Patent Application
20080070042
The properties of thermoplastic polyurethanes (TPUs) give them a distinct processing advantage for use as layers in a blanket or sleeve construction. The use of TPUs or TPU alloys provides flexibility in designing a blanket or sleeve having the desired properties for use in offset printing. Further, TPUs do not require the use of solvents in processing, which saves time, cost, and effort in adding, drying, and recovering solvents in addition to initial purchase of the solvents. Furthermore, TPUs do not cure like traditional rubber materials used in blanket constructions, affording additional process time and energy savings. TPUs also provide an advantage in that they are easily colorable and recyclable. Further, TPUs maintain their elastomeric behavior over a wide temperature range, and they have a high rebound ability and improved cohesive strength, resulting in longer life for the blanket or sleeve in which they are incorporated.
Thermoplastic polyurethanes are formed by reacting a difunctional isocyanate composition with at least one difunctional polyhydroxy compound and optionally a chain extender. Unlike cast urethanes, TPUs consist of block copolymer molecules with alternating hard and soft segments. This combination allows TPUs to have high elasticity, low glass transition temperatures, high melting points, and elastomeric character. By adjusting the ratio of hard and soft segments, many properties can be adjusted over a wide range, including tear and tensile strength, hardness, stiffness, and elasticity.
Suitable thermoplastic polyurethanes that are suitable 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. Alloys of the above-described thermoplastic polyurethanes with conventional rubber materials such as nitrile rubber, EPDM, polysulfide, and butyl rubber may also be used.
Referring now to
The base layer may alternatively be comprised of a TPU or TPU alloy which provides support when the blanket is placed under tension. Where the blanket is tensioned, the base layer should have a coefficient of friction which facilitates even tensioning of the blanket around a printing cylinder. This may be achieved with the use of TPU or TPU alloys, or TPU reinforced with fibers, a TPU/textile composite, or the use of a thermoplastic material such as Delrin™ (polyoxy-methylene). Where the blanket is non-tensioned, a metal base layer may be used, or any of the above TPU materials may be used as long as they provide the desired low elongation properties.
The compressible layer 15 is comprised of a thermoplastic polyurethane (TPU) and/or a TPU alloy. TPUs and alloys thereof can be formed into compressible layers by introducing voids within the TPU material. These voids may be induced by using techniques that include the incorporation of pre-expanded microspheres, unexpanded microspheres that expand with the thermal processing of the starting material, or the use of endothermic or exothermic blowing agents. Other suitable techniques include the incorporation and subsequent removal of leachable additives, mechanical whipping of the material, and/or the incorporation of low-boiling liquid additives.
The ability to control void gauge and percentage void content varies, depending on the method in which the voids are introduced. The use of microspheres is preferred for introducing voids into the thermoplastic polyurethane. Microspheres can be incorporated into the TPU compound prior to TPU pellet formation or as an additive during thermal processing such as extrusion as explained below.
When using pre-expanded microspheres, care must be taken so that the voids are not destroyed by thermal processing that relies on shear, such as extrusion.
The use of unexpanded microspheres is preferred for use in the present invention. Such microspheres expand with heat and can be added during extrusion and expanded as the TPU mixture exits an extrusion die as described below or subsequent to extrusion with the application of additional heat. Void gauge is controlled by the proper application of heat, the rate of cooling, and the pressure applied to the layer during layer formation and/or lamination. Percentage void content for either pre-expanded or unexpanded microspheres is a function of void gauge, the number of spheres added, and their uniform distribution within the compressible layer.
The TPU compressible layer is preferably produced using unexpanded microspheres dispersed in, for example, ethylene vinyl acetate, and a thermoplastic polyurethane having a Shore A hardness of from about 55 to 70.
Suitable methods of incorporating microspheres in a TPU are disclosed in European Patent Applications EP 1 174 459 A1 and EP 1 233 037 A2, and PCT applications WO 01/10950, and WO 00/44821, the subject matter of which are incorporated herein by reference.
Where the TPU compressible layer is produced using expanded microspheres, the temperature of the TPU during the application process should be kept below the expansion temperature of the microspheres so that the amount of expansion will remain constant during the processing of the compressible layer.
Where the TPU compressible layer is produced using unexpanded microspheres, the TPU may be heated just to or slightly above the expansion temperature of the TPU during extrusion such that the expansion occurs at or near the exit of the extrusion die. The still soft TPU is then passed through a calibrating nip to achieve the desired gauge. Alternatively, the temperature of the TPU may be kept below the expansion temperature of the microspheres during the extrusion process and subsequently brought just to or slightly above the expansion temperature of the microspheres. In this case, the softening point of the TPU should be matched relatively closely to the expansion temperature of the microspheres so that it can deform to accommodate the expansion. One method of raising the temperature of the TPU to the expansion temperature of the microspheres is to pass the extruded TPU film containing the unexpanded microspheres through a heated nip or series of heated nips so that the temperature of the composite is gradually raised to the expansion temperature of the microspheres and expansion occurs under pressure to control the total gauge of the compressible layer. This temperature exceeds the temperature reached during compounding and extrusion, allowing the material to soften and the microspheres to expand under pressure, controlling the amount of expansion.
Alternatively, endothermic and/or exothermic blowing agents may be introduced into the TPU material during initial compounding/manufacturing of the TPU and prior to TPU pellet formation or, preferably, during thermal processing. Blowing agents decompose when their activation temperature is reached and release gas upon decomposition. Endothermic blowing agents absorb energy during decomposition and tend to release less gas than exothermic agents, approximately 110 ml/g. Such blowing agents are useful in producing finer and more homogeneous foams.
Exothermic blowing agents emit energy during decomposition and tend to release more gas than endothermic agents, approximately 220 ml/g. They are useful in producing foams with larger void gauge. The void gauge and percentage void content is dependent on the amount and type of blowing agent, heat, the rate of cooling, and the pressure applied to the layer during layer formation and lamination.
Leachable additives such as various salts, sugars, or other selectively soluble materials can also be added to the TPU in the compounding stage or during thermal processing. Once the leachable additives are incorporated, voids will not be induced until the TPU layer is formed. At this point, the TPU layer must be brought into contact with an appropriate solvent that will dissolve or leach out the additives without degrading the layer. With the additives thus removed, voids remain in the layer. The gauge of these voids is determined by the gauge of the particulate additive selected, while the percentage void content is a function of the quantity and distribution of the additive and degree of removal.
Mechanical whipping of the molten TPU can also be employed to introduce voids with the layer. For example, when the TPU has been melted by thermal processing by extrusion or other means, the TPU can be agitated by mechanical means such that air or other gases are incorporated. Such mechanical means can include stirring, beating, whipping, or any other mechanical process in which air or other gases are forcibly mixed into the molten material. Alternatively, air or other gases may be injected into the molten TPU and mixed to disperse the air/gas evenly throughout. The whipped/mixed material can then be formed into an appropriate layer. Void gauge and percentage void content is mechanically controlled by the severity of the whipping/mixing process, the amount of air or gas introduced, and by the geometry of whipping/mixing equipment such as agitators, screws, and paddles.
Low-boiling liquid additives such as fluorocarbons or chlorocarbons can also be incorporated during thermal processing of the TPU. However, selection of the liquid and thermal processing parameters must be done with care so that the liquid is intermixed well within the TPU prior to boiling. When the boiling occurs, voids are formed within the material that will be retained when the TPU material cools during layer formation. The void gauge and percentage void content are determined by the amount and type of liquid added, the balance of heat and cooling, and the pressure applied to the layer during formation and lamination.
While the compressible layer has been described herein as comprising a TPU layer, it should also be appreciated that the compressible layer, in certain blanket/sleeve constructions, may comprise a polymeric rubber layer. Such a compressible polymeric rubber layer may be incorporated with voids as described above.
The compressible layer preferably has a thickness of from about 0.006 inches to about 0.100 inches (about 0.15 mm to 2.54 mm), and more preferably, from about 0.010 inches to about 0.060 inches (about 0.25 mm to 1.5 mm).
The base layer is typically about 0.010 inches to about 0.026 inches (about 0.25 mm to 0.66 mm) thick, and the printing surface layer is typically between about 0.010 inches to 0.025 inches (about 0.25 mm to 0.64 mm) thick. However, it should be appreciated that the thickness of the base layer and printing surface layer may vary, depending on the materials selected for the layers and the desired finished blanket/sleeve properties.
In the preferred method of making a printing blanket or sleeve including the thermoplastic polyurethane compressible layer 15, a base layer 12 is provided on a printing blanket or sleeve, and the thermoplastic polyurethane compressible layer is either extruded in liquid form as described above or is laminated to the base layer with the use of heat and/or adhesives. The printing surface layer 18 may be applied to the compressible layer 15 by adhesive bonding, heat lamination, or direct extrusion.
The base layer 22 is comprised of a low elongation, high tensile strength TPU and/or TPU alloy as described above. The optional image reinforcement layer 26 is positioned beneath the printing surface layer 28 and preferably comprises a hard TPU and/or TPU alloy, which functions to stabilize the printing surface layer 28 and protect the underlying compressible layer 24, when present. The thickness, hardness and elongation of the image reinforcement layer may be modified as desired by the selection of the TPU materials to provide a means of adjusting and varying the feed rate of the product as needed for the particular printing press design. This provides an improvement over textile materials which have previously been used as image reinforcement layers.
The image reinforcement layer preferably has a Shore A hardness ranging from 70 to 95, and more preferably, from about 80 to 90. This TPU material is preferably blended with other polymers or other suitable processing aids to reduce tack and aid in processing.
In the embodiment shown in
The printing surface layer preferably comprises a TPU/nitrile rubber alloy and a mineral additive such as talc. The talc is preferably included at a loading of between about 1% and 35% and functions as an aid during the mechanical surface finishing (grinding) process, i.e., it functions to reduce frictional heat build-up during grinding.
The printing surface layer preferably exhibits a Shore resilience of less than 40%, and an average surface roughness of less than about 0.5 microns. By “Shore resilience,” it is meant the vertical rebound of the layer is measured pursuant to ASTM 2632.
The desired characteristics of the printing surface profile can be provided by thermal forming either before or after applying the TPU or TPU alloy material onto the blanket/sleeve composite. Alternatively, the desired surface profile can be mechanically imparted by abrasion/grinding, or chemically etching or leaching after application of the TPU material to the blanket/sleeve composite.
In embodiments where each of the base layer, optional compressible layer, optional image reinforcement layer, and printing surface layer are comprised of TPU or TPU alloys, such layers may be provided in the form of free or supported films. The layers may be adhered to adjacent layer(s) of the blanket construction by bonding methods well known in the art, or by heat lamination or direct extrusion onto the blanket construction. The layers may also be extrusion-laminated or slot-die coated to adjacent layers, or may be co-extruded with adjacent layers. It should be appreciated that the layers may also be adhered with the use of conventional adhesives. Alternatively, the TPU materials comprising the layers may be softened by the application of heat such that they function as adhesives.
The preferred embodiments of the present invention exclude the use of fabrics as we have found that the omission of fabric layers in the construction minimizes the wicking of solvents and other chemicals from the printing press into the blanket or sleeve layers, which can cause swelling and delamination of the layers. However, fabric layers may be incorporated into the construction as long as the blanket or sleeve edges are sealed and/or the fabric is sufficiently impregnated with a suitable TPU material to prevent wicking of solvents/chemicals. Where the blanket or sleeve layers are comprised primarily of TPU or TPU alloys, edge sealing is readily achieved by heating the exposed edges of the blanket, allowing the thermoplastic material to soften and flow together. Alternatively, additional TPU or TPU alloy may be added with heat to the exposed edges. The added TPU or TPU alloy will bond readily to the blanket cross-section due to its thermoplastic nature.
Where one or more fabric layers are used as a reinforcing layer (for example, as shown in
In embodiments where the blanket or sleeve includes a compressible layer comprised of a TPU or TPU alloy foam, the blanket or sleeve should preferably exhibit a static compressibility of about 0.14 to 0.22 mm at 1060 kPA, or about 0.21 to 0.29 mm at 2060 kPa. The blanket or sleeve including the compressible layer should also exhibit a dynamic gauge loss of less than about 0.025 mm. The blanket or sleeve should also exhibit solvent/swelling resistance. Preferably, in distilled water, the blanket or sleeve should exhibit a volume swell of less than 2.5%; in 3.125% fountain solution, less than 3.0%; in 10% fountain solution, less than 3.5%; and in blanket wash, less than 2.0%.
The specific illustrations and embodiments described herein are exemplary only in nature and are not intended to be limiting of the invention defined by the claims. Further embodiments and examples will be apparent to one of ordinary skill in the art in view of this specification and are within the scope of the claimed invention.
This application claims the benefit of U.S. Provisional Application No. 60/845,949, filed Sep. 20, 2006, entitled IMAGE TRANSFER PRODUCT WITH THERMOPLASTIC POLYURETHANE FOAM AND/OR THERMOPLASTIC POLYURETHANE ALLOY FOAM, and U.S. Provisional Application No. 60/891,294, filed Feb. 23, 2007, entitled IMAGE TRANSFER PRODUCT COMPRISING THERMOPLASTIC POLYURETHANE LAYERS. The entire contents of said applications are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60845949 | Sep 2006 | US | |
| 60891294 | Feb 2007 | US |
