transmitting focus-forming layer 40 filling the gap between the microspheres 31 and the specular reflective layer 50, if the content of acrylic acid in the acrylic resin of the focus-forming layer 40 and the content of the crosslinking agent for the acrylic resin are within the ranges shown in Table 2 that result in the focus-forming layer 40 having excellent alkali resistance, the microsphere-type retroreflective sheeting 1 will have excellent alkali resistance. Furthermore, in the microsphere-type retroreflective sheeting 1, if the content of acrylic acid and 2-hydroxyethyl methacrylate in the acrylic resin of the focus-forming layer 40 and the content of the crosslinking agent for the acrylic resin are within the ranges shown in Tables 4 and 6 that result in the focus-forming layer 40 having excellent alkali resistance, the microsphere-type retroreflective sheeting 1 will have excellent alkali resistance. [Industrial Applicability]

[0092] According to the present invention, a microsphere-type retroreflective sheeting having excellent alkali resistance can be provided, which can be used in technical fields such as vehicle license plates and decorative sheets, although the fields of use of the microsphere-type retroreflective sheeting of the present invention are not limited to these. [Explanation of symbols]

[0093] 1...Microspherical retroreflective sheet 10...Surface protective layer 20...Holder layer 30...Microsphere layer 31...microsphere 40...focal formation layer 50...Specular reflective layer 60...Adhesive layer 70...protective paper

Claims

1. a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane; a specular reflective layer facing the microspheres; a light-transmitting focusing layer that fills the gap between the microspheres and the specular reflective layer; Equipped with The focus forming layer contains an acrylic resin as a main component containing acrylic acid and a melamine-based crosslinking agent, the content of the acrylic acid in the acrylic resin is 7.00 wt % or less; The content of the crosslinking agent relative to the acrylic resin is 8.3 wt % or more. A microsphere type retroreflective sheeting characterized by:

2. a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane; a specular reflective layer facing the microspheres; a light-transmitting focusing layer that fills the gap between the microspheres and the specular reflective layer; Equipped with The focus forming layer contains an acrylic resin as a main component containing acrylic acid and a melamine-based crosslinking agent, the content of the acrylic acid in the acrylic resin is 6.00 wt % or less, The content of the crosslinking agent relative to the acrylic resin is 6.2 wt % or more. A microsphere type retroreflective sheeting characterized by:

3. a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane; a specular reflective layer facing the microspheres; a light-transmitting focusing layer that fills the gap between the microspheres and the specular reflective layer; Equipped with The focus forming layer contains an acrylic resin as a main component containing acrylic acid and a melamine-based crosslinking agent, The content of the acrylic acid in the acrylic resin is 4.50 wt % or less, The content of the crosslinking agent relative to the acrylic resin is 4.2 wt % or more. A microsphere type retroreflective sheeting characterized by:

4. a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane; a specular reflective layer facing the microspheres; a light-transmitting focusing layer that fills the gap between the microspheres and the specular reflective layer; Equipped with The focus forming layer contains an acrylic resin as a main component containing acrylic acid and a melamine-based crosslinking agent, the content of the acrylic acid in the acrylic resin is 1.50 wt % or less; The content of the crosslinking agent relative to the acrylic resin is 2.1 wt % or more. A microsphere type retroreflective sheeting characterized by:

5. a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane; a specular reflective layer facing the microspheres; a light-transmitting focusing layer that fills the gap between the microspheres and the specular reflective layer; Equipped with The focus forming layer includes an acrylic resin as a main component containing acrylic acid and 2-hydroxyethyl methacrylate, and a melamine-based crosslinking agent, The content of the 2-hydroxyethyl methacrylate in the acrylic resin is 14.00 wt % or more and 28.00 wt % or less, the content of the acrylic acid in the acrylic resin is 6.00 wt % or less, The content of the crosslinking agent relative to the acrylic resin is 4.2 wt % or more. A microsphere type retroreflective sheeting characterized by:

6. a microsphere layer in which a plurality of light-transmitting microspheres are arranged in a plane; a specular reflective layer facing the microspheres; a light-transmitting focusing layer that fills the gap between the microspheres and the specular reflective layer; Equipped with The focus forming layer includes an acrylic resin as a main component containing acrylic acid and 2-hydroxyethyl methacrylate, and a melamine-based crosslinking agent, The content of the 2-hydroxyethyl methacrylate in the acrylic resin is 14.00 wt % or more and 28.00 wt % or less, the content of the acrylic acid in the acrylic resin is 3.00 wt % or less; The content of the crosslinking agent relative to the acrylic resin is 2.1 wt % or more. A microsphere type retroreflective sheeting characterized by:

7. The acrylic resin contains ethyl acrylate and methyl methacrylate.

7. The microspherical retroreflective sheeting according to claim 1, wherein the microspherical retroreflective sheeting is made of a material having a thickness of 100 nm or less.

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