Heat and pressure consolidated decorative laminates have been produced commercially in the United States and elsewhere for a number of years, and have found widespread acceptance as marresistant surfaces for wall-coverings, paneling, table tops, counter tops, and the like. These laminates contain a number of laminae that are consolidated to form a unitary structure carrying a surface decoration which can range from something as simple as a solid color to something as complex as an embossed simulated wood-grain finish.
Although the methods of preparing such laminates and the number and types of laminae can vary widely, the procedure for preparing such laminates generally involves the consolidation of one or more sheets of core stock, depending primarily on the ultimate thickness desired, in combination with a decorative or print sheet and a top coat or overlay. The core stock usually comprises an unbleached kraft paper which has been impregnated with a relatively inexpensive thermosetting resin, such as a phenolic resin, which is easily cross-linked upon the application of pressure and heat.
The decorative or print sheet has more stringent requirements, in that it is usually pigment filled, and that it must still be capable of also being impregnated with a "noble" thermosetting resin which also is cross-linkable upon the application of heat and pressure, but which exhibits no color deterioration upon the application of such heat and pressure, and which prevents any strike-through or bleeding of the resin used in the corestock. In many instances, the decorative or print sheet must also be capable of being printed with a design such as a geometric pattern, simulated wood grain, etc., which survives the consolidation step intact. Two of the most common "noble" thermosetting resins which are used to impregnate the decorative or print sheet employed in the preparation of these laminates are urea-formaldehyde resin condensates and melamine-formaldehyde resin condensates, although other resins such as polyester resins have also been so employed.
In most instances, and particularly when the decorative sheet carries a printed design, an overlay sheet is superimposed over the decorative or print sheet prior to consolidation of the laminae. The overlay sheet is generally a thin, high-quality, alpha-cellulose paper which is also impregnated with a "noble" thermosetting resin, usually of the same type used to impregnate the decorative or print sheet which is employed. The overlay sheet is usually transparentized during the consolidation step, thereby enabling the decoration and/or printing which is present on the decorative or print sheet to be readily seen in the finished laminate, yet imparting a greater degree of mar and abrasion resistance to the decorative laminate than would otherwise be obtained without such an overlay.