WO1998013200A1

WO1998013200A1 - Pressure sensitive adhesive structure for offset blankets

Info

Publication number: WO1998013200A1
Application number: PCT/US1997/017207
Authority: WO
Inventors: James T. Boyle
Original assignee: Venture Tape Corporation
Priority date: 1996-09-27
Filing date: 1997-09-25
Publication date: 1998-04-02
Also published as: AU4593897A

Abstract

An offset blanket (50) having an improved adhesive structure (40) for releasible adherence to a blanket cylinder of an offset printing apparatus. The adhesive structure (40) includes a composite of a hot-melt adhesive film (44) bonded to a layer of pressure sensitive adhesive (42) disposed on a siliconized surface of a release layer (46). A method for forming an offset blanket is also disclosed in which a layer of a pressure sensitive adhesive is bonded to the cloth side of an offset blanket utilizing a hot-melt adhesive film. In the bonding step, a composite formed of the pressure sensitive layer bonded to a hot-melt adhesive film is nipped together with the offset blanket such that the cloth side of the blanket faces the hot-melt adhesive film. Sufficient heat and pressure are applied to activate the hot-melt adhesive film.

Description

PRESSURE SENSITIVE ADHESIVE STRUCTURE FOR OFFSET BLANKETS

FIELD OF THE INVENTION This invention relates generally to rubber blankets used for offset printing, and more particularly to an adhesive structure for releasably holding these blankets on a printing cylinder.

BACKGROUND OF THE INVENTION

Offset printing or lithography is a process that is well known in the art and need not be discussed in great detail herein. Offset printing typically is performed with water-repellant and acid- resistant paste inks. Oxidizing, quick-setting, heat-setting and absoφtive inks are used for printing of various types of products, including metal cans, folding cartons, labels and rigid plastics as well as magazines, books and newspapers. In a general sense, an offset printing system typically includes two closely spaced, parallel, horizontal rotating cylinders between which a web, such as paper, cardboard or the like is fed for printing of an image. One cylinder, typically a lower one of the two rotating cylinders, is called an impression cylinder and assists in feeding the web through the nip between the two rollers and applies pressure to the web during the printing process. The other cylinder, typically an upper cylinder, is referred to as the blanket cylinder and carries a printing blanket on an outer surface thereof. The printing blanket typically has a rubber layer bonded to a woven cloth layer and is about 1/16 inch thick. Disposed above the blanket cylinder is a third, parallel, rotating cylinder frequently referred to as the plate cylinder. The plate cylinder carries on its outer surface a photochemicall? treated plate containing the image to be printed on the web. The image areas accept ink and non-image areas reject the ink but accept acidified water from a fountain solution. Disposed above and adjacent to the plate cylinder are a series of ink rollers which transfer ink from an ink fountain to the plate cylinder. Also disposed adjacent the plate cylinder is another fountain containing acidified water. A series of rollers transfers this acidified water to the plate cylinder.

In operation, ink is transferred to the photochemically treated plate whose image areas accept the ink and non-image areas reject the ink. Acidified water is also transferred to the treated plate on the plate cylinder and is accepted by the non-image areas. Typically, the plate cylinder and blanket cylinder rotate in opposite directions, and the blanket cylinder rotates in a direction opposite that of the impression cylinder to feed the web through the printing area. As a result, the ink representing the image areas on the chemically treated plate is transferred to the rubber layer of the offset blanket, and the ink image is thereafter transferred from the blanket on the offset blanket cylinder to the web. The printing plates and printing blankets must be removed and replaced with other printing plates and other blankets after completion of a printing project. The same blanket cannot be reused for printing multiple images. Therefore, the printing blanket must be affixed to the blanket cylinder in such a manner that allows its removal without damage to the blanket cylinder, and without leaving any residue behind on the cylinder.

At the present time, an adhesive is applied to the cloth layer of the offset blanket for holding the blanket onto a blanket cylinder. Unfortunately, adhesives presently used do not readily bond to the cloth layer of the blanket, unless the cloth layer is first primed with a solvent- based primer. A transfer adhesive is laminated to the previously primed cloth layer. While this method provides a satisfactory adhesive layer, the solvent-based primer typically is a hazardous material which causes both health problems to the worker and disposal problems. Moreover, this priming and laminating step is labor intensive, thus increasing the costs of and time required for manufacturing the offset blanket.

SUMMARY OF THE INVENTION The foregoing shortcomings of the adhesive system presently used for offset blankets, and the method of applying an adhesive system to an offset blanket are overcome by the present invention in which a dry, pressure sensitive adhesive structure is transferred directly to the cloth surface of an offset blanket using a hot melt or heat sealable adhesive film.

In one aspect of this invention, the adhesive structure applied to an offset blanket comprises a layer of a pressure sensitive adhesive coated onto a heat sealable adhesive film. Preferably a release layer covers the exposed surface of the pressure sensitive adhesive. The heat sealable film is formed of a hot melt adhesive which is applied as a clear, dry, non-tacky film.

In another aspect of this invention, the process of making this adhesive structure includes the step of applying a liquid, pressure sensitive adhesive onto a release layer, such as a web formed of a silicon coated release paper, and transporting the liquid adhesive and release layer through a series of thermal curing stations to produce a cured layer of pressure sensitive adhesive. Thereafter, a layer of heat sealable adhesive film is bonded to the cured pressure sensitive adhesive layer at a lamination station. The resulting product is wound onto a master roll.

The foregoing adhesive structure is then applied to the woven layer of the offset blanket. This process typically is performed at the factory where the offset blankets are manufactured. The previously laminated offset blanket containing a woven layer and a rubber layer is sheet fed toward two nip rollers with the woven cloth surface facing upwardly. The previously-formed adhesive structure described above, is then simultaneously fed to the same nip rollers in sheet form with the heat sealable film facing downwardly to be nipped into engagement with the woven cloth surface. The rollers are heated to activate the heat sealable adhesive, thereby bonding the heat sealable adhesive directly to the cloth layer. The pressure sensitive adhesive bonded to the other side of the heat sealable film, it is available for releasably securing the offset blanket to a blanket cylinder, after removal of the release layer covering the pressure sensitive adhesive. The resulting blanket may then be cut into desired sizes and shapes in a known manner.

The present invention obviates the step of priming the cloth surface with a solvent-based primer prior to affixing the adhesive to the cloth. The elimination of this step significantly reduces the time and costs of manufacture of offset blankets, as well as provides an improved bond between the pressure sensitive adhesive layer and the cloth surface of the blanket. Moreover, the pressure sensitive adhesive used bonds to the blanket cylinder with adequate strength to avoid any printing plate lifting or slippage, but removes cleanly with no residue after the printing job has been completed. The adhesive structure of this invention may be provided directly to the manufacturer of the offset blanket in a roll forfri which allows its ready use and application to the cloth side of the offset blanket in an automated fashion without the need to make significant changes to existing manufacturing machinery and processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages and features of this invention will be more clearly appreciated from the following detailed description, when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a perspective, schematic view illustrating an offset printing process with which this invention is to utilized; Fig. 2 is a cross-sectional side view of a cutaway portion of a prior art offset blanket;

Fig. 3 is a cross-sectional side view of a cutaway portion of an adhesive structure in accordance with this invention;

Fig. 4 is a cross-sectional side view of a cutaway portion of an offset blanket carrying the adhesive structure of this invention;

Fig. 5 is a side schematic view illustrating the process of manufacturing the adhesive structure of this invention; and

Fig. 6 is a side schematic view illustrating the process of applying the adhesive structure of this invention to an offset blanket.

DETAILED DESCRIPTION With reference now to the drawings, and more particularly to Fig. 1 thereof, the use and application of this invention will be described with particular reference to a conventional offset printing apparatus and process. Fig. 1 illustrates in schematic fashion an exemplary, conventional offset printing apparatus 1 1 which includes blanket cylinder 10, impression cylinder 12, plate cylinder 14, ink rollers 16, ink fountain 18, water dampeners 20, and water fountain 22. Blanket cylinder 10 and impression cylinder 12 typically are rotated in opposite directions. In the example shown in Fig. 1 , blanket cylinder 10 is rotated in a counterclockwise direction while impression cylinder 12 is rotated in a clockwise direction to feed a web 24 formed of paper, cardboard, metal, plastic or the like from left to right as shown in Fig. 1 , through the nip between cylinders 10 and 12. Plate cylinder 14 rotates in a direction opposite that of blanket cylinder 10. or in a clockwise direction as shown in Fig. 1. All of cylinders 10, 12 and 14 typically are horizontally aligned and disposed parallel to one another and cylinders 10, 12 and 14 typically are journaled at their ends in a support structure (not shown) so that the three cylinders are disposed vertically one above the other, with cylinder 12 on the bottom and cylinder 14 on top, as shown in Fig. 1. However, as is well known, other configurations are possible.

Affixed to plate cylinder 14 is a printing plate 26 which has an image etched thereon which is to be printed on web 24. Plate 26 has been photochemically treated so that the image areas on the plate will accept ink and non-image areas will reject ink but will accept acidified water. Ink rollers 16 transfer ink from ink fountain 18 to plate 26 on plate cylinder 14. Typically, the ink is a water- repellent and acid-resistant paste ink. Typical inks used are oxidizing, quick-setting, heat-setting and absorptive inks. Similarly, water dampeners 20 transfer acidified water from fountain 22 to plate 26. Disposed on blanket cylinder 10 is offset blanket 28 which is substantially aligned with printing plate 26, so that as cylinders 14 and 10 rotate with respect to one other, plate 26 always engages or is pressed against blanket 28 at the same location on blanket 28.

In operation, ink from ink fountain 18 is transferred via rollers 16 onto plate 26 where it adheres to the image areas on plate 26. At the same time, acidified water is transferred from fountain 22 via rollers 20 to plate 26 where it adheres to the non-image areas of plate 26. As plate cylinder 14 rotates with respect to blanket cylinder 10, the ink adhering to the image areas of plate 26 is transfenred onto offset blanket 28. Offset blanket 28 in turn transfers the ink onto web 24 to print an image thereon.

Fig. 2 illustrates a typical structure of an existing offset blanket 28. Top layer 30 typically is formed of a rubber material, and is the layer which accepts ink from printing plate 26. Offset blanket 28 also includes a layer 34 which is formed of a woven cloth material which is affixed to layer 30 by a layer 32 of laminating adhesive. A pressure sensitive adhesive layer 36 is affixed to the exposed surface of cloth layer 34. As previously discussed, adhesive layer 36 typically only will adhere to cloth layer 34 after cloth layer 34 has been primed with a solvent based primer. Adhesive layer 36 removably affixes blanket 28 to the outer surface of blanket cylinder 10.

The adhesive structure 40 of this invention will now be described with particular reference to Fig. 3. Structure 40 includes pressure sensitive adhesive layer 42, a heat sealable adhesive layer 44 disposed on one side of layer 2 and a release layer 46 disposed on the other side of layer 42. Layer 44 typically is a clear, dry, non-tacky film formed of a hot melt adhesive.

In one aspect of the invention, adhesive layer 42 must bond to blanket cylinder 10 with sufficient strength to prevent offset blanket 28 from inadvertently lifting off the outer surface of cylinder 10 and to prevent any slippage of offset blanket 28 along the surface of cylinder 10.

However, adhesive layer 42 must allow blanket 28 to be peeled off the outer surface of cylinder 10 and must remove cleanly from the outer surface of cylinder 10 leaving no residue behind after the printing job has been completed. Therefore, the adhesive chosen must not build to a permanent bond, and must have a somewhat lower tack than other pressure sensitive adhesives. In one embodiment, adhesive layer 42 may be a two component, solvent based adhesive system in which the primary component is an acrylic polymerized from monomers such as methyl methacrylate and butyl methacrylate. Suitable solvents include toluene, isopropanol, heptane, hexane and any other solvent capable of dissolving an acrylic. The solvent content typically is in the range from about 40% to about 44%, although higher and lower solvent contents are acceptable. The viscosity of the adhesive may be in the range of from about 1000 to about 10,000 centipoise at the indicated solvent content although a preferred viscosity is in the range of from about 2000 to about 4000 centipoise. The peel adhesion of this adhesive when coated at one mil and tested in accordance with PSTC-1 is between about 2.5 and about 3.5 pounds per inch. The Williams Plasticity Index typically is between about 4.0 and about 5.0 millimeters. An acceptable, commercially available candidate for adhesive layer 42 is an adhesive such as Avery 2531 made by Avery Dennison in Mill Hall, Pennsylvania 17751.

Layer 44 may include an adhesive resin from a family comprising an ethylene-acrylic acid copolymer, an ethylene- vinyl acetate copolymer, or a homopolymer such as low density polyethylene. Layer 44 may also be a coextrusion of a polyethylene film and an ethylene-vinyl acetate film. Layer 44 preferably comprises a hot melt adhesive whose activation temperature is about 185°F or above. Layer 44 typically is about 3 to 4 mils in thickness, although a lesser film thickness would be acceptable. A typical, final coating weight for this heat sealable adhesive is about 3.17 ounces per square yard. A typical film which is an acrylic polymerized from monomers such as methyl methacrylate and butyl methacrylic has the following approximate properties as measured by ASTM D-882 for a thickness of 3.5 mils: an ultimate tensile strength of 2200 in the machine direction (MD) and 700 in the transverse direction (TD); ultimate elongation of 300; 2% Secant modulus of 6200 (MD) and 6100 (TD). The Elmendorf Tear strength in grams per mil as measured by ASTM D-1922 is about 140 (MD) and 600 (TD). The Dart Drop impact test as measured by ASTM D-1709 is about 550 grams. An acceptable, commercially available candidate for adhesive layer 44 is DAF 821 produced by the Dow Chemical Company in Midland Michigan 48674.

Adhesive layer 44 may also be a low density homopolymer such as low density polyethylene. A typical adhesive of this type can be either a low slip or a high slip film available in thickness of 1.25 mils. A typical density is about 0.92 grams per cubic centimeter. Typical treatment levels are 38 dynes per square centimeter, although a 42 dyne per square centimeter material is also acceptable. An acceptable, commercially available candidate for a low density polyethylene film is a 103.33 grade film available from Deerfield Plastics Company, South Deerfield, Massachusetts 01373 or a REG 101 film available from Favorite Plastics of Old Saybrook, Connecticut.

Layer 46 may be formed of any conventional release material. A typical, preferred material for layer 46 is a silicone coated paper having a release of about 40 grams per inch. A commercially acceptable candidate for layer 46 is an HV90 which includes a coating of a 368/478 silicone with a coating weight of 90 grams per square meter. Such a product is available from 4P of Rube, Germany.

Fig. 4 illustrates an offset blanket 50 having the structure of Fig. 3 bonded thereto. Blanket 50 includes rubber layer 52, a layer of a laminating adhesive 54, a layer of woven cloth 56, heat sealable adhesive layer 54, pressure sensitive adhesive layer 60 and release layer 62. Rubber layer 52 is identical to rubber layer 30, adhesive layer 54 is identical to adhesive layer 32, and cloth layer 56 is identical to layer 34, all of Fig. 2. With respect to Fig. 3, heat sealable adhesive layer 58 is identical to layer 44, while layers 60 and 62 of Fig. 4 are identical to respective layers 42 and 46.

The method and apparatus used for formation of structure 40 of Fig. 3 will now be described with particular reference to Fig. 5. Coating apparatus 70 includes adhesive application station 72, curing ovens 74 and lamination station 76. Apparatus 70 also includes supply roll 84, supply roll 86 and takeup roll 88. Application station 72 includes rollers 78, 80 and 82, and container 92 filled with a liquid adhesive. Lamination station includes rollers 91 and 92. Web 90, which ultimately forms release layer 46 of Fig. 3 or layer 62 of Fig. 4, is supplied from roll 84. Web 90 passes from roll 84 over rollers 77 and 78 and between rollers 80 and 78. Roller 78 is spaced from roller 80, and roller 82 remains stationary. Roller 80 typically rotates in the same direction as roller 78, or in a counterclockwise direction as shown in Fig. 5. Thus, the top of roller 78 and web 90 move in direction opposite that of the neighboring portions of the bottom of roller 80. As roller 80 rotates, adhesive is lifted from container 92 and carried to web 90 where the adhesive is transferred to web 90. The spacing between rollers 80 and 82 may be adjusted to adjust the coating weight of the adhesive on web 90, and roller 82 uniformly meters the amount of adhesive transferred to web 90. The opposite rotation of adjacent portions of rollers 78 and 80 and the controlled spacing between rollers 80 and 82 ensure that the adhesive coating is uniformly controlled along the length and width of web 90. In an alternative embodiment, application station 72 may include a knife (not shown) disposed a fixed distance from roller 80 to meter the adhesive onto web 90. In another embodiment, application station 72 also may be comprised of a slotted extrusion die configuration (not shown) in which adhesive is forced through a slot in a die and deposited onto web 90.

Curing ovens 74 are conventional electric or gas fired chambers that heat the web 90 and adhesive to a predetermined curing temperature. A typical temperature is about 250 °F. Rollers 92 in lamination station 76 preferably are rubber coated steel nip rollers which laminate the web 94 of heat sealable adhesive film to the cured adhesive on web 90.

In operation, web 90 passes through the spacing between rollers 78 and 80 in application station 72 where adhesive is applied to a silicon coated side of web 90 in the desired coating weights and thicknesses by roller 80, as described. Alternatively, adhesive may be metered on with a knife (not shown), or extruded onto web 90 using a slotted extrusion die (not shown). Typically, the adhesive is applied onto what becomes an outwardly or upwardly facing side of web 90 as shown in Fig. 5. A preferred wet coating weight is about 2.4 ounces per square yard. Preferably, the adhesive is applied such that it has a 3.0 mil wet thickness so that the final, cured coating weight of the adhesive layer which later becomes adhesive layer 42 of Fig. 3 or adhesive layer 60 of Fig. 4, is about 1.2 ounces per square yard and so that the final cured thickness of the adhesive layer is about 1.5 mils. Thereafter, the adhesive coated web 90 is passed through curing ovens 74 which heat the adhesive and web 90 to a temperature of about 250 °F. Curing ovens 74 vaporize the solvents and cure the adhesive in a known manner. Web 90 with its adhesive coating is then passed through lamination station 76. At the same time, web 94 formed of a heat-sealable adhesive film is retrieved from roll 86 and passed over roller 91 and through lamination station 76. Web 94 ultimately becomes layer 44 of Fig. 3 or layer 58 of Fig. 4. Web 94 and web 90 together pass through nip rollers 92 such that web 94 is urged into contact with the adhesive on web 90. The pressure produced by rollers 92 activates the pressure-sensitive adhesive on web 90 to bind web 94 to web 90. The resulting laminated product is then rolled onto take-up roll 88. When the finished product is wound onto roll 88, the exposed adhesive web 94 does not bind to itself or to any other part of the finished product, since this adhesive is activated only by the application of heat above a specified temperature. Therefore, web 90 need not have a release coating such as silicon on both sides thereof. Typically, roll 88 is cut into a plurality of rolls 96 of tape (see Fig. 6) of the finished laminate having the desired width. These rolls 96 of tape contain the structure of Fig. 3.

The apparatus 100 and process for applying tape 96, or the structure of Fig. 3, to an existing offset blanket 98 to form the structure of Fig. 4 will now be described with particular reference to Fig. 6. Offset blanket 98 previously has been formed in a manner well-known to those skilled in the art. As previously discussed, a layer of rubber is affixed to a woven cloth layer through the use of a laminating adhesive. The blanket itself and the method of manufacture thereof do not form any part of this invention. Offset blanket 98 is fed to nip rollers 102 which are heated. At the same time, web 104 from roll 96, which is formed of web 90 laminated to web 94, is fed to rollers 102 such that heat-sealable film web 94 is placed into intimate contact with the woven cloth layer of blanket 98. Rollers 102 provide sufficient heat that the hot melt adhesive in web 94 is activated, and the combination of heat and pressure applied by rollers 102 adhesively bonds web 104 to blanket 98. The resulting product is then cut into appropriately sized sections, or is accumulated on a roll in a manner well-known to those skilled in the art.

It can be seen from the foregoing description that no priming of the cloth layer on the offset blanket is necessary for affixing a pressure-sensitive adhesive thereto. Rather, a hot-melt adhesive in the form of a heat-sealable adhesive film is bonded to the pressure-sensitive adhesive layer and in turn bonds the pressure-sensitive adhesive layer to the cloth layer of the offset blanket, thus obviating the need for priming. As a result, the assembly process for the offset blankets is considerably simplified, reducing the time and cost required for preparation of offset blankets.

Modifications and improvements will occur within the scope of this invention to those of ordinary skill in the art, and the above description is intended to be exemplary only. The scope of this invention is defined only by the following claims and their equivalents. What is claimed is:

Claims

1. An offset blanket for use in offset printing operations for transferring ink from a printing plate to a web, said blanket comprising: a layer of rubber material having a first surface for receiving the ink and for transferring the ink to the web, said rubber layer having a lower surface opposite said first surface; a fabric layer bonded to said lower surface of said rubber layer; a layer of a pressure-sensitive adhesive; and a layer of a hot-melt adhesive disposed between said fabric layer and said layer of pressure- sensitive adhesive, said hot-melt adhesive adhesively binding said pressure-sensitive adhesive to said fabric layer.

2. The offset blanket as recited in claim 1 further comprising a release layer releasably secured to said layer of a pressure-sensitive adhesive.

3. The offset blanket as recited in claim 1 wherein said layer of pressure-sensitive adhesive is formed of a cured acrylic adhesive.

4. The offset blanket as recited in claim 3 wherein said pressure-sensitive adhesive comprises an acrylic polymerized from butyl methacrylate.

5. The offset blanl et as recited in claim 3 wherein said pressure-sensitive adhesive comprises an acrylic polymerized from methyl methacrylate.

6. The offset blanket as recited in claim 1 wherein said hot-melt adhesive comprises an ethylene-acrylic acid co-polymer.

7. The offset blanket as recited in claim 1 wherein said hot-melt adhesive comprises an ethylene-vinyl acetate co-polymer.

8. The offset blanket as recited in claim 1 wherein said hot melt adhesive comprises a low-density polyethylene.

9. A method for forming an offset blanket for use in an offset printing operation for transferring ink from a printing plate to a web, said method comprising the steps of: adhering a rubberized layer to a cloth layer; bonding together a layer of a pressure-sensitive adhesive and a film of a hot-melt adhesive to form a composite; and affixing the film of a hot melt adhesive of the composite to the cloth layer.

10. The method as recited in claim 9 wherein said bonding step comprises the steps of: curing a layer of a pressure-sensitive adhesive; placing together the hot-melt adhesive film and the layer of cured pressure-sensitive adhesive; and applying pressure to urge the hot-melt adhesive film against the layer of cured pressure- sensitive adhesive to activate the pressure-sensitive adhesive to bond the layer of pressure-sensitive adhesive to the hot-melt adhesive film.

1 1. The method as recited in claim 10 wherein said curing step comprises the steps of: applying liquid, uncured, pressure-sensitive adhesive to a silicon-coated face of a release layer; and passing the silicon-coate^"d release layer and liquid adhesive through ovens to cure the adhesive.

12. The method as recited in claim 9 wherein said affixing step comprises the steps of: placing together the composite and the cloth layer of the offset blanket such that the hot-melt adhesive film is touching the cloth layer; and applying heat and pressure to urge the composite against the cloth layer sufficiently to activate the hot-melt adhesive film to bind the composite to the cloth layer.