Patent Application
20080057188
The methods described herein to make a printing blanket or sleeve having a texturized polyurethane surface utilize thermosetting or thermoplastic polyurethanes. We have found that the use of such polyurethanes provides advantages over the use of prior art rubber compounds in that the polyurethanes may be used as 100% solids materials. Accordingly, there is no solvent which needs to be removed prior to curing. In addition, the polyurethane surface may be applied in a single pass and cured or hardened inline at relatively high speeds without the need for expensive equipment.
Where the polyurethane compound comprises a thermosetting urethane (also referred to herein as a cast urethane or two-part urethane), the urethane is typically supplied in the form of a 100% solids material which may be warmed to a liquid state and then applied to the base ply or a substrate web or sleeve by dip coating, spray coating, reverse roll coating, knife coating, or slot die coating. The thermosetting polyurethanes are generally based on polyesters or polyethers. Polyester-based polyurethanes are preferred for use due to their chemical resistance. Depending on the specific urethane employed, the curing mechanism may comprise heat, UV, or moisture curing. Optionally, the application of heat may be used to activate and/or accelerate curing.
Suitable polyurethane casting compositions for use in the present invention are described in U.S. Pat. No. 3,211,701, the disclosure of which is hereby incorporated by reference. Such compositions comprise the reaction product of an isocyanate-terminated prepolymer with an organic chain extender or crosslinking agent (which may be a polyamine or a polyhydric alcohol) with a functionality of at least 2 and a molecular weight from 18 to 600. The isocyanate-terminated prepolymer is prepared from a hydroxyl-terminated polyester, polyether, or polybutadiene polyol or mixtures thereof having a molecular weight of 300 to 6000 and a functionality of at least 2 and optionally, a hydroxyl containing chain extending agent with a functionality of at least 2 and a molecular weight of 18 to 600, with an excess of organic diisocyanate.
If desired, cure-blocked and/or delayed-cure polyurethanes may be used which are preferably derived from either blocked isocyanates or blocked or delayed action curatives, depending on the casting method employed. Where the polyurethanes are derived from blocked isocyanates, a prepolymer such as those described above for two-component systems is reacted with a blocking group such as methylethyl ketoxime, caprolactam or other active hydrogen-containing compound prior to adding a chain extender or crosslinking agent to the system. Curing is initiated only after the mixture is applied to a substrate and heat is supplied. In the presence of heat, the blocking group is released from the original isocyanate group, thus allowing the isocyanate group to react with other active hydrogen containing entities in the matrix. Examples of suitable cure-blocked and/or delayed-cure polyurethanes include the MEKO and Caytur type systems from Chemtura.
Suitable moisture-cure polyurethanes for use in the present invention include urethane prepolymers which are isocyanate-capped polyols, such as polyesters, polyethers and polyester/polyols that do not contain any internal cross-linking agent (i.e., water cross-links the polymer and gives the desired physical properties). Typical prepolymers for moisture cured polyurethanes are the same as those described above for two component cast polyurethane systems, but normally the final free NCO content of the prepolymer for a moisture cured systems will be 5% or less while typical prepolymers used in two component cast systems range from greater than 2% up to about 12%. Preferred moisture curable polyurethanes for use are commercially available from Bayer, Futura, Sika and others.
A typical UV or radiation-curable polyurethane system contains an oligomer, which may or may not contain reactive functional groups (such as double bonds), a crosslinking agent, a reactive diluent for viscosity control, and a photosensitizer or photoinitiator. By selecting an oligomer which contains at least two points of reactive unsaturation, or a reactive diluent which contains at least two points of reactive unsaturation, a crosslinking agent may be eliminated. Control over the properties of the cured systems can be exercised via the structure of the oligomer backbone, including such factors as degree of chain-branching, types of functional groups, number and types of unsaturated bonds, molecular weight, etc.; functionality and level of crosslinking agents; nature and level of reactive diluent; kind and level of the sensitizer or photoinitiator; and the like. An exemplary oligomer is an unsaturated urethane oligomer obtained by reacting an isocyanate-functional prepolymer with unsaturated compounds containing an isocyanate-reactive active hydrogen group. The unsaturated urethane oligomers are typically the reaction product of at least one organic isocyanate compound having at least two isocyanate groups; at least one polyether or polyester polyol with a functionality of at least 2 (similar to those described above); and at least one unsaturated addition-polymerizable monomeric compound having a single isocyanate-reactive active hydrogen group such as hydroxyl ethyl(propyl)-(methyl)acrylate. Before any polymerization can occur, free radicals must first be produced via the photoinitiator. The production of free radicals by the photoinitiator is a wave length function of the actinic radiation. Once the radicals are formed, propagation of polymer growth rapidly advances through chain reaction. Suitable UV or radiation curable polyurethanes are available from companies such as Sartomer, Radcure and others.
Where the polyurethane compound comprises a thermoplastic polyurethane, such polyurethanes are typically supplied as 100% solids materials which are melted and applied as a viscous liquid to the base ply or sleeve by extrusion or slot die coating, or by heated, reverse roll coating. Alternatively, the thermoplastic polyurethane may be applied as a heat laminated film. The thermoplastic polyurethanes do not require curing as they regain all of their physical properties upon cooling and reformation as a solid after coating. Suitable thermoplastic polyurethanes for use in the present invention are polyester or polyether-based and include those commercially available from Huntsman Polyurethanes, Dow and Bayer. Polyester-based polyurethanes are preferred for use due to their chemical resistance.
It should also be appreciated that alloys of the above-described thermosetting or thermoplastic polyurethanes with conventional rubber materials such as nitrile rubber, EPDM, polysulfide, and butyl rubber may also be used.
Referring now to
In an alternative embodiment illustrated in
Referring now to
In one embodiment of the invention, after the substrate web or base sleeve has been coated with polyurethane by one of the above-described methods, dusting particles are deposited onto the polyurethane surface, preferably while the surface is still formable, e.g., prior to curing, where a thermosetting polyurethane is used or where a thermoplastic polyurethane is used, while the temperature of the polymer is above its softening point.
For optimum results, it is preferred that the polyurethane surface be at least partially cured or cooled prior to dispensing the dusting particles. In embodiments where a thermosetting polyurethane coating has been applied, the pot life of the coating may be such that partial curing occurs at ambient temperature prior to dispensing the dusting particles. Alternatively, the web or sleeve may be passed through a pre-curing station (not shown) downstream from the coating apparatus to initiate and/or accelerate the cure to an appropriate level prior to dispensing the dusting particles. Such a pre-curing station would include an appropriate curing source (e.g., UV, IR, RF, or radiated heat) of energy.
Where a thermoplastic polyurethane has been applied, the surface will be formable while its temperature is in excess of its glass transition temperature (Tg). In most instances, deposition of dusting particles can occur shortly after initial application lamination of the thermoplastic polyurethane. However, it may be necessary to warm the coating prior to deposition to provide the optimum surface for accepting the dusting particles.
As illustrated in
Preferred dusting materials for use in the present invention include starch, salt, sugar, and other organic particulates. Inorganic particulates may also be used but are preferably treated with a release agent such as silicone to facilitate their removal. It should be appreciated that the dusting materials used must be inert to the polyurethane chemistry and they must have a melting point which exceeds any processing temperatures encountered while the dusting material is being applied, embedded, and/or while the polyurethane is being cured. The dusting materials must also be inert to both the surface chemistry and heat used during curing and embedding such that the dusting material is not melted, chemically distorted, or chemically bonded to the polyurethane surface, allowing the material to be readily removed after curing.
The dusting particles are preferably provided in a size which provides the resulting depressions with an average diameter of from approximately 3 microns to about 65 microns and an average depth of from about 3 to about 65 microns.
After the dusting particles 52 have been deposited onto the surface of the polyurethane on web 14, the web is passed between opposing rollers 56 and 58 which impart pressure such that the particles are pressed into the polyurethane surface 54. Where the polyurethane comprises a thermosetting polyurethane, after the particles are pressed into the polyurethane surface, the polyurethane is preferably substantially completely cured by an appropriate curing source 60 (such as UV, infrared, heat, etc.). The curing operation fixes the dust particles in position in the polyurethane surface.
Alternatively, where the polyurethane comprises a thermoplastic polyurethane, after the particles are pressed into the surface, the surface is cooled to below the Tg of the polyurethane.
The web 14, including the fully cured or cooled polyurethane surface 12, is then preferably passed under a rotating brush roller 62 which brushes the surface of the web 14 to remove the dusting particles 52. Where the substrate comprises a sleeve, the brush roller preferably interfaces with the sleeve surface as it is rotated.
We have found that some organic dusting materials are difficult to remove with the use of brushing alone. While organic materials such as salt and sugar can be dissolved and washed away readily with water, organic materials such as starch require additional treatment before they can be dissolved and washed away. When starch is used as a dusting material, the particles are preferably washed first with a solution of sodium hydroxide and water which breaks down the starch into a sugar so that it can be dissolved and removed from the polyurethane surface. Such a solution can be provided in the form of a bath in a reservoir (not shown) associated with brush rollers 62 such that the solution is applied using the brushes.
After removal of the dusting particles, in the areas where the embedded particles were previously present, the polyurethane surface includes a plurality of spaced apart depressions or ink reservoirs. Such depressions provide improved ink carrying and transferring characteristics.
In an alternative embodiment illustrated in
As previously described, curing or hardening of the polyurethane is preferably initiated prior to contact with the embossed roll such that the urethane is formable only by pressure by the time in comes into contact with the embossed roll. In this embodiment, curing may be controlled by the application of heat, moisture, or a UV light source, depending on the type of polyurethane used.
While the polyurethane is preferably cured and embossed by the above methods, it should be appreciated that prior methods used for curing rubber surfaces may be adapted to cure and emboss the polyurethane surfaces, although such methods employ more costly equipment and require more time. For example, the drum or belt in a Rotacure or AUMA may be replaced with an embossed drum or belt and employed such that the polyurethane surface is in contact with the embossed surface during heating. Alternatively, an embossed release liner may be substituted for the typical smooth release liner which is interleaved in a standard drum wrapping and curing system.
Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 60/840,909, filed Aug. 29, 2006, entitled METHOD OF MAKING A PRINTING BLANKET OR SLEEVE INCLUDING A TEXTURIZED POLYURETHANE PRINTING SURFACE. The entire contents of said application are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60840909 | Aug 2006 | US |
