Adhesive sheet and its use for base plate roll

The present invention relates to a novel adhesive sheet utilized for attaching a printing plate to a base plate roll, a use thereof and a process for detaching a printing plate from a base plate roll.

A printing plate leaf made of resin is attached to a base plate roll to form a plate roll by means of a double faced adhesive sheet and continuous printing is made by rotating the plate roll.

Adhesives generally utilized for attaching the printing plate leaf to the base plate roll have strong adhesive ability and the printing plate leaf is not easily detached from the base plate roll after the printing is finished. Occasionally, the printing plate leaf made of resin is torn up during the operation of detaching. A part of the adhesive and the adhesive sheet usually remains on the surface of the base plate roll or the printing plate and removal of the remaining things takes time.

Patent abstract of Japan; Vol. 13; No. 519(M-895)(3867) corresponding to JP-A-1210343 discloses a process of printing which is performed by fixing a lithographic plate to a transparent plate cylinder using a photopeel-type adhesive. After printing, light is projected to the rear surface of the lithographic plate through the surface of a transparent plate cylinder, and the lithographic plate is removed from the plate cylinder by decreasing the adhesion of the photopeel-type adhesive.

CH-A-497 280 discloses the fixation of character fonts during typing operation by means of a watersoluble adhesive.

US-A-4,871,631 discloses a flexographic printing relief-plate, which has a relief layer, an interlayer and a base layer which is provided with a double-sided adhesive sheet. The double-sided adhesive sheet transmits actinic light and is firmly bonded to the base layer on one side. On the opposing side, the adhesive sheet has an adhesive force of from 1 to 6 N/25 mm which is less than the adhesive force on the side bonded to the base layer. The adhesive sheet permits the flexographic relief printing plate to be repeatedly adhered to and removed from the printing cylinder without damaging the plate.

It was considered by the present inventors that the antinomic problem that a printing plate is firmly attached to a base plate roll during the printing operation and easily detached after the printing operation is finished can be solved only by changing the adhesive ability of the adhesive utilized between the time of attachment and the time of detachment. For the purpose of changing the adhesive ability of the adhesive utilized, a mechanism of decreasing the adhesive ability by curing the adhesive by the action of electromagnetic wave and another mechanism of decreasing the adhesive ability by swelling the adhesive by water were considered. These considerations led to the completion of the present invention.

The present invention accordingly has an object to provide an adhesive sheet utilized for a plate roll in which a printing plate is firmly attached during the printing operation, the printing plate is easily detached from the base plate roll after the printing operation is finished and remaining adhesive is easily removed. Another object of the invention is to provide a use of an adhesive sheet for attaching a printing plate to the base plate roll.

Still another object of the present invention is to provide a process for detaching a printing plate made of resin from a base plate roll.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

In accordance with the present invention there is provided an adhesive sheet comprising a removable sheet utilized for attaching a printing plate made of resin to a base plate roll which is characterized in that the adhesive sheet comprises a photo curable adhesive which loses adhesive ability by curing by irradiation of electromagnetic wave or electron beam, or a water swelling adhesive which loses adhesive ability by swelling by water, and the adhesive is coated on one face of the removable sheet.

In a preferred embodiment the adhesive sheet of the present invention is characterized in that the adhesive sheet comprises a supporting sheet, a removable sheet and adhesive layers coated on both faces of the supporting sheet, that the adhesive layer coated at least on one face of the supporting sheet comprises a photo curable adhesive which loses adhesive ability by curing by irradiation of electromagnetic wave or electron beam and that the removable sheet is attached at least on one face of the adhesive sheet.

In another preferred embodiment the adhesive sheet of the present invention is characterized in that the adhesive sheet comprises a supporting sheet, a removable sheet and adhesive layers coated on both faces of the supporting sheet, that the adhesive layer coated at least on one face of the supporting sheet comprises a water swelling adhesive which loses adhesive ability by swelling by water and that a removable sheet is attached at least on one face of the adhesive sheet.

In a still further preferred embodiment the adhesive sheet according to the present invention is characterized in that the adhesive sheet comprises a supporting sheet, a removable sheet and adhesive layers coated on both faces of the supporting sheet, that the adhesive layer coated on one face of the supporting sheet comprises a photo curable adhesive which loses adhesive ability by curing by irradiation of electromagnetic wave or electron beam, that the adhesive layer coated on the other face of the supporting sheet comprises a water swelling adhesive which loses adhesive ability by swelling by water and that the removable sheet is attached at least on one face of the adhesive sheet.

In accordance with the present invention there is further provided a use of the above adhesive sheets of the invention for attaching a printing plate made of resin to a base plate roll.

Further there is provided a use of an adhesive sheet of the present invention, which comprises a photo curable adhesive, which loses adhesive ability by curing by irradiation of electromagnetic wave or electron beam, for attaching a printing plate made of resin to a base plate roll, wherein the printing plate made of resin is transparent to electromagnetic waves.

According to the present invention there is also provided a process for detaching a printing plate made of a resin which is attached with a based plate roll by means of a photo-curable adhesive, wherein the adhesive is irradiated with electromagnetic waves or electron beams, whereby the adhesive loses its adhesive ability, and the printing plate is detached characterized in that the printing plate made of resin is transparent to electromagnetic waves and that the adhesive is irradiated with electromagnetic waves or electron beams from the surface of the printing plate made of resin.

The kind of the printing plate made of resin and the base plate roll utilized in the invention is not particularly limited and is selected suitably.

The adhesive utilized in one embodiment of the invention is not particularly limited so long as the adhesive loses its adhesive ability by irradiation of electromagnetic wave, such as ultraviolet light, visible light, γ-ray, X-ray, infrared light and the like, or electron beam. When electromagnetic wave having weaker penetrating power through substances, such as ultraviolet light, visible light and infrared light, is utilized, the printing plate made of resin is desirably more transparent.

The loss of adhesive ability in the invention means a remarkable decrease of adhesive strength. It is the condition that the adhesive strength is so lower than the original strength that the printing plate can be detached easily by hands.

Examples of the photo curable adhesive of the invention are: (1) an adhesive comprising polymer having two or more photo reactive unsaturated bonds as the main component of the adhesive and (2) an adhesive which is prepared by compounding a photo sensitive monomer and like other reactive low molecular weight compounds into a conventional adhesive.

Examples of the photo curable adhesive (1) are: a polymer prepared by reacting glycidyl (meth)acrylate with acid groups in copolymer of acrylic ester or methacrylic ester and a small amount of acrylic acid or methacrylic acid; and a polymer prepared by reacting half-urethane with a mixture of β-hydroxyethyl (meth)acrylate having hydroxyl group in the molecule with copolymer of acrylic ester or methacrylic ester and a small amount of acrylic acid or methacrylic acid. The half-urethane is prepared by the reaction of polyisocyanate with a monomer containing hydroxyl group. Photo reactive groups are introduced into the polymer by these reactions.

Examples of the photo curable adhesive (2) are: a compound prepared by mixing a photo sensitive monomer with conventional rubber or acrylic adhesive in the amount from 1 to 50 weight %.

Examples of the photo sensitive monomer are: (meth)arylic esters having two or more double bonds in the molecule, such as 1,4-butylene glycol diacrylate, 1,6-hexane glycol diacrylate, neopentyl glycol diacrylate, (poly)ethylene glycol diacrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tetracrylate and the like.

It is desirable that a photo sensitizer is added to the photo curable adhesive to enhance the reaction rate of curing. Examples of such photo sensitizer are: diacetyl, benzil, benzophenone, benzoin, bromoacetophenone, chloroacetone, benzoquinone, anthraquinone, 2-ethylanthraquinone, benzoin isopropyl ether, benzoin ethyl ether and the like other organic carbonyl compounds. The photo sensitizer is generally utilized in the amount from 0.1 to 10 weight % of the adhesive.

After printing is finished, the plate roll is irradiated by electromagnetic wave or electron beam from the surface of the printing plate made of resin attached on the base plate roll. Because the adhesive between the printing plate and the base plate roll is cured by the irradiation, the adhesive ability of the adhesive is lost and the printing plate made of resin is easily detached from the base plate roll.

The source of the irradiation is not particularly limited so long as the irradiation cures the adhesive and can be appropriately selected according to the adhesive utilized. Ultraviolet light is preferable because of efficiency of the irradiation time and simplicity of apparatus.

The kind of the water swelling adhesive of another embodiment of the invention is not particularly limited so long as the adhesive loses adhesive ability by swelling with water.

An example of the water swelling adhesive is an adhesive described in Japanese Patent Publication Showa 49-23294 which is described in the following.

The adhesive comprises (A) one or more than one kinds of nonionic surface active agent which comprises ethylene oxide group as the hydrophilic group and (B) one or more than one kinds of copolymer composed of 40 to 99 mole % of one or more than one kinds of constituent unit having the formula (I) and 60 to 1 mole % of one or more than one kinds of constituent unit having the formula (II) and, optionally according to the needs, carboxyl groups comprised in the component (B) are crosslinked by a crosslinking agent reacting with carboxyl group, such as zinc acetate and magnesium chloride; <chemistry id="chem0001" num="0001"><img file="EP0452808B1_D0001.tif" /></chemistry> wherein R₁ is hydrogen, chlorine or lower alkyl group, R₂ is hydrogen, chlorine, lower alkyl group, -OCOR₃, -OR₃, -COOR₃, phenyl, tolyl, -CN, -CONH₂, -CONHCH₂OH, -CH₂OH, -C₂H₄OH, -OH or pyridyl and R₃ is alkyl group of 1 to 8 carbon atoms; <chemistry id="chem0002" num="0002"><img file="EP0452808B1_D0002.tif" /></chemistry> wherein X, Y and Z may be the same kind or different kinds selected from hydrogen, chlorine, lower alkyl group, -COOR₄, -COOH, -CH₂COOH or -CH₂COOR₄ and R₄ is alkyl group of 1 to 4 carbon atoms. Alkaline compounds, such as potassium hydroxide, may be added when the adhesive is crosslinked by the crosslinking agent

Examples of the nonionic surface active agent which comprises ethylene oxide group as the hydrophilic group of the component (A) are: <ul id="ul0001" list-style="none"><li>(1) OH(C₂H₄O)_a(C₃H₆O)_b(C₂H₄O)_cH, wherein a, b and c are integers from 20 to 80;</li><li>(2) RA(C₂H₄O)_dH, wherein R is alkyl group of 6 to 18 carbon atoms, alkylphenyl group having alkyl group of 4 to 20 carbon atoms or a group having the formula (III) <chemistry id="chem0003" num="0003"><img file="EP0452808B1_D0003.tif" /></chemistry> wherein A is oxygen, sulfur or -COO-, -CONH-, -CON(C₂H₄O)_dH-, -PO₄H- or -PO₄(C₂H₄O)_dH- and d is an integer from 2 to 80; and</li><li>(3) <chemistry id="chem0004" num="0004"><img file="EP0452808B1_D0004.tif" /></chemistry> wherein R is alkyl group of 6 to 18 carbon atoms and x, y and z are integers of 2 to 40.</li></ul>

Preferable examples of the nonionic surface active agent which comprises ethylene oxide group as the hydrophilic group of the component (A) are: polyethylene glycol nonylphenyl ether, polyethylene glycol sorbitane monooleyl ester, polyethylene glycol lauryl ether, block copolymer of polyethylene glycol and polypropylene glycol, polyethylene glycol lauryl phenyl ether, polyethylene glycol t-butyl phenyl ether and the like.

The constituent unit having the formula (I) of the component (B) is introduced by using a monomer, such as vinyl esters like vinyl acetate, methacrylic esters like ethyl acrylate, ethylene, styrene, methyl vinyl ether, vinyl chloride and acrylonitrile. The constituent unit having the formula (II) of the component (B) is introduced by using a monomer having carboxyl group, such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, aconitic acid, monoalkyl maleic acid, monoalkyl fumaric acid, monoalkyl itaconic acid and the like.

It is required that the component (A) and the component (B) are compatible and the adhesive prepared swells with water and, therefore, that the ratio of the component (A) and the component (B) is within a definite range as described in the cited reference.

Epoxy compounds of glycidyl derivative may be optionally utilized with the component (A) and the component (B).

Another example of the water swelling adhesive is an adhesive described in Laid Open Japanese Patent Publication Showa 59-157162 which is described in the following.

The adhesive comprises (A) water soluble polymer and (B) (meth)acrylic ester monomer which gives water soluble or water swelling polymer by polymerization.

An example of the water soluble polymer of the component (A) is polymer prepared from a monomer having the following formula (V): <chemistry id="chem0005" num="0005"><img file="EP0452808B1_D0005.tif" /></chemistry> wherein R₅ is hydrogen or methyl group, R₆ is alkylene group of 2 to 4 carbon atoms, R₇ is alkyl group of 1 or more than 1 carbon atoms and n is an integer of 1 or more than 1, such as (meth)acrylic ester, acrylic acid, vinylpyrrolidone, acrylamide, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, vinyl methyl ether and the like. A water soluble polymer prepared by copolymerizing 30 weight part or less of acrylic ester, vinyl acetate, styrene and the like monomer with 100 weight parts of the monomer having the formula (V) may be optionally utilized as the water soluble polymer of the component (A).

Example of (meth)acrylic ester monomer of the component (B) are: (meth)acrylic ester, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate and the like.

Still another example of the water swelling adhesive is the adhesive described in Laid- Open Japanese Patent Publication Showa 56-70077 which is described in the following.

The adhesive comprises (A) a salt of a copolymer prepared by neutralizing carboxylic acid in a copolymer comprising 10 to 40 mole% of monomer having carboxyl group, 60 to 90 mole % of alkyl (meth)acrylate in which the alkyl group has 4 or more carbon atoms and 0 to 20 mole % of other vinyl monomer and (B) a hydrophilic compound having the formula: R₈O(R₉O)_mR₁₀, wherein R₈ is alkyl group of 1 to 4 carbon atoms, R₉ is alkylene group of 2 to 4 carbon atoms, R₁₀ is hydrogen or alkyl group or acetyl group of 1 to 4 carbon atoms and m is an integer of 1 to 6.

When the printing operation is finished and the printing plate made of resin attached to the base plate roll is detached, water is added to the adhesive sheet remaining on the face of the base plate roll or the printing plate. The adhesive sheet is swelled by water. The adhesive sheet and adhesive layer are removed completely with ease.

According to one embodiment of the present invention the adhesive sheet comprises a layer of the photo curable adhesive and a removable sheet on one or both faces of the adhesive layer.

The adhesive sheet comprising a layer of the photo curable adhesive and a removable sheet only on one face of the adhesive layer can be stored in the form of roll. It is desirable that the faces of the removable sheet have such characteristics that a face of the removable sheet is more easily removed from the adhesive than the other face of the removable sheet.

When the ease of removal of the adhesive from the removable sheet is different between the two faces of the removable sheet, the problem that the adhesive coated originally on one face of the removable sheet is separated on the both faces of the removable sheet in the roll can be surely prevented during the operation. When the adhesive sheet is utilized, it is pulled out from the roll, the adhesive face of the adhesive sheet is pressed to the outer face of the base plate roll or to the inner face of the printing plate and the removable sheet is removed from the adhesive. The printing plate is attached to the base plate roll by pressing it against the base plate roll and the plate roll is prepared.

When the adhesive sheet comprises a layer of the photo curable adhesive and a removable sheet on both faces of the adhesive layer, it is desirable that the removable sheet at one face of the adhesive layer is more easily detached from the adhesive than the removable sheet at the other face of the adhesive layer by the same reason as described. When the adhesive sheet is utilized, the removable sheet which can be removed from the adhesive more easily is first removed from the adhesive sheet and the adhesive sheet is pressed to the outer face of the base plate roll or to the inner face of the printing plate and the remaining removable sheet is removed from the adhesive. The printing plate is attached to the base plate roll by pressing it against the plate roll and the plate roll is prepared.

When the operation of printing is finished by using this adhesive sheet of the invention, the plate roll attached with the printing plate made of resin is irradiated by, for example, ultraviolet light for 20 to 30 seconds from the surface of the printing plate made of resin. The photo curable adhesive on the inner face of the printing plate is cured by the irradiation and loses adhesive ability. Thus, the printing plate is easily detached from the plate roll by band. Remaining adhesive on the faces is removed easily.

In a further embodiment the adhesive sheet of the invention comprises a supporting sheet and two layers of adhesive coated on both faces of the supporting sheet and at least one of the two layers of adhesive comprises the photo curable adhesive. The adhesive sheet can be stored by attaching a removable sheet on one or both faces of the adhesive sheet.

The supporting sheet is not particularly limited. Examples of the supporting sheet are films of polyethylene terephthalate, nylon, polypropylene, polycarbonate and the like other materials. When ultraviolet light, visible light and infrared light are utilized for irradiation, films having higher transparency are preferable so that the light transmission is easier.

It is also preferable that the surface of the supporting sheet is activated by corona discharge or by like other methods to enhance the affinity of the adhesive and the surface.

When this adhesive sheet of the invention having a removable sheet on one face of the sheet is utilized, the adhesive sheet is attached to the outer face of the base plate roll or to the inner face of the printing roll by the face on which the adhesive is exposed. The removable sheet is removed and the printing plate is attached to the base plate roll by pressing to the base plate roll. It is desirable that the adhesive layer of photo curable adhesive is placed to the face of the base plate roll.

When the printing plate is attached to the base plate roll by utilizing this adhesive sheet of the invention having photo curable adhesive on one face and conventional adhesive on the other face and the adhesive layer of the photo curable adhesive is placed to the face of the base plate roll, the adhesive on the surface of the base plate roll is cured by irradiation of ultraviolet light and the printing plate can be detached from the base plate roll while the adhesive sheet is still attached to the printing plate.

Printing plates made of resin are often disposed after the use. This adhesive sheet of the invention having photo curable sheet only on one face is suitable for easy disposal because the adhesive sheet need not be removed.

When a printing plate made of resin is used again after it is utilized for printing, this adhesive sheet of the invention having photo curable adhesive on the both faces is suitable for use.

In a still further embodiment the adhesive sheet of the invention is utilized for the plate roll and comprises a layer of water swelling adhesive and a removable sheet attached on one or both faces of the layer of the adhesive.

When the removable sheet is attached only on one face of the adhesive layer, the adhesive sheet can be stored in the form of roll. It is desirable that the faces of the removable sheet have such characteristics that a face of the removable sheet is more easily removed from the adhesive than the other face of the removable sheet.

When the ease of removal of the adhesive from the removable sheet is different between the two faces of the removable sheet, the problem that the adhesive coated originally on one face of the removable sheet is separated on the both faces of the removable sheet in the roll can be surely prevented during the operation. When the adhesive sheet is utilized, it is pulled out from the roll, the adhesive face of the adhesive sheet is pressed to the outer face of the base plate roll or to the inner face of the printing plate and the removable sheet is removed from the adhesive. The printing plate is attached to the base plate roll by pressing it against the base plate roll and the plate roll is prepared.

When the adhesive sheet comprises a layer of the water swelling adhesive and a removable sheet on both faces of the adhesive layer, it is desirable that the removable sheet at one face of the adhesive layer is more easily detached from the adhesive than the removable sheet at the other face of the adhesive layer by the same reason as described. When the adhesive sheet is utilized, the removable sheet which can be removed from the adhesive more easily is first removed from the adhesive sheet and the adhesive sheet is pressed to the outer face of the base plate roll or to the inner face of the printing plate and the remaining removable sheet is removed from the adhesive.

The printing plate is attached to the base plate roll by pressing it against the base plate roll and the plate roll is prepared.

When the operation of printing is finished by using this adhesive sheet of the invention, the printing plate made of resin is detached from the base plate roll. Adhesive remaining on the surfaces of the base plate roll and printing plate made of resin is treated with water. The adhesive is swelled by water and can be removed very easily by wiping with cloth or the like.

In a further embodiment the adhesive sheet of the invention comprises a supporting sheet and two water swelling adhesive layers coated on both faces of the supporting sheet. As the supporting sheet, a porous sheet is preferred because water penetrates easily in the sheet.

When the adhesive sheet of the invention having the photo curable adhesive only on one face is coated with the water swelling adhesive on the other face, the adhesive sheet formed corresponds to the adhesive sheet of a still further embodiment of the invention. The plate roll is prepared by utilizing this adhesive sheet in the same way as in the embodiment, wherein the adhesive sheet comprises a supporting sheet and two layers of adhesive coated on both faces of the supporting sheet and at least one of the two layer of adhesive comprises the photocurable adhesive.

The base plate roll attached with the printing plate by utilizing this adhesive sheet of the invention through the photo curable adhesive on the outer face of the base plate roll and the water swelling adhesive on the inner face of the printing plate is, after finishing the operation of printing, irradiated with ultraviolet light and the printing plate is removed from the face of the base plate roll while the adhesive sheet is still attached to the printing plate.

When the printing plate is dipped into water, the adhesive is swelled and the adhesive sheet is removed spontaneously from the printing plate. The remaining printing plate can be utilized again after drying.

When the base plate roll is attached with the printing plate through the water swelling adhesive on the outer face of the base plate roll and the photo curable adhesive on the inner face of the printing plate, the printing sheet remains on the surface of the base plate roll and can be removed by swelling with water.

The former method is preferable because the printing plate is quickly detached from the base plate roll.

Example 1

Polyethylene terephthalate film of 50µ thickness activated by corona discharge on both faces was used as the supporting sheet for a double faced tape.

A removable sheet of polyethylene terephthalate film of 38µ thickness coated with silicone on one face was coated with a photo curable adhesive to 15µ thickness by a knife coater on the face coated with silicone. The adhesive was an acrylic adhesive comprising 40 weight % of trimethylolpropane (meth)acrylate and 5 weight % of benzoin was further added as the photo sensitizer.

The removable sheet coated with the photo curable adhesive was laminated to the both faces of the supporting sheet and a double face adhesive sheet was prepared.

One of the removable sheets on the adhesive sheet was removed and the adhesive sheet was pressed to a printing plate made of resin (Torayleaf®, a product of Toray Co., Ltd., 1 mm thickness). The other removable sheet was removed and the printing plate was attached to a stainless steel base plate roll by using the double face adhesive sheet.

After standing for 24 hours, the plate roll was irradiated with ultraviolet light by a 35 mW/cm² ultraviolet lamp for 20 seconds over the printing plate made of resin. Total dose of ultraviolet light irradiated was 700 mJ/cm².

After the irradiation, the printing plate made of resin was easily detached by fingers. The adhesive sheet remained attached to the face of the printing plate when the printing plate was detached from the base plate roll and was removed from the printing plate easily by fingers.

The same adhesive sheet as the above was laminated to the same printing plate as the above and used for adhesion test without attaching to the base plate roll. After standing for 24 hours, the adhesive strength was 1700 g/25 mm width. Then, ultraviolet light was irradiated to the outer face of the printing plate made of resin in the same condition as the above. Ten minutes after the irradiation, the adhesive strength decreased to 90 g/25 mm width.

Example 2

A removable sheet coated to the thickness of 50µ with an acrylic water swelling adhesive which comprised a copolymer of ethyl acrylate, vinyl acetate and methacrylic acid and polyethylene glycol lauryl ether and was crosslinked by zinc acetate in the presence of potassium hydroxide was utilized in place of one of the two removable sheets coated with adhesive utilized in Example 1. A porous sheet of polyethylene terephthalate treated by corona discharge was utilized as the supporting sheet. A printing plate was attached to a base plate roll by the same method as in Example 1.

After standing for 24 hours, ultraviolet light was irradiated by the same method as in Example 1. The printing plate was detached from the base plate roll easily by fingers.

When the printing plate was detached from the base plate roll, the adhesive sheet was firmly attached to the inner face of the printing plate. After dipping the printing plate in water for 3 hours, the adhesive sheet was removed from the printing plate easily by fingers.

To summarize the advantages obtained by the invention, the base plate roll attached with the printing plate made of resin has advantages that the printing plate attached is easily detached, the adhesive sheet and remaining adhesive are easily removed after the operation of printing is finished and thus efficiency of the operation of printing is enhanced.