The production of high pressure laminates is well known, having been carried out for many years. Generally, the procedures for forming such laminates involve providing phenolic (i.e., phenol-formaldehyde) resin impregnated paper core sheets and melamine (i.e. melamine-formaldehyde) resin decorative and overlay sheets, and pressing the stacked resin impregnated sheets under heat on the order of 230.degree.-310.degree. F and under pressure of approximately 800-1600 psi until the resins have become thermoset, thereby providing an extremely hard, attractive and permanent surfacing material known as a "high pressure laminate" meetimg NEMA Standards. These high pressure laminates have, for many years, found use as counter and table tops, bathroom and kitchen work surfaces, furniture and cabinet surfacing, wall paneling and partitionings, doors, etc.
Most general purpose decorative high pressure laminates, of about 1/16 inch thickness, are formed of an assembly comprising a top ply of .alpha.-cellulose paper, about 20-30 lbs. ream weight, impregnated with a partially cured water solution of melamine-formaldehyde condensate; a print ply therebeneath, normally also an .alpha.-cellulose paper, pigment filled, with or without decorative printing on the surface thereof, ranging in weight from 50 to 125 lbs. ream weight and also impregnated with a partially cured amino resin condensate, usually the same melamine resin as used in the overlay ply, and therebeneath a plurality (e.g. six) core plys which are normally 100-130 lbs. ream weight Kraft paper, impregnated with a water or alcohol soluble partially cured phenolic resin condensate. All of such partially cured resin condensates are referred to as being in the "B-stage"; in this stage they are thermoplastic and will flow under heat and pressure during the high heat, high pressure laminating procedure.
The above described assembly is repeated, back-to-back and front-to-front using separators therebetween, each two laminates being separated by a pressing plate die, until a "book" containing, typically, 10 assemblies is obtained. The book is inserted in a high pressure press, and the laminates are cross-linked under the conditions of heat and pressure indicated above. During the pressing cycle, the resins flow and then cure, consolidating the individual plys and forming a comprehensive and infusible cross-linked product. After cure, the book is removed from the press and the laminates are separated, trimmed, and their backs or bottom surfaces sanded to improve adhesion for subsequently gluing to various substrates.
In the early years, generally only glossy surface laminates were produced (i.e. using polished molding surfaces), though some attempts were made to provide laminates with some slight surface irregularities to give satin or "textured" finishes. In more recent years it has become increasingly desirable to provide various types of irregular surfaces, ranging from relatively shallow depressions such as textured (e.g., satin, matte or semi-gloss) surfaces, to relatively deeply "sculptured" or embossed surfaces which have a noticeable three-dimensional effect, such as wood grain, leather, slate, abstract patterns, creative designs, etc. Particularly with the rising costs of natural products, it has become more desirable to provide deeply sculptured high pressure laminates which simulate natural materials such as wood, leather and slate.
Today, high pressure decorative laminates meeting NEMA Standards are manufactured with smooth glossy surfaces, with textured surfaces, or with deeply sculptured or embossed surfaces. In the latter case, a textured surface is usually superimposed on the embossed surface, and these textured surfaces are normally obtained by utilization of a release, parting or separating membrane or sheet of known type which is located, during the laminating operation, between the upper surface of the uppermost laminate and the molding plate die. In the production of deeply sculptured surfaces which simulate natural products such as slate, leather and wood, it is particularly desirable that the textured surfaces be superimposed over the face of the simulated product, since this provides a softer and more naturally appearing product.
In the manufacture of deeply embossed or sculptured surfaces having surface depth variations which duplicate natural products such as slate, leather, wood and the like, it has in the past generally been necessary to use either extremely expensive etched steel or stainless laminating die plates or, alternatively, thermoset pressing plate dies such as described in the Michaelson et al U.S. Pat. Nos. 3,303,081 and 3,311,520. While the use of such dies provides the necessary embossing,, many simulated natural products, such as wood grain, must have appropriate color in registry with the surface embossing or debossing. Unfortunately, the problem of registration of color and embossing has proven very difficult, conventional registry techniques such as used in the printing industry being unreliable on a consistent basis in the manufacture of high pressure laminates, and also being extremely expensive, often involving complex electronic sensing equipment.
Other methods of providing laminates having registered color and texture have utilized valley printing, namely the inking of high portions of an engraved plate and pressing into the resin while curing. This operation, as well as the mechanical registration of an embossing roll or plate with a print, is difficult to carry into production and/or does not always give a uniform product. In addition, inks may not be compatible with or as durable as the base resin in which case the product will not meet NEMA Standards.
In more recent years, a number of other techniques have been developed in an attempt to solve the problems of registration of color and embossing in the manufacture of deeply sculptured high pressure laminates. Certain of these techniques have been found to be highly useful in the preparation of certain specific types of configuration, e.g. see U.S. Pat. Nos. 3,700,537 and 3,698,978. Other recent techniques have been more versitile from the viewpoint of the design of the product (note, e.g., U.S. Pat. Nos. 3,814,647; 3,732,137; 3,802,947 and 3,661,672), but some of these recent techniques have been less than fully desirable because they require special materials and/or are difficult to carry out successfully with the result that the products are sometimes inconsistent or the manufacturing operation is more expensive than is desirable.
In copending application Ser. No. 528,776*/(incorporated here by reference) there is disclosed a method of providing registered color and embossing in which the overlay sheet is impregnated with melamine resin containing dispersed pigment, and this overlay sheet is then used in the conventional lay-up over a conventional melamine impregnated print sheet of contrasting color. While the process exemplified in application Ser. No. 528,776 produces a laminate of exceptional appearance particularly when the product is a mottled pattern in which the darker color overlies the lighter color, e.g. simulated leather, such exemplified process does have certain disadvantages. First, it is difficult to carry out and therefore requires exceptional care to impregnate the overlay sheet, without having longitudinal streaks of pigment concentrations. This is due to the mechanics of impregnation and is not a fundamental problem.
Secondly, and more fundamental is the fact that the process exemplified in Ser. No. 528,776 provides a product which may not have as great an abrasion resistance as is desired. Because much of the pigment is concentrated near the the overlay surface, and the finished laminate surface is heavily embossed, the peaks show loss of color earlier than desirable in the simulated wear tests unless this wear is compensated by usng extraordinary measures, e.g. an extra impregnated overlay sheet, i.e. two overlay sheets. While the use of the second overlay sheet considerably improves the abrasion resistance, this solution reduces the crispness of the color contrast in the fine embossing details with the result that the product is sometimes not as attractive as is desired.
A third disadvantage of the technique exemplified in Ser. No. 528,766 is that by incorporating the dispersed pigment in the overlay by means of impregnation including first saturating the overlay with pigment dispersed resin and then wiping excess from its surface, the saturating characteristics of the overlay paper have undue influence over the color uniformity of the final product. That is, the normal density and wicking properties of the paper that vary from point-to-point in a sheet may cause variation in resin-pigment concentration at different locations across the area of the sheet. In a conventional overlay, this does not cause problems because there is enough resin to provide suitable physical properties even in low concentration areas, and the variation is not visible because the resin is almost transparent. When the pigmented resin is used, however, concentration variations sometimes result in a mottled appearance in the finished laminate if the pigment is not strongly opaque and, therefore, the technique exemplified in Ser. No. 528,776, while giving a highly realistic mottled simulated product such as leather, is not as suitable for other designs containing larger smooth areas where color variation is not desirable, if the pigment is not strongly opaque.