A number of processes exist in which a plastic film or sheet is formed on or against a release sheet and then separated from the release sheet after taking steps, such as cooling or curing, to set the film or sheet. Curing, where necessary, may be accomplished by heat, by peroxide catalyst, or by U.V. radiation or by electron beam radiation. The release sheet provides a surface from which the set plastic material can be readily separated and imparts to the surface of the plastic material the quality of finish of the release surface. For example, a desired textured surface can be provided on the surface of the plastic material by forming on or against a release sheet having the mirror image of the desired textured surface.
One example of such forming processes is "casting", wherein a resinous material, such as polyvinyl chloride or polyurethane resin, in a flowable state is deposited or "cast" onto the release sheet surface, heated, cured and cooled to consolidate the resinous material into a continuous self-supporting film, and stripped from the support. The release sheet is normally provided with a desired surface effect, such as high gloss, texturing or an embossed configuration, and the surface effect is replicated on the cast film.
Another example of such forming processes is "panel pressing" of decorative plastic laminates, which can be either of the high pressure or low pressure type. In high pressure panel pressing, decorative laminates are conventionally prepared by assembling in a stacked relationship a plurality of core sheets, each of which is a web of paper impregnated with a resinous material, such as phenolic resin. Immediately positioned above the core sheet assembly is a decorative sheet, which is a resin saturated sheet having a solid color or a suitable design thereon. Superimposed above the decorative sheet is generally an overlay sheet which is a thin sheet of fine paper impregnated with a noble thermosetting resin, such as a melamine formaldehyde resin or an unsaturated polyester resin and the like (and is generally the same resin used to impregnate the decorative sheet). The entire assembly of core sheets, decorative sheet, and overlay sheet is placed between platens in a press and consolidated by application of heat and pressure. Generally, a release sheet having the desired surface effect to be reproduced in the surface of the overlay sheet is placed against the overlay sheet during pressing. High pressure laminates after being consolidated are usually further glued to a structural substrate, such as particle board or plywood. Low pressure panel pressed decorative laminates are made in a similar manner to high pressure laminates, but generally involve lamination of the decorative sheet directly to particle board or other structural substrate.
Other pressing processes where a plastic film or sheet is formed on or against a release sheet may not include the lamination step, but only texturing a moldable plastic surface which is already laminated. For example, a plastic film could be coated directly onto particle board or plywood and then textured by pressing against a release sheet having the desired textured pattern in its surface. (See, for example, U.S. Pat. No. 4,113,894 to Koch.)
Other uses for release sheets include heat transferable printed designs and pressure sensitive adhesive coated webs. The heat transferable printed designs are printed on the release sheet with a polyvinyl chloride plastisol ink or offset printing ink and overcoated with a polyvinyl chloride plastisol. When placed against a receptive surface, such as a T-shirt, and heated, the printed design and overlayer are transferred to the receptive surface. On the other hand, pressure sensitive coated webs are typically adhesive coated tapes, labels or decals and the like which are attached to a release surface for easy removal when it is desired to permanently attach them. The release surface must permit temporary attachment of the pressure sensitive adhesive, but also permit easy removal.
Other uses of release sheets similar to the panel pressing area include use as an interleaver between groups of laminae pressed at the same time in back to back configuration to form two distinct decorative laminates. The release sheet in this case separates the laminates from each other and thereby permits more than one to be pressed at the same time between the same platens. (See, for example, U.S. Pat. No. 4,030,955 to Antonio et al.)
Release sheets are typically made by coating, treating, or impregnating a paper sheet or other substrate with a release coating of such materials as polymethylpentene, polypropylene, polyfluorocarbons, silicone oil, thermoset silicone resins, and other conventional release agents. Surface effects on the release sheet are conventionally provided by any one of a number of techniques. The release coating can be dried to a smooth surface gloss, or surface effects such as texturing or embossing can be provided in the coating by mechanical means, applied either to the surface of the paper before coating or to the paper after the coating is applied. Another technique employed for producing a release coating with a textured surface is to extrude a molten thermoplastic film such as polypropylene or polymethypentene, onto a paper surface, cool it and then pass it between matched steel embossing rolls. In all cases a satisfactory release paper must have its release coating securely adhered to the substrate so that it will remain with the substrate when the sheet or film formed on or against it is stripped.
One disadvantage of these typical prior art techniques is that the pattern of the embossing rolls or other mechanical means is not completely replicated in the surface of the release coating. That is, the entire embossure depth of the embossing rolls or other mechanical means is not reproduced in the release coating, often providing only about 60% actual replication. This shortcoming is particularly acute in producing fine patterns such as wood grain or leather grain, where the finer parts of the pattern can be lost in the replication process.
The disadvantages associated with the prior art techniques of providing only about 60% actual replication was virtually eliminated with the inventions of U.S. Pat. No. 4,289,821 and U.S. Pat. No. 4,322,450 (both of which are hereby incorporated by reference herein). These patents disclose coating a substrate with an electron beam curable release coating and then irradiating the coating while it is in contact with a replicative surface having the desired surface effect. The irradiation takes place through the substrate since the coating must be kept against the replicative surface. This method can produce a release coating which simulates the replicative surface almost 100%. Curing the coating against a surface, however, results in poorer release properties than one cured by irradiating the coating out of contact with the replicative surface. An alternate method disclosed in the patents which improves the release properties includes the additional steps of applying a second coating of electron beam curable material over the first layer already at least partially cured and then curing the second layer. This alternate method improves the release properties by curing a fresh coating layer out of contact with a replicating surface, but it reduces reproduction fidelity significantly.