Field of Invention The present invention relates to an adhesive sheet which comprises elastic microspheres in an adhesive layer and is adhered to a substrate having an uneven surface, a method for producing such adhesive sheet, and a structure having an adhered adhesive sheet. In particular, the present invention relates to an adhesive sheet which has a good adhering property when it is adhered to a substrate exposed outdoors such as an outer wall of a building of mortar or concrete.

Background of Invention

The following adhesive sheets (a) to (d) are known as adhesive sheets (also known as adhesive films) such as a decorative sheet or a retroreflective sheet, which are adhered to a substrate having an uneven surface such as a wall, for example:

(a) An adhesive sheet disclosed in Japanese Patent Publication JP-A-2- 32180 comprises a backing layer which comprises a nonwoven fabric and a flexible resin, and an adhesive coated on said backing layer. This adhesive sheet intends to improve a follow-up property ofthe sheet to the uneven surface of the substrate by the use ofthe above backing layer having good flexibility. When it is adhered to a porous substrate such as mortar or concrete materials and used outdoors, water or vapor containing water which is absorbed by the substrate migrates to an interface between the adhesive layer and the substrate, but cannot escape from the interface, whereby the adhesive layer and in turn the adhesive sheet is blistered by an osmotic pressure. Since the surface ofthe substrate is very rough, it is difficult to decorate the surface by, for example, printing.

(b) A repeelable adhesive tape disclosed in Japanese Patent Publication JP-U-572946 comprises a backing layer, a viscoelastic layer formed on the backing layer, and an adhesive layer which contains microspheres and is formed on said viscoelastic layer. This adhesive tape intends to improve the following-up property

to the uneven surface ofthe substrate by the function ofthe viscoelastic layer But, since an average particle size ofthe microspheres is as small as 1 to 10 μm, the tape has insufficient adhesion force to a substrate surface having relatively large unevenness such as the mortar or concrete material In addition, since the viscoelastic layer is additionally formed, the production steps may be complicated

Here, a degree of unevenness" is expressed by a surface roughness, and is defined in Japanese Industrial Standards JIS B 0601 (1994) When an arithmetical average surface roughness (Ra) measured at a cut-off value of 8 mm and an evaluated length of 40 mm is about 20 to 100 μm, a surface is ranked as "a surface having a relatively large degree of unevenness" or "a rough surface"

(c) A pressure sensitive adhesive sheet disclosed in Japanese Patent Publication JP-A-7-3222 and Japanese Patent Publication JP-A-7-48549 comprises, in general, a pressure sensitive adhesive layer which is photocured, and microspheres surfaces of which are treated to lower their surface energy and which are contained in said layer This pressure sensitive sheet intends to improve its following-up property to the uneven surface ofthe substrate by surface treating nontacky microspheres having an average diameter of 5 to 200 μm to make mobility ofthe microspheres in the pressure sensitive layer easy when the sheet is adhered However, since the microspheres are embedded in the adhesive layer and do not form convex parts, the adhesion ofthe sheet to the uneven surface cannot be improved

(d) An adhesive tape disclosed in Japanese Patent Publication JP-U-1- 70848 comprises a backing layer tape and an adhesive which is formed on the backing layer in a relatively regular uneven pattern such as a net pattern But, the formation ofthe regular pattern is difficult, and a contact area is not increased effectively on the substrate the surface unevenness of which is not necessarily regular, such as the mortar or concrete material

As adhesive sheets having an adhesive layer which contains tacky elastic microspheres, the following adhesive sheets (e) and (f) are known (e) An adhesive film disclosed in Japanese Patent Publication JP-A-6-

287525 comprises a film backing and an adhesive layer which is formed on said film

and comprises tacky elastic microspheres having a volume average diameter of 100 to 300 μm and an adhesive, whereby an air trapped between the film and a flat surface of a substrate can escape when the adhesive film is adhered to the substrate. The microspheres are contained in the adhesive discretely or in a "cluster state" (which will be defined later) in which the microspheres are dispersed in the form of minute bulks consisting of two or more agglomerated microspheres. When the elastic microspheres are contained discretely, the adhesive film cannot have a sufficient adhesion force on the surface ofthe substrate which has a relatively large degree of unevenness and irregular patterns and configurations, such as the mortar or concrete material This Japanese KOKAI Publication suggests that the formation ofthe clusters ofthe elastic microspheres is advantageous to escape bubbles, but does not describe or teach limitation in relation to the clusters which provide good adhesion to the uneven surface, and a method for forming advantageous thereto, (f) A repeelable adhesive material disclosed in Japanese Patent Publication JP-A-6172739 contains only tacky microspheres having an average particle size of 1 to 200 μm, or such tacky microspheres and a nontacky binder This adhesive material does not have a film-form backing, and the formation of clusters ofthe elastic microspheres is not suggested. Since a production method of this adhesive material does not include any means for coating the adhesive material directly on a backing, formation of clusters of tacky microspheres is very difficult

In addition, noninclusion of a tacky polymer makes the effective formation ofthe cluster of microspheres difficult

Summary of Invention The conventional adhesive sheets do not have sufficient adhesion force on the substrate having a rough surface having a relatively large degree of unevenness, and irregular shape and configuration ofthe unevenness such as the mortar or concrete material As the results, the adhesive sheet tends to peel off from the substrate as the time passes. Since the mortar and concrete materials are water absorbing porous materials, when the adhesive sheet is adhered thereto, vapor containing water which

penetrated in the substrate migrates to the adhesion interface to cause the blister of the adhesive layer As the result, the adhesive layer tends to peel off from the substrate

In view ofthe above problems, one aspect ofthe present invention provides an adhesive sheet which can maintain a sufficient adhesion force on a substrate having a relatively large degree of unevenness and irregular shape and configuration of unevenness and comprising a water-absorbing porous material such as a mortar or concrete material, and can prevent blister of an adhesive layer caused by vapor containing water which migrates from the inside ofthe substrate to an adhesion interface

Another aspect ofthe present invention provides a method for producing an adhesive sheet which can form clusters of elastic microspheres useful in the adhesive sheet

An adhesive sheet ofthe invention comprises a backing layer, and an adhesive layer which is formed on said backing layer and contain tacky elastic microspheres wherein a) said adhesive layer has convex adhesive parts containing one or more clusters of at least two said elastic microspheres and said, and b) a contact area rate between said adhesive layer and a plate surface is from 20 to 90 % when said contact area rate is measured by adhering said adhesive sheet to a flat glass plate surface under the pressure of 2 kg/25 cm ²

Brief Description ofthe Drawing

Fig 1 schematically shows the adhesive sheet having the clusters of microspheres according to the present invention

Embodiments ofthe Invention

An adhesive sheet of the invention is schematically shown in Fig 1 The adhesive sheet 1 has discrete adhesive parts 3 containing the convex clusters ofthe microspheres 2 Since the convex adhesive parts contain the cluster or clusters of the elastic microspheres, sufficient adhesive force is attained even on a substrate having a rough surface such as the mortar or concrete material

•A-

"Tacky elastic microsphere" and "microsphere" mean particles having a volume average diameter of less than 500 μm and at least a surface of which has tackiness, and exhibits rubbery elasticity as a whole The rubbery elasticity ofthe microspheres is necessary for the increase ofthe adhesion force ofthe adhesive layer to the uneven surface and the formation ofthe passageways which prevent the blister ofthe adhesive layer. In addition, the microspheres themselves make it easy and sure to form the cluster structure, and improve the adhesion force ofthe adhesive layer to the uneven surface

"Cluster of elastic microspheres" means an agglomerate ofthe elastic microspheres which are dispersed in a bulk form of two or more ofthe microspheres in a matrix ofthe tacky polymer to form a convex adhesive part having an irregular shape and irregular configuration That is, the adhesive layer comprises "island" parts (discrete phases) comprising the cluster(s) and the tacky polymer, and relatively flat "sea" parts (continuous parts) comprising the tacky polymer but no cluster

"Adhesive layer" means is a layer comprising microspheres and tacky polymer, and having plural convex adhesive parts with the irregular shapes

"Tacky polymer" means a polymer which has tackiness at room temperature and can be used as a pressure-sensitive adhesive Each convex adhesive part comprises an agglomerate ofthe elastic microspheres which are arranged in a two-dimensional plane, an agglomerate ofthe elastic microspheres which are three dimensionally arranged, or both When the adhesive sheet is adhered to a substrate, the agglomerated elastic microspheres in the cluster are shifted to deform the convex adhesive part in a shape which is suitable to be adhered to the uneven surface ofthe substrate However, a degree of this deformation should be configured so that passageways which communicate with the atmosphere are formed and maintained between the uneven surface and the adhesive layer The degree ofthe deformation can be controlled by the tackiness of the elastic microspheres, and the inclusion ofthe elastic microspheres and the tacky polymer in a specific ratio.

The irregular shape ofthe convex adhesive part serves to improve the adhesion ofthe adhesive layer to the substrate having the irregular uneven surface The irregular shape ofthe convex adhesive part is derived from the irregularly shaped and arranged cluster ofthe elastic microspheres The cluster ofthe elastic microspheres can be formed by coating an adhesive coating composition on a coating surface ofthe backing layer having a specific wetting index to form the adhesive layer The cluster formation may be accelerated by controlling a viscosity ofthe adhesive coating composition in a specific range

When the adhesive sheet ofthe present invention is adhered to the surface of the substrate, the above described "sea-island" structure (continuous phase/discontinuous phase) serves to provide passageways which are formed by the substrate surface and the adhesive layer and communicate with the atmosphere, whereby the blister ofthe adhesive layer caused by the vapor containing water which migrates from the substrate is prevented To form such passageways, a contact area rate between the adhesive layer and a flat glass surface, which specifies a degree of contact between them and will be defined later, is adjusted in the specific range, when it is measured by adhering the adhesive sheet to the flat glass surface.

The contact area rate is easily regulated by maintaining the ratio ofthe elastic microspheres to the tacky polymer in the specific range, using the elastic microspheres having the specific average particle size, and adjusting a coating weight ofthe adhesive layer in a specific range

1 Elastic Microspheres Preferably, microspheres for the present invention have a volume average diameter from 10 to 300 μm, preferably from 50 to 250 μm When the volume average diameter is in the range between 50 μm and 250 μm, the adhesive layer follows the uneven surface very well, an apparent contact area rate increases, and consequently the sufficient adhesion force is achieved

The volume average diameter is obtained by measuring diameters of 1000 microspheres using an optical microscope and an image analyzer, and calculating an average value according to the following equation

Volume average diameter (μm) = ∑(d, ⁴ x n,)/∑(d, ³ x n,) wherein d, is a diameter (μm) of a microsphere having an i-th largest diameter, and n, is the number of microspheres having the diameter d,

The microsphere may be solid or hollow having at least one void therein Polyacrylate is preferred as a material for the microspheres, since it is readily available, and its rubbery elasticity and tackiness are easily controlled A compressive modulus ofthe microsphere is preferably in the range between 1 x IO ⁴ and 1 xlO ⁶ dyne/cm ² When the compressive modulus is in this range, the microspheres themselves and/or their clusters are effectively deformed, and the adhesive layer is effectively adhered to the uneven surface

The compressive modulus is measured using a RSA 1 1 viscoelastic spectrometer (manufactured by RHEOMETRIX) at 20°C That is, temperature dependency ofthe modulus is measured by changing temperature from -80°C to 150°C with applying a compression strain having a frequency of 1 rad/sec , and a measured value at 20°C is used as the compressive modulus

2 Method for Producing Elastic Microspheres

The microspheres may be produced by any of known methods such as suspension polymerization, emulsion polymerization, seed polymerization, and so on As an example, a method for producing the microspheres of polyacrylate by emulsion polymerization will be explained briefly In a reactor equipped with a mechanical stirrer, deionized water, an acrylic monomer, a radical polymerization initiator, and other optional additives are added, and an interior ofthe reactor is purged with an inert gas Then, the reactor is heated to a specific temperature to initiate polymerization ofthe monomer while stirring In general, a stirring rate is from 10 to 700 rpm, and a reaction temperature is from 30 to 120°C A reaction time is usually from several hours to several ten hours

As the acrylic monomer, a mixture of an alkyl acrylate (e g isooctyl acrylate, 2-ethylhexyl acrylate, isononyl acrylate, etc ) and an acrylic unsaturated acid (e g acrylic acid, methacrylic acid, itanonic acid, maleic acid, etc ) is used A weight ratio ofthe alkyl acrylate to the acrylic unsaturated acid is preferably from 99 1 to 90 10 When an amount ofthe alkyl acrylate is too small, the tackiness is lowered, while it is too large, the rubbery elasticity is decreased In either case, the adhesion force to the uneven surface tends to decrease It is possible to crosslink the polyacrylate by the addition of a crosslinking agent comprising a bifunctional acrylate such as divinyl benzene l,4-dibutyl-2diacrylate to the above mixture The production method ofthe polyacrylate microspheres is disclosed in, for example, US Patent No 4,994,322

The produced microspheres are isolated by filtration and used in general, while the as-produced aqueous dispersion containing the microspheres after the reaction can be used, and the tacky polymer is added to the dispersion to obtain the adhesive coating composition

3 Tacky Polymer

Tacky polymers such as polyacrylates, polyurethanes, polyolefins, polyesters, and so on may be used As in the case of conventional pressure- sensitive adhesives, a tackifier may be used in combination with the tacky polymer

A molecular weight ofthe tacky polymer is in a range in which the desired tackiness is achieved In general, a weight average molecular weight is from 10,000 to 100,000

4 Adhesive Layer

The irregular shaped convex adhesive parts achieve the development of good adhesion to the uneven surface, and the formation ofthe passageways which communicate with the atmosphere and prevents the blister ofthe adhesive layer

A mixing ratio ofthe microspheres to the tacky polymer is preferably from 20 to 500 wt parts, more preferably from 100 to 400 wt parts ofthe former per lOO wt parts of the latter When the amount ofthe former is less than 20 wt parts,

it is difficult to form the cluster (which will be explained later) having an effective size, while it exceeds 500 wt parts, it may be difficult to form the passageways which communicate with the atmosphere between the uneven surface and the adhesive layer so that the prevention ofthe blister ofthe adhesive layer due to water and so on will be difficult

5 Backing Layer

Any conventional material can be used as a backing layer For example, a sheet of paper, a meta! film, a plastic film and so on can be used As the plastic film, a synthetic polymer such as polyvinyl chloride, polyester, polyurethane, polyacrylate, etc can be used A thickness ofthe film is usually from 10 to 1500 μm

A surface ofthe backing layer on which the adhesive layer is formed may be surface treated to increase the bond force ofthe adhesive layer to the backing layer, or to control the wetting index which will be explained later

6 Cluster of Elastic Microspheres

In general, the cluster comprises at least two microspheres The cluster having a size effective to the preferred adhesion property contains 10 to 200 microspheres A percentage ofthe clusters having such effective size in the whole adhesive layer is preferably at least 80 % of all the clusters The formation, shape and size ofthe clusters ofthe microspheres in the adhesive layer can be confirmed by the observation by an optical microscopy using reflected light, at a magnification of 10 to 100 times One of suitable methods for forming the clusters comprises adjusting the wetting index on the coating surface ofthe backing layer in the range between 35 and 70 dyne/cm ² , coating the adhesive coating composition on the coating surface and drying it to form the adhesive layer

When the wetting index is too low, coating irregularity appears in the adhesive phase so that the adhesion force to the rough surface tends to decrease,

while the too large wetting index may make it difficult to form the effective clusters From these view points, the preferred wetting index is from 40 to 60 dyne/cm ²

The wetting index ofthe backing layer can be adjusted by the surface treatment ofthe backing layer such as corona treatment, plasma treatment, chemical treatment, etc In the case ofthe plastic film, its wetting index can be adjusted by a kind and an amount of an additive such as a plasticizer which is contained in the plastic film As a method for adjusting the wetting index, the corona discharge treatment is preferred, since contamination ofthe adhesive layer by the chemical can be avoided in comparison with the chemical treatment In the case ofthe corona discharge treatment, a discharge energy is generally from 0 1 to 3 kW, and a discharge frequency is from 5000 to 35,000 Hz While the corona discharge can be carried out in the air or an atmosphere of, for example, nitrogen

In the chemical treatment, ( 1 ) a solution of an acid, an alkali or an organic amine, or (2) a solution of a trichloroacetic acid or a phenol compound can be used as a chemical Such chemical is applied on the coating surface ofthe backing layer, optionally heated, and dried

The wetting index is measured by a wetting test method according to JIS 6768 (1977) at 23±2°C, 50±5 %RH

The control ofthe viscosity of the adhesive coating composition is also advantageous for the easy formation ofthe clusters Preferably, the viscosity ofthe adhesive coating composition is from 100 to 6000 cps When the viscosity is lower than 100 cps, the adhesive layer having a sufficient coating weight may not be formed, while it is higher than 6000 cps, the adhesive layer in which the microspheres are uniformly arranged tends to be formed, so that the effective clusters may not be formed

As explained above, the mixing ratio ofthe microspheres to the tacky polymer, and the tackiness of the microspheres may be the factors which control the formation ofthe effective clusters

A coating weight suitable for the formation ofthe effective clusters is from 5 to 150 g/m ² , more preferably from 50 to 100 g/m2

7 Contact Area Rate

It is necessary to maintain a ratio ofthe real contact area to the apparent contact area between the adhesive layer and the substrate, that is, the contact area rate, in a specific range for achieving both the high adhesion force to the uneven surface, and the prevention ofthe blister ofthe adhesive layer when the adhesive sheet is adhered to the substrate having the porous surface But, it is very difficult to define a degree of contact between the adhesive layer and the substrate by the measurement ofthe contact area percentage on the uneven surface Then, the contact area rate is defined by a contact area percentage between the adhesive layer and a flat surface which is measured by adhering the adhesive sheet to a flat surface of a glass plate

Concretely, the adhesive layer ofthe adhesive sheet is adhered to a flat surface of a glass plate such as a slide glass and pressed by a roller of 2 kg by reciprocating it over the adhesive sheet one time When the glass plate is illuminated by a white light from the glass surface on which the adhesive sheet is not adhered, and the reflected light is observed through a polarized light filter, domains in which the convex adhesive parts and the glass surface are contacted are seen dark, while non-contact domains are seen white A photograph of such observed state is taken, and a total area ofthe contact domains and the area of the whole observed field (corresponding to the apparent contact area) are measured, and their ratio (the total area ofthe contact domains/the area ofthe whole observed field) is expressed in "percentage", which is used as the contact area rate"

The above measurement can be done using an optical microscope equipped with a Polaroid camera In this case, the area ofthe observed field is usually 1 cm ² A surface roughness Ra ofthe used glass plate is 0 1 μm or less

The contact area rate measured as above is preferably from 20 to 90 % more preferably from 40 to 80 %, in particular from 50 to 70 % The contact area rate in this range can achieve both the adhesive property ofthe adhesive sheet to the uneven surface and the effective formation ofthe passageways which escape the vapor containing water trapped between the adhesive sheet and the substrate

The contact area rate increases, as the size ofthe microspheres decreases or the coating weight ofthe adhesive layer increases, while it decreases as the content ofthe microspheres in the tacky polymer increases Accordingly, the contact area rate can be controlled in the above range by adjusting these parameters in the desired ranges

8 Production Method of Adhesive Sheet

The adhesive sheet ofthe present invention can be produced by coating the adhesive coating composition on one or both ofthe surfaces ofthe backing layer, and drying it to form the adhesive layer In this method, the clusters are effectively formed by the use ofthe above described measures

The adhesive coating composition is prepared by mixing the microspheres, the tacky polymer, the solvent, and optional additives in a mixing apparatus such as a homomixer, a planetary mixer, etc. to obtain a mixture in which all the components are homogeneously dispersed The prepared coating composition is coated on the backing layer, and dried to form the adhesive layer Any conventional means such as a knife coater, a roll coater, a die coater, a bar coater, and so on can be used The drying is carried out at a temperature of 60 to 180°C A drying time is usually from several ten seconds to several minutes As the solvent, water or an organic solvent may be used For the formation ofthe effective clusters, an aqueous coating composition is suitable In this case, the drying temperature is usually 100°C or higher In addition, a co-solvent which is partly miscible with water may be added Examples of useful co-solvent are alkylene glycol monoalkyl ether esters such as 3-methyl-3methoxybutyl acetate As the additive, any conventionally used additive may be used as long as the effects ofthe present invention are not impaired Examples ofthe additives are a viscosity modifier, an anti-foaming agent, a leveling agent, a UV light absorber, an antioxidant, a pigment, etc

After the formation ofthe adhesion layer, it may be coated with a silicone- treated liner for the protection of the adhesion layer

9 Adhesive Sheet

The adhesive sheet ofthe present invention can be used as a decorative sheet and adhered to a wall, a floor, or a ceiling of a building In particular, the adhesive sheet ofthe present invention is useful, when the surface is covered with a mortar, a concrete, or a wall paper, and has a large degree of unevenness the shape and size of which are irregular

Since the adhering surface of the adhesive layer of the adhesive sheet according to the present invention has many convex adhesive parts containing the clusters of the microspheres, when it is adhered to the substrate, the surface ofthe substrate and the adhering surface are spot adhered to form continuous passageways Since the adhesive sheet of the present invention has such structure, it can remove water which migrated in the substrate from outside through the passageways into the atmosphere, when the adhesive sheet is adhered to the porous substrate which absorbs water Therefore, the blister of the adhesive layer caused by such water can be prevented If the blister reaches the backing layer, appearance of the adhered sheet is deteriorated, and the blister causes the peeling off of the sheet from the substrate Therefore, the adhesive sheet of the present invention is very useful when it is used on the substrate which is exposed outdoors such as the outer wall ofthe building made of the mortar or concrete The adhesive sheet ofthe present invention has the much larger adhesion force to the mortar surface than the conventional adhesive sheets, for example, 2 kg/2 5 cm or larger), and seldom suffers from the generation of the blister of the adhesive layer caused by the exudation ofthe vapor containing water

The present invention together with such effects will be illustrated by the following Examples

EXAMPLES

In Examples 1-14 and Comparative Examples 3 and 4, the following elastic microspheres, tacky polymer, and backing layer film were used

Elastic Microsphere

As described above, the elastic microspheres were produced by the suspension polymerization in water as a medium A monomer composition contained IOA (isooctyl acrylate) and AA (acrylic acid) in a weight ratio of 96 4 A volume average diameter in each Example is shown in Table 1

The compressive modulus was adjusted to 7 x 10 ⁵ dyne/cm ² for all the diameters of the microspheres The compressive modulus was measured under the above described measuring conditions with a sample which was produced by removing the solvent from the suspension containing the microspheres, molding them in a cylindrical sample with a diameter of 5 mm and a height of 7 mm, and attached to parallel plate jigs Tacky Polymer

As a tacky polymer, an aqueous dispersion tackifier El 000 manufactured by SOKEN CHEMICAL Co , Ltd was used, which had a monomer composition of butyl acrylate and acrylic acid in a weight ratio of 94 6

Backing Layer Film

A backing layer was a polyvinyl chloride film of 70 μm in thickness having a polyester carrier of 50 μm in thickness This backing layer was produced by casting a solution of a homopolymer of vinyl chloride Its coating surface was treated by corona discharge to adjust its wetting index to a value in Table 1

Preparation of Sample Sheet

To the tacky polymer (100 wt parts), the microspheres of the weight parts shown in Table 1 was added, and mixed by a homomixer, and the mixture was coated on the backing layer film with a knife coater so as to achieve the coating weight of Table 1 after drying After drying it at 105°C for 5 minutes, a silicone liner (PD 530 BH manufactured by TOKUSHU SEISHI) was laminated to obtain a liner-laminated adhesive sheet

In Comparative Examples 1 and 2, conventional products were used That is, SCOTCHCAL film UPF-1 (manufactured by 3M) and FARAON sheet (manufactured by FARAON) were used in Comparative Examples 1 and 2, respectively Both are commercially sold as concrete masking films

With each ofthe adhesive sheets, the following properties were measured Contact Area Rate

On a flat surface of a slide glass of 76 mm in length, 26 mm in width and 1 mm in thickness (MICRO SLIDE GLASS HAKUROKUMA No 1 manufactured by MATSUNAMI GLASS INDUSTRIES, Co , Ltd ), the adhesive layer ofthe adhesive sheet of about 5 cm x about 2 cm, from which the liner had been removed, was adhered and pressed by reciprocating a roller of 2 kg over the sheet one time to obtain a sample Then, the contact area rate was measured with the sample according to the above described method This flat surface ofthe slide glass had Ra ofabout O OOl μm

Bubble Escapabilitv

After peeling off the liner, the adhesive sheet of 10 cm x 10 cm was placed on a flat acrylate plate, and squeezed by a squeezer towards the center ofthe adhesive sheet to gather bubbles A roller of 2 kg was rolled over the bubbles several times, and a degree ofthe bubble escape was observed When all the bubbles escaped, the sheet was ranked "Good", while when a part of bubbles remained, the sheet was ranked "Bad" Adhesion Force

An adhesive sheet of 15 cm x 2 5 cm was adhered to a mortar plate manufactured by Nippon Test Panel which had been degreased with isopropanol at

20°C, 65 %RH (Ra ofthe rough surface being in the range between 20 and 100 μm) according to JIS Z 0237 (1980), and kept at the same temperature and the same humidity for 48 hours Then, a 180 degree peel strength was measured at a peeling rate of 30 cm/rnin Blister

An adhesive sheet of 15 cm x 2 5 cm was adhered to a rough surface ofthe same mortar plate as used in the measurement of adhesion force with a rivet brush, and dipped in water kept at 40°C for one week When the sheet was not peeled off, it was ranked "No" When the slight blister was formed but the sheet was not peeled off, the sheet was ranked "Slight" When the blister was formed and the sheet was peeled off, it was ranked "Yes"

Viscosity of Coating Composition

A viscosity of each adhesive coating composition before coating was measured by a B-type viscometer (Spindle No 4) at 23°C at a rotation speed of 60 φm

The results are shown in Table 1

Table 1

Microsphere Microspheres Cluster Wetting Contact Coating Viscosity of Adhesion force Blister Bubble diameter (wt parts) index area rate weight coating (kg/2 5cm) escapabihty

(μm) (dyne/ cm ² ) (%) (g m ² ) composition (csp)

C Ex 1 . No _ 100 133 . 1 2 Yes Bad

C Ex 2 - « No _ 100 47 _ 0 7 Yes Bad

C Ex 3 135 10 No 47 100 132 1000 5 1 Yes Bad

C Ex 4 135 900 Yes 47 92 127 200 1 1 Yes Bad

C Ex 5 135 100 Yes 37 17 36 200 0 4 No Good

C Ex 6 40 100 No 47 50 19 150 0 1 No Good

∞

The invention is not limited to the above embodiments The claims follow