WO2003002680A1

WO2003002680A1 - Adhesive sheet and marking film

Info

Publication number: WO2003002680A1
Application number: PCT/US2002/020137
Authority: WO
Inventors: Tsuneyoshi Saitoh; Hideto Endoh
Original assignee: 3M Innovative Properties Company
Priority date: 2001-06-26
Filing date: 2002-06-26
Publication date: 2003-01-09
Also published as: JP2003027016A; JP4700846B2

Abstract

There is provided an adhesive sheet which can effectively prevent a sheet from being damaged such as a crack in a protective layer under a low-temperature environment, and which, at the same time, is provided with excellent protection effect (pollution resistance and weather resistance). An adhesive sheet includes: a flexible base material (1), an adhesive layer (3) disposed on a back surface of said base material (1), and a protective layer (2) disposed on a front surface of said base material (1) and made of a hydrophilic film containing a curing resin and a hydrophilic agent of an inorganic oxide. The base material (1) contains a layer containing a first polyurethane resin having a reaction product of polyester polyol and a polyfunctional isocyanate compound.

Description

ADHESIVE SHEET AND MARKING FILM

Field of the Invention

The present invention relates to an adhesive sheet that is used as a marking and particularly to a marking film for use in low temperature environments.

Background of the Invention

Generally, a protective layer is disposed on the surface of an adhesive sheet, such as a film or sheet used outdoors including a pressure sensitive adhesive, in order to improve pollution resistance of the surface of the adhesive sheet. The protective layer is generally a film containing an acrylic resin or a fluorine-based copolymer. Paint for forming such a film is commercially available.

However, since the surface of a protective layer formed using commercially available paint has a comparatively high water contact angle (generally 70° or more), such a protective layer has the following problems relating to pollutants.

Hydrophilic pollutants (stains caused by sludge, sand dust, dust, raindrop marks, etc.): Since the surface of the protective layer repels water, waterdrops easily fall off the protective layer. However, pollutants adhere to the surface of the protective layer in the area in which waterdrops dried, thereby forming stains in the shape of a spot. Moreover, since the surface of the protective layer has low hydrophilicity, pollutants adhering to the surface are rarely removed by natural washing effects such as rain.

Lipophilic pollutants (exhaust gas, smoke, etc.): Since the surface of the protective layer has comparatively high lipophilicity, lipophilic pollutants tend to adhere to the surface of the protective layer. There are films containing a hydrophilic agent (hydrophilicity providing agent) (hydrophilic film) that are used as the protective layer for a marking film. Such a film is disclosed in literature given below.

For example, Japanese Patent Application Laid-open No. 11-267585 discloses a clear coat paint comprising (A) 30-90 wt% of a resin component, (B) 10-70 wt% of a curing agent component, and (C) 1-50 wt% of a hydrophilic agent (nonvolatile content ratio). As the hydrophilic agent of the component (C), hydrophilic agents of an inorganic oxide such as organosilicate, organosilicate condensate, and inorganic oxide sol (aluminium oxide sol, silicon oxide sol, zirconium oxide sol, antimony oxide sol, and the like) are disclosed. This clear coat paint can provide improved protection effects such as pollution resistance and weather resistance to an article covered with a film of this paint by combining the curing resin (cured resin) and the inorganic oxide-based hydrophilic agent. Such a paint is also disclosed in Japanese Patent Application Laid-open No. 9-302257. These publications disclose directly applying the hydrophilic paint to an article such as the body of a vehicle and a traffic sign. However, these publications do not disclose use of such a film as the protective layer of an adhesive sheet.

U.S. Patent No. 5,820,978 discloses an article such as a reflective sheet including a layer for thinly spreading water (water-spreading layer) as a surface protective layer, wherein the water-spreading layer contains a silicon oxide compound having a metal oxide film on the surface thereof. The metal of the metal oxide film is selected from the group consisting of aluminum, gallium, germanium, tin, indium, arsenic, antimony, and vanadium. Such a water-spreading layer may be used as the hydrophilic protective layer. In some conventional adhesive sheets, a protective layer which does not contain the inorganic oxide-based hydrophilic agent, but contains a curing resin is provided on the surface of a base material. For example, Japanese Patent Application Laid-open No. 1-225551 discloses a protective layer formed of a cured film of paint containing an electron beam curable oligomer. Japanese Utility Model Laid-open NO. 5-76732 discloses a decorative film (decorative sheet) having a protective layer cured using electron beams. Japanese Patent Application Laid-open No. 2000-191993 discloses an adhesive sheet including a protective layer secured on the surface of a base material and an adhesive layer disposed on the back surface of the base material, wherein the protective layer contains a curing resin and transparent beads dispersed in the curing resin.

As described in these publications, the base material used in a conventional adhesive sheet or decorative sheet is a plastic film such as polyvinyl chloride, polyester, polyurethane, acrylic polymer, fluorine-based polymer, or polyolefin.

The hydrophilic film containing an inorganic oxide-based hydrophilic agent and a curing resin excels in protection effects such as pollution resistance and weather resistance. However, such a hydrophilic film has the following problems when combined with a conventional adhesive sheet.

An adhesive sheet bonded to the surface of an adherend used outdoors such as the wall of a building or a sign is bonded (constructed) in an outdoor temperature environment. However, in the case where the sheet is bent during construction in a low temperature environment outdoors in winter (for example, winter in Japan), or impact is applied to the sheet due to a stone colliding with the surface of the sheet or the like during or after construction, damages to the sheet such as cracks in the protective layer may occur. Such cracks may be formed across the direction of the thickness of the entire adhesive sheet, whereby the adherend may be exposed by the cracks in the adhesive sheet after construction. It is believed that a protective layer containing both an inorganic oxide-based hydrophilic agent and a curing resin have low impact resistance at a low temperatures, and damages, such as cracks easily occur in an adhesive sheet provided with such a protective layer. Accordingly, an object of the present invention is to provide an adhesive sheet which can effectively prevent damages to the sheet such as cracks in a protective layer in a low-temperature environment, and is provided with excellent protection effects (pollution resistance and weather resistance), and a marking film using an adhesive film.

Summary of the Invention Briefly, the present invention provides an adhesive sheet comprising: a flexible base material, an adhesive layer disposed on a back surface of the base material, and a protective layer disposed on a front surface of the base material and made of a hydrophilic film containing a curing resin and a hydrophilic agent of an inorganic oxide; wherein the base material contains a layer containing a first polyurethane resin having a reaction product of a polyester polyol and a polyfunctional isocyanate compound.

In the adhesive sheet of the present invention, it is preferable that low-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 5°C, of the base material be larger than that of the protective layer measured in the same manner, and the base material be thicker than the protective layer. Low-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 5°C, of the base material is preferably within the range from 50% to 100%. It is preferable that low-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 5°C, of the base material be 50% or more, and ordinary-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 25°C, of the base material be 200% or less. It is preferable that the base material of the adhesive sheet of the present invention comprise: (i) a lower layer containing the first polyurethane resin, and (ii) an upper layer which is disposed between the lower layer and the protective layer, which adheres to the protective layer, and which contains a second polyurethane resin having a reaction product of a polycarbonate polyol and a polyfunctional isocyanate compound. The lower layer is preferably thicker than the upper layer.

The present invention also provides a marking film made of an adhesive sheet, wherein the protective layer is substantially transparent, the lower layer of the base material is a transparent layer not containing pigment substantially or a white layer containing a white pigment, the upper layer of the base material is a colored layer containing a pigment except for a white pigment.

Brief Description of the Drawings FIG. 1 is an explanatory cross-sectional diagram showing one embodiment of an adhesive sheet according to the present invention.

Description of the Preferred Embodiments) In an adhesive sheet of the present invention, a flexible base material includes a layer containing a polyurethane having a reaction product of a polyester polyol and a polyfunctional isocyanate compound (first polyurethane resin) (first polyurethane resin layer).

This effectively prevents damages to the sheet such as cracks in the protective layer in a low temperature environment. Moreover, since the adhesive sheet includes the protective layer formed of the hydrophilic film, excellent protection effects (pollution resistance and weather resistance) can be obtained.

In the present invention, the protective layer formed of the hydrophilic film containing a curing resin and an inorganic oxide-based hydrophilic agent exhibits excellent protection effects in the same manner as a conventional protective layer. Since the base material contains a polyester polyol-based polyurethane (first polyurethane resin), the base material has large elongation at a low temperature (about 5°C). Therefore, impact directly applied to the protective layer can be effectively absorbed by the base material as a cushion in a low-temperature environment. Moreover, since the entire sheet has high flexibility, occurrence of cracks across the direction of the thickness of the adhesive sheet can be effectively prevented.

The low-temperature elongation, specifically, breaking elongation measured at 5°C of the base material is preferably 50-100%. If the low-temperature elongation is less than 50%, occurrence of cracks in a low temperature environment may not be effectively prevented. If the low-temperature elongation exceeds 100%, the sheet may be deformed or break due to an excessive amount of elongation when constructed at ordinary temperature (about 25°C) or more. Therefore, the low-temperature elongation is particularly preferably 60-95%. In the present specification, the low-temperature elongation is a value measured by subjecting a test specimen of a film with a thickness of about 35 μm to a tensile test at 5°C and a tensile speed of 300 mm/min.

The breaking elongation of the base material measured at 25°C and a tensile speed of 300mm/min. (ordinary-temperature elongation) is preferably 200% or less. If the ordinary-temperature elongation exceeds 200%, the sheet may be deformed or break due to an excessive amount of elongation when constructed at ordinary temperature (about 25°C) or more. Therefore, the ordinary-temperature elongation is particularly preferably 180% or less. The low-temperature elongation is a value measured by subjecting a test specimen of a coating film with a thickness of about 35 μm to a tensile test at 25°C and a tensile speed of 300 mm/min.

In order to effectively increase the protection effects, it is preferable that the protective layer be comparatively hard. Therefore, the low-temperature elongation of the protective layer is set lower than the low-temperature elongation of the base material measured in the same manner. In this case, it is advantageous that the thickness of the base material is greater than the thickness of the protective layer in order to increase the impact absorption effect. Adhesive sheet

An adhesive sheet (100) according to the preferred embodiment of the present invention has a structure shown in FIG. 1. As shown in FIG 1, the adhesive sheet (100) includes a flexible base material (1), an adhesive layer (3) disposed on the back surface of the base material (1), and a protective layer (2) disposed on the front surface of the base material (1) and formed of a hydrophilic film containing a curing resin and a hydrophilic agent of an inorganic oxide. The base material (1) may be formed of a single layer containing the first polyurethane resin. However, the base material (1) is preferably formed of a laminate consisting of two or more layers. In the example shown in FIG. 1, the base material (1) is a laminate (laminated film) consisting of a lower layer (11) and an upper layer (12) adhering to each other. Specifically, a front surface (111) of the lower layer (11) adheres to aback surface (122) of the upper layer (12). The protective layer (2) adheres to a front surface (121) of the upper layer (12). The adhesive layer (3) adheres to aback surface (112) of the lower layer (11).

When the base material (1) consists of a laminated film consisting of the lower layer (11) which is disposed on the side of the adhesive layer (3), and the upper layer (12) which adheres to the lower layer (11) and disposed on the side of the protective layer (2), it is preferable that the lower layer (11) contain the first polyurethane resin and have a thickness greater than the thickness of the upper layer (12). Impact absorption effects at a low temperature can be effectively obtained by disposing the cushion layer having a relatively large thickness in the lowest layer close to the adhesive layer (3). Moreover, flexibility of the entire sheet can be effectively increased even in a low temperature environment. The upper layer (12) adhering to the protective layer (2) preferably contains a hard polyurethane resin in comparison with the lower layer (11), specifically, a second polyurethane resin, which is a reaction product of a polycarbonate polyol and a polyfunctional isocyanate compound. This enables adhesion between the entire base material (1) and the protective layer (2) to be effectively increased through the upper layer (12), even if the curing resin in the protective layer (2) is comparatively hard and has a low-temperature elongation differing from the low-temperature elongation of the lower layer (11) of the base material to a large extent. This effectively prevents damages to the sheet due to removal of the protective layer (2) from the base material (1) when the sheet is bent or impact is applied to the sheet in a low temperature environment.

Low-temperature elongation of the upper layer (12) of the base material is preferably smaller than the breaking elongation (low-temperature elongation) of the lower layer (11) of the base material measured in the same manner. The low-temperature elongation of the lower layer (11) measured using the above-described method is usually 40-120%, and preferably 50-110%. The low-temperature elongation of the upper layer (12) is usually 5-40%, and preferably 7-30%. The ordinary-temperature elongation of the lower layer (11) measured using the above-described method is usually 120-250%, and preferably 130-230%. The ordinary-temperature elongation of the upper layer (12) is usually 30-150%, and preferably 40-140%.

When the base material (1) is formed of the upper layer (12) and lower layer (11), the adhesive sheet of the present invention is suitable for use as a marking film. In this embodiment, the lower layer (11) of the base material is preferably a transparent layer containing substantially no pigment or a white layer containing a white pigment. The upper layer (12) is preferably a pigment-containing layer colored other than white. The lower layer (11) contains a polyurethane formed from a polyester polyol. Such a polyester-based polyurethane has comparatively low weather resistance. Therefore, in the case where a layer containing the polyester-based polyurethane is colored and used so as to be observable, fading of color (discoloration) may occur with the passage of time. A polycarbonate polyol-based polyurethane excels in weather resistance. Therefore, it is preferable to allow the upper layer (12) to be observable as the coloring layer. In this case, since the upper layer (12) (coloring layer) usually has a thickness smaller than that of the lower layer (11), the color of the lower layer (11) is easily observed through the upper layer (12). Therefore, the degree of vividness of the color of the upper layer (12) can be effectively increased by making the lower layer (11) transparent or white.

In an embodiment wherein the marking film of the present invention is bonded to an opaque adherend, the lower layer (11) is preferably white in order to effectively increase the degree of vividness of the color of the coloring layer by shading the color of the adherend. In an embodiment wherein the case of bonding the marking film to a light-transmitting adherend and allowing the coloring layer to be observed from the surface of the adherend using light from the back surface of the adherend, the lower layer (11) is preferably transparent or white translucent (diffuse translucent). Base material

The base material (1) is either a single-layer film formed only of a layer containing the first polyurethane resin (first polyurethane resin layer), or a laminated film consisting of the first polyurethane resin layer and another layer.

The raw material of the first polyurethane resin usually includes at least one polyester polyol. The polyester polyol refers to a polymer or oligomer having a polyester bond in the molecule and containing a hydroxyl group at least on both terminals.

The OH equivalent of the polyester polyol is not particularly limited. The OH equivalent of the polyester polyol is usually 200-30,000, and preferably 500-10,000. If the OH equivalent is too small, impact absorption effects at a low temperature may be decreased. If the OH equivalent is too large, the sheet may be plastic deformed or break due to an excessive amount of elongation when constructed at ordinary temperature (about 25°C) or more. The "OH (hydroxyl group) equivalent" of the polyol used herein refers to a molecular weight of the polyol per OH (hydroxyl group).

The polyfunctional isocyanate compound has two or more isocyanate functional groups in the molecule. A conventional polyfunctional isocyanate compound may be used insofar as the compound has a specific NCO equivalent. The NCO equivalent of the isocyanate compound included in the raw material of the first polyurethane resin is usually 50-2,000, and preferably 70-1,000. If the NCO equivalent is too small, impact absorption effects at a low temperature may be decreased, although this also depends upon the OH equivalent of the polyol. If the NCO equivalent is too large, the sheet may be plastic deformed or break due to an excessive amount of elongation when constructed at ordinary temperature (about 25°C) or more. The "NCO (isocyanate group) equivalent" of the polyfunctional isocyanate compound used herein refers to a molecular weight of the isocyanate compound per NCO (isocyanate group). There are no specific limitations to the number of functional groups in the molecule of the polyfunctional isocyanate compound insofar as the entire base material or the first polyurethane resin layer have sufficient low-temperature elongation whereby impact absorption can be obtained. The number of functional groups is preferably 2-2.8. As the polyester polyol, a polyester polyol having a polyester unit formed by

(1) one or more dicarboxylic acids selected from the group consisting of adipic acid, hexamethylenedicarboxylic acid, isophthalic acid, and orthophthalic acid, and (2) one or more diols selected from the group consisting of 1,6-hexanediol, ethylene glycol, propylene glycol, tetramethylene glycol, and caprolactonediol in the main chain, and containing a hydroxyl group at least on both terminals of the main chain may be used. A polyol obtained by ring-opening polymerization of caprolactone may also be used. Atriol such as 1,1,1-trimethylolpropane or glycerol maybe incorporated into the main chain in addition to the diol compound so that a hydroxyl group is present in the side chain. In order to increase impact absorption, use of a polyester polyol obtained by using an aliphatic dicarboxylic acid as the raw material is preferable.

As the isocyanate compound, one or more diisocyanates selected from the group consisting of isophorone diisocyanate, MDI (diphenylmethane diisocyanate), hydrogenated MDI, and 1,6-hexanediol diisocyanate may be used. A compound synthesized from a starting raw material containing such a diisocyanate may also be used. For example, (A) a compound obtained by a urethanization reaction of the above triol (1,1,1-trimethylolpropane or the like) and the above diisocyanate, (B) a compound having a biuret structure or an isocyanurate structure obtained by reacting the above diisocyanates, and the like may be utilized. A polyfunctional isocyanate compound obtained by the reaction of the above compound and a diol such as polycaprolactonediol may also be used to adjust the NCO equivalent.

The first polyurethane resin layer may contain a polyurethane formed using a polyol other the polyester polyol insofar as the impact absorption effects at a low temperature are not impaired. As examples of such a polyol, a polyether polyol, acrylic polyol, and the like can be given.

In the case where the base material (1) is formed of a laminated film, a layer combined with the first polyurethane resin layer usually has a thickness smaller than the thickness of the first polyurethane resin layer. As described above, this layer is preferably disposed between the first polyurethane resin layer and the protective layer (2). There are no specific limitations to the configuration of such a layer. Preferably, such a layer contains the second polyurethane resin formed of a reaction product of a polycarbonate polyol and a polyfunctional isocyanate compound (second polyurethane resin layer).

The raw material of the second polyurethane resin includes one or more polycarbonate polyols. The polycarbonate polyol refers to a polymer or oligomer having an alkylene polycarbonate skeleton or an aralkylene polycarbonate skeleton shown by "-O-R-O-(C=O)-" in the molecule and containing a hydroxyl group at least on both terminals. The OH equivalent of the polycarbonate polyol is not particularly limited. The OH equivalent of the polycarbonate polyol is usually 200-30,000, and preferably 500-10,000.

R in the above formula represents an aromatic or an aliphatic hydrocarbon group. R preferably represents an aliphatic hydrocarbon group, and particularly preferably a linear aliphatic hydrocarbon group. The aliphatic hydrocarbon group effectively increases adhesion of the second polyurethane resin layer to the protective layer (2) and the first polyurethane resin layer. Moreover, weather resistance of the second polyurethane resin layer can be particularly effectively increased. The isocyanate compound which is reacted with the polycarbonate polyol is preferably an aliphatic polyfunctional isocyanate compound in order to increase weather resistance of the second polyurethane resin layer. As the polyfunctional isocyanate compound used for forming the second polyurethane resin, the polyfunctional isocyanate compound given above may be used. The NCO equivalent of the isocyanate compound is usually 100-2,000, and preferably 200-1,000. There are no specific limitations to the number of functional groups in the molecule of the polyfunctional isocyanate compound insofar as impact absorption as the base material is not impaired. The number of functional groups is preferably 2-2.8. The second polyurethane resin layer may contain a polyurethane formed using a polyol other than the polycarbonate polyol insofar as the effect of the present invention is not impaired. As examples of such a polyol, a polyether polyol, acrylic polyol, and the like can be given. The base material is formed as follows, for example. Paint containing a polyurethane resin or a raw material for forming a polyurethane resin is provided. The polyurethane resin included in the paint may be a polyurethane resin of which the reaction is completed in advance. A raw material containing a polyol and a polyfunctional isocyanate compound may be included in the paint, and the reaction may be allowed to proceed in the film of the paint. The nonvolatile content (polyurethane, polyol, polyfunctional isocyanate compound, and the like) included in the paint is usually 15-85 mass%, and preferably 30-80 mass%.

The paint thus provided is applied to the surface of a substrate and heated to dryness to form a base material. The paint is applied using a printing method such as silk printing or application method using a knife coater, bar coater, or the like. The amount of application is usually 10-80 g/m², and preferably 12-70 g/m². The drying temperature is usually 60- 180°C. In order to promote the reaction between the isocyanate compound and the polyol, a heat treatment may be performed at a temperature of 30-60°C for 1-21 days.

In the case where the base material (1) is a laminated film, the base material is formed by layering the film of the first polyurethane resin paint and the film of the second polyurethane resin paint in the same manner as described above. For example, the first polyurethane resin layer is formed on the substrate from the film of the first polyurethane resin paint, and the second polyurethane resin paint is applied onto the first polyurethane resin layer and dried.

The thickness of the entire base material (1) thus obtained is usually 20-200 μm, and preferably 30-150 μm. In the case where the base material (1) is a laminated film, the thickness of the first polyurethane resin layer is usually 20-150 μm, and preferably 30-130 μm. The thickness of the second polyurethane resin layer is usually 1-100 μm, and preferably 2-70 μm.

A coloring agent such as a pigment may be included in the layers which make up the base material (1). The content of pigment is usually 1-80 mass% of the total mass of the layer. Additives such as a UV absorber, heat stabilizer, and plasticizer may be added in addition to the coloring agent insofar as the effect of the present invention is not impaired. Protective layer

The protective layer (2) is formed of a hydrophilic film containing a curing resin and a hydrophilic agent of an inorganic oxide as the essential components. The "hydrophilic film" refers to a film of which the surface has hydrophilicity. The water contact angle of the surface of the hydrophilic film is less than 70°, and preferably within a specific range of 65° or less. If the water contact angle of the surface of the protective layer is 65° or less, pollution resistance can be increased effectively and maintained for a comparatively long period of time.

The combination of the curing resin and the inorganic oxide-based hydrophilic agent is effective for controlling the water contact angle of the surface of the protective layer within a specific range of 65° or less. The inorganic oxide-based hydrophilic agent increases protection effects such as pollution resistance as described above. The surface of the protective layer can be prevented from becoming unnecessarily hydrophilic (for example, water contact angle of less than 35°) by combining the hydrophilic agent with the curing resin.

The lower limit of the water contact angle of the surface of the protective layer is not particularly limited from the viewpoint of pollution resistance. However, there may be a case where another adhesive sheet may be bonded in layers (over-laminated) to the surface of the protective layer of the adhesive sheet of the present invention. In this case, it is preferable to set the water contact angle of the surface of the protective layer to 35° or more in order to effectively prevent a decrease in waterproof adhesion of the over-laminated adhesive sheet. In many cases, another adhesive sheet (second adhesive sheet) is over-laminated on the surface (surface of the protective layer) of the adhesive sheet (first adhesive sheet) bonded to the surface of an adherend used outdoors such as the wall of a building and a sign. For example, the first adhesive sheet is bonded to the surface of a sign as an underlay, and the second adhesive sheet cut into the shape of a design, characters, or the like is bonded to the surface of the first adhesive sheet to form a bonded structure. In this case, the second adhesive sheet in contact with the hydrophilic film can be bonded with sufficient adhesion in a usual state. However, when the bonded structure is exposed to water such as rain for a long period of time, adhesion (peel resistance) between the surface of the protective layer (hydrophilic film) of the first adhesive sheet and the second adhesive sheet, specifically, waterproof adhesion may be decreased. Therefore, the water contact angle of the surface of the protective layer is preferably 35° or more from this point of view. In order to increase pollution resistance and to prevent a decrease in waterproof adhesion of the second adhesive sheet in good balance, the water contact angle of the surface of the protective layer is preferably 40-64°. It is preferable that the curing resin in the protective layer do not substantially contain a fluorine-based polymer but contain a non-fluorine-based polymer (polymer containing no fluorine atom in the molecule). This is particularly advantageous for effectively preventing a decrease in waterproof adhesion of the second adhesive sheet.

The above water contact angle refers to an initial value when starting to use the adhesive sheet. It is preferable that not only the initial value, but also the water contact angle of the adhesive sheet after subjected to a test using a sunshine weatherometer (WOM) for 1000 hours be within the above range. The water contact angle is a contact angle between water and the surface of the protective layer measured using a contact angle meter by dropping a waterdrop onto the surface of the protective layer. Water used for the measurement is usually purified water obtained by distilling ion exchanged water.

The thickness of the protective layer (2) is usually 0.1-15 μm, and preferably 1-10 μm. If the thickness of the protective layer (2) is too small, the strength of the protective layer (2) may be decreased, whereby the protection effects may be decreased. If the thickness of the protective layer (2) is too great, flexibility and elongation of the entire adhesive sheet may be decreased since the protective layer (2) contains the curing resin. The inorganic oxide-based hydrophilic agent is a compound containing at least one of (i) a silicon oxide containing compound such as an organosilicate compound (organosilicate, organosilicate condensate, and the like), and (ii) an inorganic oxide sol such as a silicon oxide sol, aluminum oxide sol, zirconium oxide sol, or antimony oxide sol. The hydrophilic agent is capable of controlling the water contact angle of the protective layer within a specific range. Compounds disclosed in Japanese Patent Application Laid-open No. 9-302257 and Japanese Patent Application Laid-open No. 11-267585 are preferable as such a compound.

As the hydrophilic agent, an organosilicate compound is preferable. The organosilicate compound is used in the form of a silane coupling agent (silicon oxide-based surface treating agent) carried on the surface of an inorganic oxide sol such as a silica sol, or a silica sol in which the surface of particles is covered with an organic polymer. The average particle diameter of the sol is usually 100 nm or less.

The amount of hydrophilic agent is usually 1-50 parts by mass, preferably 3-40 parts by mass, and particularly preferably 5-30 parts by mass for 100 parts by mass of the curing resin. If the amount of hydrophilic agent is too small, the water contact angle of the surface of the protective layer may exceed the specific range, whereby the protection effects such as pollution resistance may be decreased. If the amount of hydrophilic agent is too great, the water contact angle of the surface of the protective layer may be less than the specific range, whereby waterproof adhesion of the second adhesive sheet may be decreased. Curing resin in protective layer

The protective layer (2) contains the curing resin as described above. The curing resin contains a cured polymer. In the case of adding a curing agent for curing the polymer, the curing resin refers to a resin composition consists of a mixture of the above polymer and the curing agent.

Use of a curing resin containing a non-fluorine-based polymer is preferable. The non-fluorine-based polymer increases waterproof adhesion of the second adhesive sheet as described above. As the non-fluorine-based polymer, an acrylic polymer, polyolefin polymer, polyester polymer, polyurethane polymer, and silicone (including modified silicone such as silicone polyurea) polymer may be used. These polymers are curable polymers having a functional group capable of reacting with the curing agent such as a hydroxyl group, carboxyl group, epoxy group, and amino group, or having a photocurable functional group.

There are no specific limitations to the curing agent in the case where the curing resin includes the curable polymer and the curing agent. As the curing agent, an isocyanate compound and an epoxy compound are preferably used. The ratio of the curable polymer to the curing agent is usually 40-95 parts by mass to 5-60 parts by mass, and preferably 30-90 parts by mass to 10-70 parts by mass.

In addition to the above components, a UV absorber, a stabilizer, or other additives may be added to the protective layer (2) in order to improve weather resistance, stability, and other properties.

There are no specific limitations to the formation method of the protective layer (2). It is preferable to form the protective layer (2) by applying a coating solution prepared by dissolving or dispersing all the components including the hydrophilic agent, polymer, and curing agent in a solvent to the surface of the base material (1), and drying the applied solution to form a film. The solution is preferably dried under comparatively mild conditions at a temperature of 50-90°C for 3-60 minutes.

Adhesive layer

As the adhesive layer (3) disposed on the back surface of the base material (1), an adhesive layer used in a conventional adhesive sheet may be used. The adhesive layer (3) usually contains an adhesive such as a pressure sensitive adhesive, a heat sensitive adhesive (including a hotmelt adhesive), or a solvent reactivated adhesive. As the adhesive, use of an acrylic pressure sensitive adhesive is preferable taking into consideration workability, construction capability, weather resistance, and cost. AUV absorber and a heat stabilizer may be added to the pressure sensitive adhesive in order to improve weather resistance. A crosslinking agent (curing agent), a tackifier, a plasticizer, and the like maybe added in order to improve adhesion. Use of an adhesive having a peel strength (180° peel, 300 mm/min.) of 5-50 N/25 mm is preferable, because the adhesive sheet may be bonded to an adherend having a warped surface or to an adherend for which shape conformity is needed.

As the adhesive, a polyolefin adhesive, polyester adhesive, polyurethane adhesive, silicone (including modified silicone such as silicone polyurea) adhesive, and epoxy adhesive may be used in addition to the acrylic adhesive. The adhesive layer may be formed of a film of a coating solution including the adhesive. The thickness of the adhesive layer is not particularly limited. The thickness of the adhesive layer is usually 5-500 μm, and preferably 10-300 μm.

Example Weather resistance: A test specimen prepared in the same manner as in evaluation of low-temperature impact resistance was exposed to ultraviolet rays at a dose of 70 mW for 300 hours using a metal halide lamp accelerated weathering machine. The appearance of the adhesive sheet after exposure was observed with the naked eye. A case where one of interlayer delamination, cracks, and change in color was observed in comparison with the adhesive sheet stored as a control specimen without subjecting to the test was evaluated as "Bad". A case where none of interlayer delamination, cracks, and change in color was observed and there was almost no change in appearance in comparison with the control specimen was evaluated as "Good".

Pollution resistance: A test specimen prepared in the same manner as in evaluation of low-temperature impact resistance was allowed to stand outdoors. The degree of staining of the test specimen after four months was observed with the naked eye. A case where almost no staining was observed in comparison with an adhesive sheet stored indoors as a control specimen was evaluated as "Good". A case where staining was observed was evaluated as "Bad".

Water contact angle: A waterdrop was dropped onto the surface of the protective layer of the adhesive sheet. The contact angle between the surface of the protective layer and water was measured using a contact angle meter ("CA-Z" manufactured by Kyowa Interface Science Co., Ltd.) according to the procedure described in the manual. Water used for the measurement was purified water obtained by distilling ion exchanged water. The initial value is a value measured for the adhesive sheet before use. The value after WOM 1000 h is a value measured after subjecting the adhesive sheet to a sunshine weatherometer accelerated weather resistance test for 1000 hours according to the JIS standard.

A base material was formed on a substrate (PET film with a thickness of 50 μm) as described below.

As paint for forming a lower layer of the base material, paint (nonvolatile content: 75 mass%) containing 100 parts by mass of a first polyurethane resin produced by reacting a polyester polyol formed using an aliphatic dicarboxylic acid as the raw material and an aliphatic polyfunctional isocyanate compound, and 100 parts by mass of a white pigment was prepared. This paint was applied to the substrate using a knife coater. The applied paint was heated to dryness at 65°C for one minute and at 85°C for one minute to form a lower layer of the base material with a thickness of 35 μm.

As paint for forming an upper layer of the base material, paint (nonvolatile content: 52 mass%) containing 100 parts by mass of a second polyurethane resin produced by reacting a polycarbonate polyol and an aliphatic polyfunctional isocyanate compound, 50 parts by mass of a blue pigment, 50 parts by mass of a yellow pigment, and 25 parts by mass of a white pigment was prepared. This paint was applied to the surface of the lower layer on the substrate using a knife coater. The applied paint was heated to dryness at 65°C for one minute, at 85°C for one minute, and at 160°C for one minute to form an upper layer of the base material with a thickness of 10 μm. The base material including the substrate was obtained in this manner. In this base material, the ratio of the thickness of the lower layer to the thickness of the upper layer was 7:2. The low-temperature elongation of the resulting base material was 80%. The low-temperature elongation is a value measured by preparing a laminated film (thickness of each layer was adjusted so that the total thickness was about 35 μm while maintaining the ratio of the thickness) using the above paint, and subjecting a test specimen (length: 10 cm, width: 25 mm) of the laminated film to a tensile test in a constant temperature environment of 5°C at a tensile speed of 300 mm/min. along the longitudinal direction. The ordinary-temperature elongation of the base material was 170%. The ordinary-temperature elongation is a value measured in a constant temperature environment of 25°C in the same manner as in the measurement of low-temperature elongation. Paint for a protective layer was applied to the surface (of the upper layer) of the base material thus obtained using a knife coater. The paint was heated to dryness at 100°C for three minutes to cause the protective layer to adhere to the surface of the base material. The base material including the protective layer was thus obtained. The thickness of the protective layer was 3 Dm.

The paint for the protective layer was prepared by mixing curing agent paint (containing a polyfunctional isocyanate compound) and hydrophilic paint ("Belclean Clear No. 5000" manufactured by Nippon Oil and Fats Co., Ltd.). The mixing ratio of the hydrophilic paint (nonvolatile content: 46 mass%) to the curing agent paint (nonvolatile content: 70 mass%) was 65:35 (mass ratio).

The hydrophilic paint used in this example contained an organosilicate compound-based hydrophilic agent (silane coupling agent carried on the surface of a silica sol) and an acrylic polyol resin.

After removing the substrate from the base material including the protective layer, an adhesive layer with a liner provided separately was bonded to the back surface (of the lower layer) of the base material to complete an adhesive sheet of this example. The adhesive layer was a layer containing an acrylic pressure sensitive adhesive and having a thickness of 30 μm.

The low-temperature elongation of the lower layer of the base material, the upper layer of the base material, and the protective layer used in this example was 91%, 10%, and 7%, respectively. The low-temperature elongation is a value measured using a test specimen with a thickness of 35 μm, a length of 10 cm, and a width of 25 mm formed of the film used for each layer by using the above-described method. The ordinary-temperature elongation of the lower layer of the base material, the upper layer of the base material, and the protective layer was 180%, 125%, and 12%, respectively. The ordinary-temperature elongation is a value measured using a test specimen with a thickness of 35 μm, a length of 10 cm, and a width of 25 mm formed of the film used for each layer using the above-described method.

The adhesive sheet of this example was evaluated as described below. The results are shown in Table 1. Comparative Example

An adhesive sheet of this example was completed by forming a protective layer on a base material (thickness: 45 μm) formed of a film of the above paint for the upper layer of the base material. The adhesive sheet of this example was evaluated in the same manner as in

Example 1. The results are shown in Table 1. Evaluation method

Low-temperature impact resistance: The adhesive sheet of each example was bonded to the surface of an aluminum substrate with a length of 250 mm, a width of 25 mm, and a thickness of 3 mm in a thermostat at 25°C to obtain a test specimen.

The test specimen was allowed to stand at 5°C for 24 hours and subjected to an impact resistance test at 5°C as described below. A 1 lb (about 454 g) weight was caused to collide with the test specimen from 50 inches (about 1.27 m) above the surface of the adhesive sheet on the test specimen with a sharp tip facing downward. The surface of the adhesive sheet after collision was observed with the naked eye. A case where the adhesive sheet was deformed in a state in which the adhesive sheet adhered to the substrate and no cracks were observed on the surface of the protective layer was evaluated as "Good". A case where cracks were observed in the protective layer and/or the base material was evaluated as "Bad". In the adhesive sheet of Comparative Example, cracks occurred across the direction of the depth of the sheet and the surface of the substrate was exposed by the cracks.

Table 1

As described above, the adhesive sheet according to the present invention can effectively prevent damages to the sheet such as cracks in the protective layer in a low-temperature environment, and is provided with excellent protection effects (pollution resistance and weather resistance). A marking film which can be suitably used outdoors in all seasons can be provided by using the adhesive sheet of the present invention. Explanation of Symbols

1: Base material, 2: protective layer, 3: adhesive layer, 11: lower layer of base material, 12: upper layer of base material, 100: adhesive sheet, 111: front surface of lower layer of base material, 112: back surface of lower layer of base material, 121: front surface of upper layer of base material, 122: back surface of upper layer of base material

Claims

What is claimed is:

1. An adhesive sheet comprising: a flexible base material, an adhesive layer disposed on a back surface of said base material, and a protective layer disposed on a front surface of said base material and made of a hydrophilic film containing a curing resin and a hydrophilic agent of an inorganic oxide; wherein said base material contains a layer containing a first polyurethane resin having a reaction product of polyester polyol and a polyfunctional isocyanate compound.

2. An adhesive sheet according to claim 1, wherein low-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 5°C, of said base material is larger than that of said protective layer measured in the same manner, and said base material is thicker than said protective layer.

3. An adhesive sheet according to claim 1, wherein low-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 5°C, of said base material is within the range from 50% to 100%.

4. An adhesive sheet according to claim 1, wherein low-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 5°C, of said base material is 50% or more, and ordinary-temperature elongation, which is defined as breaking elongation measured at a tensile speed of 300 mm/min. at 25°C, of said base material is 200% or less.

5. An adhesive sheet according to claim 1, wherein said base material comprising: a lower layer containing the first polyurethane resin, and an upper layer which is disposed between the lower layer and said protective layer, which adheres to said protective layer, and which contains a second polyurethane resin having a reaction product of a polycarbonate polyol and a polyfunctional isocyanate compound; wherein the lower layer is thicker than the upper layer.

6. A marking film made of an adhesive sheet according to claim 5, wherein said protective layer is substantially transparent, the lower layer of said base material is a transparent layer not containing pigment substantially or a white layer containing a white pigment, the upper layer of said base material is a colored layer containing a pigment except for a white pigment.