Method for controlling laminate gloss

Abstract: A method for forming a glossy decorative laminate is disclosed. The method is achieved by stacking a decorative laminate sheet assembly (10) having a top layer, casting a polypropylene release sheet (12) wherein the polypropylene release sheet (12) is processed with chill rollers treated in a manner which improves the gloss characteristics of the resulting decorative laminate, positioning the polypropylene release sheet (12) on the top layer of the decorative laminate sheet assembly (10), applying heat and pressure to the decorative laminate sheet assembly (10) and the polypropylene release sheet (12) sufficient to bond the decorative laminate (10) and releasably bond the polypropylene release sheet (12) to the top layer of the decorative laminate sheet assembly (10), and removing the polypropylene release sheet (12) from the top layer of the decorative laminate sheet assembly (10) to reveal a decorative laminate exhibiting desirable gloss characteristics.

Description: This application is a continuation-in-part of U.S. Patent Application Serial No. 09/092,480, filed June 5, 1998, entitled "Texture Release Sheet, Method Of Making Texture Decorative Laminates Therewith, And Decorative Laminate Lay-Ups Including Such A Sheet", which is currently pending. The invention relates to a method for producing glossy decorative laminates, as well as controlling laminate gloss, using a cast polypropylene release sheet. High pressure decorative laminates are currently manufactured with smooth glossy surfaces, textured surfaces, or deeply sculpted and embossed surfaces. As general background, decorative laminates prepared by heat and pressure consolidation have been produced commercially for a number of years, and have found widespread acceptance in the building and furniture industry as counter and tabletops, bathroom and kitchen work surfaces, wall paneling, flooring products, partitions and doors. These decorative laminates may be described as containing a number of laminae consolidated to form a unitary structure carrying a surface decoration. The surface decoration may range from something as simple as a solid color to something as complex as an embossed simulated wood grain finish. Decorative laminates generally include plural layers of synthetic resin impregnated paper sheets bonded under heat and pressure to form a unitary structure. In normal practice, a decorative laminate sheet assembly, from the bottom up, includes a core of one or more phenolic resin impregnated sheets, above which lies a decorative melamine impregnated sheet. The decorative sheet may be further covered with a melamine impregnated overlay. The core, or base, functions to impart rigidity to the laminate and usually includes a solid substrate which may, or may not, be formed prior to the initial laminating steps. Prior to stacking, the sheets of the core member are impregnated with a water alcohol solution of phenol formaldehyde, dried and partially cured in a hot oven, and finally cut into shapes. The core may, for example, include a plurality of sheets of 41-68 kg (90-150 pound) phenolic resin impregnated kraft paper and a substrate. The kraft paper is impregnated throughout and bonded with a substantially completely cured phenolic resin which has been converted to a thermoset state during the initial laminating step. The substrate may be a pre-cured plastic laminate, such as glass fiber-reinforced thermoset polyester resin laminates and the like, a wood product, such as hardboard, wood waste or particle boards, plywood and the like, a mineral base board, such as, cement-asbestos board, sheet rock, plaster board, and the like, or a combination of substrates. The decorative sheet provides the laminate with an attractive appearance. The decorative sheet also dictates the surface characteristics of the decorative laminate. For example, the composition of the decorative sheet dictates the decorative laminate's resistance to chemical agents, heat, light, shock and abrasion. Decorative sheets are commonly manufactured from high quality 50-125 ream weight, pigment filled, alpha cellulose paper impregnated with a water alcohol solution of melamine-formaldehyde resin. The resin impregnated decorative sheets are subsequently dried, partially cured, and finally cut into sheets. The pigment filled, alpha cellulose paper of the decorative sheet, may include a solid color, a decorative design, or a photo-gravure reproduction of natural materials, such as, wood, marble, leather, etc. The aesthetic characteristics of the cellulose paper are revealed as the laminate's decorative design upon completion of the decorative laminate. Decorative laminates are generally manufactured by placing the resin impregnated core and decorative sheet between steel plates and subjecting the laminate stack to temperatures in the range of about 85-110 bars (800-1600 psi) for a time sufficient to consolidate the laminate and cure the resins (generally 25 minutes to an hour). The pressure and heat force the resin in the paper sheets to flow, cure and consolidate the sheets into a unitary laminated mass referred to in the art as a decorative high pressure laminate. Finally, the formed decorative laminate is bonded to a reinforcing substrate, such as, plywood, hardboard, asbestos board, particle board or the like. Generally, more than one laminate is formed at one time. Multiple laminates are formed by inserting a plurality of assembled sheets in a stack. Release sheets are positioned between the assembled sheets to separate the various laminates stacked together. After consolidation, the release sheets allow the individual laminates to be separated. In recent years, textured decorative laminates have become very popular. The textured surfaces range from relatively shallow depressions, such as, textured (e.g., satin, matte or semigloss) surfaces, to relatively deeply sculpted or embossed surfaces having a noticeable three-dimensional effect, such as, wood grain, leather, slate, abstract patterns, creative designs etc. In instances where glossy textured decorative laminates are manufactured, very expensive techniques are currently required to ensure the desired texture and gloss are imparted to the decorative laminate. While the physical characteristics, ease of manufacture, and expenses have long made polypropylene a desirable material for use as a release sheet in the production of decorative laminates, polypropylene release sheets have historically generated decorative laminates exhibiting limited gloss. For example, cast polypropylene release sheets have historically produced relatively dull decorative laminates exhibiting laminate gloss of less than six. Similarly, blown polypropylene release sheets generally only generate a gloss of around 10 to 12. EP-A-0 327 095 describes a transfer sheet used for preparing thermosetting resin decorative material. The transfer sheet includes a nonglossy base sheet which may be made of polypropylene or, preferably, polyester film. The transfer sheet also has a highly glossy pattern layer of an ink containing a curable resin and silicone resin on the surface of the matte base sheet. The transfer sheet is superposed on a resin impregnated paper so that the pattern layer on the transfer sheet is brought into contact with the resin impregnated paper. The glossy pattern layer together with the unglossy matte surface of the base layer (where the pattern layer is not present) is transferred to the surface of the resin impregnated paper to produce a decorative material having varied gloss. As such, the base sheet does not provide a glossy surface, but rather imparts a matte or unglossy surface to the surface of the resin impregnated paper. As such, a need exists for a method for producing decorative laminates exhibiting excellent gloss characteristics. The present invention provides such a method. It is, therefore, an object of the present invention to provide a method for forming a glossy decorative laminate. The method is achieved by stacking a decorative laminate sheet assembly having a top layer, providing a polypropylene release sheet wherein the polypropylene release sheet is processed with chill rollers which have been treated with electropolishing or with a co-deposition coating of a polytetrafluoroethylene and electroless nickel, so that said decorative laminate exhibits controlled gloss characteristics, positioning the polypropylene release sheet on the top layer of the decorative laminate sheet assembly, applying heat and pressure to the decorative laminate sheet assembly and the polypropylene release sheet sufficient to bond the decorative laminate and releasably bond the polypropylene release sheet to the top layer of the decorative laminate sheet assembly, and removing the polypropylene release sheet from the top layer of the decorative laminate sheet assembly to reveal a decorative laminate exhibiting desirable gloss characteristics. Other objects and advantages of the present invention will become apparent from the following detailed description when viewed in conjunction with the accompanying drawings, which set forth certain embodiments of the invention. Figure 1 is a schematic of the laminate lay-up including the polypropylene release sheet in accordance with the present invention. Figure 2 is a schematic of the laminate lay-up after pressing and heating. Figure 3 is a schematic of the resulting decorative laminate. The detailed embodiments of the present invention are disclosed herein. It should be understood, however, that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, the details disclosed herein are not to be interpreted as limited, but merely as the basis for the claims and as a basis for teaching one skilled in the art how to make and/or use the invention. With reference to Figure 1, a decorative laminate lay-up 1 including a decorative laminate sheet assembly 10 and a polypropylene release sheet 12 is disclosed. The decorative laminate lay-up 1 is used in the manufacture of a glossy decorative laminate 18 in accordance with the present invention. The decorative laminate lay-up 1 is assembled by first stacking a decorative laminate sheet assembly 10 having a top layer 14. That is, the sheets used in the production of the decorative laminate are stacked in preparation for the heating and pressure steps used to consolidate the laminate. A polypropylene release sheet 12 is also cast. The polypropylene release sheet 12 is treated with chill rollers coated in a manner which improves the gloss characteristics of the resulting decorative laminate. The polypropylene release sheet 12 is then positioned on the top layer 14 of the laminate sheet assembly 10 to form the laminate lay-up 1, and heat and pressure are applied to the laminate sheet assembly/polypropylene release sheet 16 sufficient to bond the layers of the decorative laminate sheet assembly 10, and releasably bond the polypropylene release sheet 12 to the top layer 14 of the decorative laminate sheet assembly 10 (see Figure 2). Finally, the polypropylene release sheet 12 is removed from the top layer 14 of the formed decorative laminate 18 to reveal a decorative laminate exhibiting desired gloss characteristics (see Figure 3). In most instances the gloss is controlled to produce a glossy laminate in the range of 10 to 30. However, since the desired gloss of laminates vary, the gloss may be controlled in any desirable manner and need not result in a laminate gloss of 10 to 30 The decorative laminate 18 may be any of the many resin based decorative laminates known to those of ordinary skill in the art. In accordance with one embodiment of the present invention, and as discussed above, the decorative laminate 18 includes a decorative sheet, or top layer, 14 and a core 20 as shown in Figure 3. More specifically, the decorative laminate 18 includes a core 20 composed of one or more layers of phenolic resin impregnated kraft paper 20a and a decorative sheet 14 composed of a pattern layer 22 and a melamine overlay layer 24 incorporating Al 2 O 3 for wear resistance. As discussed above, a substrate may be subsequently bonded to the final decorative laminate 18 to add rigidity to the laminate; for example, the substrate may be chosen from a variety of materials, namely, plywood, particle board, chipboard, medium density fiberboard etc. Although a preferred decorative laminate is disclosed above, a wide variety of decorative laminates may be employed. As is well known to those of ordinary skill in the art, cast polypropylene release sheets are manufactured by extruding, rolling, cooling and cutting the polypropylene release sheets. In accordance with the present invention, the polypropylene release sheet 12 may be generally described as either a propylene homopolymer or a copolymer ofpropylene and any other α-olefin. The melt flow index of the polypropylene utilized in accordance with the present invention is selected to provide the desired physical properties necessary to produce textured decorative laminates exhibiting high gloss. Generally, the melt flow index will be in the range of 1 to 20 as measured utilizing ASTM D-1238. The melt flow index is preferably in the range of 5 to 15, and more preferably in the range of 7 to 12, as measured utilizing ASTM D-1238. Non-limiting commercial examples of polypropylenes suitable for use in the present invention include EOD97-06 and EOD96-01, available from Fina (Dallas, Texas) and ESCORENE polypropylene PD 4443, available from Exxon Chemical. EOD97-06 and EOD96-01 have a melt flow of 12 g/10 min. (ASTM D-1238 Condition "L"), density of 0.91 g/cc (ASTM D-1505), tensile strength of 400 bars (5800 psi) (ASTM D-638), elongation of 10 percent at yield, tensile modulus of 19 305 bars (280000 psi) (ASTM D-638), flexural modulus of 18 616 bars (270 000 psi) (ASTM D-790), melting point of 167°C (333°F) (DSC), and a recrystallization point of 127°C (261°F). ESCORENE polypropylene PD 4443 has a melt flow of 7.25 g/10 min. (ASTM 1238), density of 0.90 g/cm 3 (ASTM D792), tensile strength of 228 bars (3300 psi) in the machine direction, elongation at yield of 5% in both the machine and transverse directions, tensile strength at break of 552 bars (8000 psi) in the machine direction and 483 bars (7000 psi) in the transverse direction, elongation at break of 660 percent in both the machine and transverse directions, haze of 2/5%, gloss of 85% and a coefficient friction of 0.3. As is well known to those of ordinary skill in the art, various processes and catalysts exist for the production of polypropylene. It is, therefore, contemplated that various processes and catalysts may be utilized. For example, traditional Ziegler-Natta catalyst systems may be employed in accordance with the present invention. Ziegler-Natta catalyst systems utilize a transition metal compound cocatalyzed by an aluminum alkyl. Having been around since the early 1950's, the general manufacture methods for Ziegler-Natta type polyolefin catalysts, as well as general methods of making and subsequent use, are well known in the polymerization art. More specifically, a Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium, with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst component is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum, and may even include an electron donor. Examples of such catalyst systems are shown in the following U.S. patents: 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562.173. These are just a few of the hundreds of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene. "Metallocene" catalysts may also be used in accordance with the present invention. "Metallocene" catalysts include a metallocene and an aluminum alkyl component. The transition metal compound of a metallocene catalyst system has two or more cyclopentadienyl ring ligands. Accordingly, titanocenes, zirconocene and hafnocenes have all been utilized as the transition metal component in such "metallocene" containing catalyst systems for the production of polyolefins. As discussed above, conventional cast polypropylene release sheets are extruded through a die and treated with chill rollers to cool the extruded sheet under controlled conditions. The present invention employs chill rollers treated in a predetermined manner to impart desirable hardness and smoothness characteristics to the extruded polypropylene release sheets. When these polypropylene release sheets are use in the manufacture of decorative laminates, the resulting decorative laminates exhibit a controlled gloss. Specifically, it has been found that chill rollers chromed in a high temperature bath, for example, 49°C (120°F), produce polypropylene release sheets resulting in high gloss laminates. Where texture is first applied to the chill rollers (as will be discussed below in greater detail), the chrome application treatment includes engraving the polished roll with a microscopically smooth, shiny tool (no acid is used in the engraving process). It has been found that chill rollers treated with a co-deposition of TEFLON, a polytetrafluoroethylene, and electroless nickel produce polypropylene release sheets generating ideal results. Similarly, chill rollers treated with electropolishing also provide ideal results. In addition, chill rollers treated with an underlayer of nickel, and a co-deposition of TEFLON and electroless nickels improve the results provided by simple chill rollers treated with a co-deposition of TEFLON and electroless nickel. In accordance with the preferred embodiment of the present invention, the co-deposition coating of TEFLON and electroless nickel, as well as the nickel underlayer with a co-deposition coating of TEFLON and electroless nickel, are preferably achieved using Series 200 "Infused Matrix" coating techniques of ENDURA COATINGS, Inc., Warren, Michigan. With regard to the electropolishing of the chill rollers, electropolishing is performed using the techniques described in "ELECTROPOLISHING a User's Guide To Applications, Quality Standards and Specifications", January 1998, Eighth Edition, Delstar Corporation. Each of the chill rollers discussed above imparts a smooth microscopic surface to the cast polypropylene release sheet. It is believed that treating chill rollers to increase the hardness and smoothness characteristics of the resulting polypropylene release sheets optimizes the production of high gloss decorative laminates. In fact, it is believed that the hardness and smoothness characteristics of the cast polypropylene release sheet are critical to the gloss of the resulting decorative laminate. As such, and discussed above, the cast polypropylene release sheet should have a melt flow index in the range of 1 to 20 as measured utilizing ASTM D-1238. The melt flow index is preferably in the range of 5 to 15, and more preferably in the range of 7 to 12. In addition, the resulting polypropylene release sheet should exhibit a smooth microscopic surface with no irregularities. As those of ordinary skill in the art will certainly appreciate, a variety of chill roller coating and/or treating techniques may be employed so long as the resulting polypropylene exhibits the desired characteristics discussed above. The temperature of the chill roller may also be utilized to control the gloss of a resulting laminate manufactured in accordance with the present invention. For example, the an increase in the temperature of the chill roller within an acceptable range will provide and increase in the gloss of a resulting laminate manufactured in accordance with the present invention. In this way, slight changes in the gloss of a laminate may be readily controlled by simply increasing or decreasing the temperature of the chill roller. It is believed that the change in temperature affects the crystalline structure of the resulting polypropylene release sheet, and ultimately the gloss of the laminate manufactured using the polypropylene release sheet. The polypropylene release sheets discussed above may also be used to simultaneously impart texture to the decorative laminate. Commonly owned European Patent Application EP 99 106 627.5 filed on March 31, 1999 and published on December 8, 1999 under EP 0 962 315, corresponding to U.S. Patent Application Serial No. 09/092,480, entitled "Texture Release Sheet, Method Of Making Texture Decorative Laminates Therewith, And Decorative Laminate Lay-Ups Including Such A Sheet", filed June 5, 1998 discloses a useful texturing technique which may be simultaneously employed with the present gloss method to create textured and glossy decorative laminates. However, texture may be simultaneously applied with gloss by any suitable processing method capable of forming a textured release sheet for applying texture to a decorative laminate. In accordance with the preferred embodiment of the present invention, extrusion and casting are preferably employed to produce both high gloss and texture. Briefly, the polypropylene is forced through an extrusion die in a melted state and subsequently cooled into a sheet as the melted extrudate passes between chill rollers. The chill rollers provide the desired texturing to the front surface of the sheet. The textured sheet is manufactured with a smooth back surface. When this texturing technique is used in accordance with the present invention, the chill rollers are first textured to impart a desired texture to the treated polypropylene sheet. The textured chill rollers are then finished by applying chrome or by coating with a co-deposition of TEFLON, a polytetrafluoroethylene, and electroless nickel. Similarly, the chiller rollers may be coated with an underlayer of nickel, and co-deposition of TEFLON and electroless nickel. Alternately, the chill rollers may be treated with electropolishing alone, or the chill rollers may be electropolished before the application of the nickel/TEFLON co-deposition or the chrome. Regardless of the method for forming the textured release sheet, it is critical that texture is imparted to the polypropylene release sheet generally while the polypropylene is still above the softening temperature, preferably while still above the melting temperature. The resulting textured release sheet will have a textured front surface and a smooth back surface. The texturing useful in the invention may take any form, dimension or pattern imaginable. In general, the peak-to-valley heigh R z of the texturing surface may be any desirable, but is preferably greater than 2,54 µm (100 µ inches) more preferably greater than about 5.08 µm (200 µ inches), and even more preferably greater than 7.62 µm (300 µ inches). The thickness of the textured release sheet useful in the invention may be varied within a wide range of limits. If the sheet is too thin, the texture will tend not to transfer. Economic factors tend to set the upper limit for the thickness of the sheet. In general, the thickness of the textured release sheet will be at least 0.0254 mm (1mil) and preferably at least 0.0381 mm (1,5 mils), and more preferably at least 0.0508 mm (2mils). The textured polypropylene release sheet provides a textured surface to decorative laminates and also functions as a release sheet between conventional decorative laminate lay-ups. With this in mind, the present polypropylene sheets may be used in the gang processing of laminates or the present polypropylene sheets may be used in the manufacture of single laminate sheets to simply impart a desired texture and gloss. Whether the present polypropylene release sheet is employed to add texture to the decorative laminate, or not, the polypropylene sheet is applied to the decorative laminate in the following manner. A typical decorative laminate sheet assembly includes several layers of a thermosetting resin impregnated core stock (preferably kraft paper) supporting a thermosetting resin impregnated decorative layer, which may be further overlaid with a thermosetting resin impregnated overlay sheet. The thermosetting resin employed is preferably a phenolic resin for the core stock, and preferably a clear melamine formaldehyde resin for the decorative and overlay sheets. The textured surface of the release sheet is oriented against the top most layer of the decorative laminate sheet assembly, be it the decorative sheet or overlay sheet, with the smooth back surface of the textured release sheet positioned against the core stock of an adjacent lay-up or steel plate. Upon pressing between suitable laminate press plates, the textured release sheet imparts texture to the top most layer of the first lay-up, and serves to allow release between the first lay-up and the adjacent lay-up. A platen press assembly, as is well known in the lamination art, provides the necessary heat and/or pressure during lamination. In practice, there may be utilized, as desired and/or necessary, antioxidants, antiblock agents, slip agents, cross linking agents, stabilizers, ultraviolet ray absorbers, lubricants, foaming agents, antistatic agents, organic and inorganic flame retardants, plasticizers, dyes, pigments, talc, calcium carbonate, carbon black, mica, glass fibers, carbon fibers, aramid resin, asbestos, as well as other fillers as are known in the art. The polypropylene utilized in the present invention may be further blended with, or incorporated into, one or more other thermoplastics. Suitable other thermoplastics include polyolefins, especially polyethylenes, other polypropylenes, polyesters, polyacrylamindes, polyamides, polycarbonates, polyurethanes, polyacrylonitriles, and butadiene styrenes. Preferred polyethylenes include linear low density polyethylenes. While the preferred embodiments have been shown and described, it will be understood that there is no intent to limit the invention by such disclosure, but rather, is intended to cover all modifications and alternate constructions falling within the scope of the invention as defined in the appended claims.

Claims: A method for forming a decorative laminate, comprising the steps of: stacking a decorative laminate sheet assembly having a top layer; providing a polypropylene release sheet; positioning the polypropylene release sheet on the top layer of the decorative laminate sheet assembly; applying heat and pressure to the decorative laminate sheet assembly and the polypropylene release sheet sufficient to bond the decorative laminate and releasably bond the polypropylene release sheet to the top layer of the decorative laminate sheet assembly; and removing the polypropylene release sheet from the top layer of the decorative laminate sheet assembly to reveal said decorative laminate, characterized in that the step of providing said polypropylene release sheet comprises casting said release sheet wherein said release sheet is processed with chill rollers, said chill rollers having been treated with a co-deposition coating of a polytetrafluoroethylene and electroless nickel, so that said decorative laminate exhibits controlled gloss characteristics. The method according to claim 1, wherein the chill rollers are treated with an underlayer of nickel. A method for forming a decorative laminate, comprising the steps of: stacking a decorative laminate sheet assembly having a top layer; providing a polypropylene release sheet; positioning the polypropylene release sheet on the top layer of the decorative laminate sheet assembly; applying heat and pressure to the decorative laminate sheet assembly and the polypropylene release sheet sufficient to bond the decorative laminate and releasably bond the polypropylene release sheet to the top layer of the decorative laminate sheet assembly; and removing the polypropylene release sheet from the top layer of the decorative laminate sheet assembly to reveal said decorative laminate, characterized in that the step of providing said polypropylene release sheet comprises casting said release sheet wherein said release sheet is processed with chill rollers, said chill rollers having been treated with electropolishing , so that said decorative laminate exhibits controlled gloss characteristics. The method according to any one of claims 1 to 3, wherein the top layer of the decorative laminate sheet assembly includes a pattern layer and an overlay layer. The method according to any one of claims 1 to 4, wherein the melt flow index of the polypropylene is between 1 and 20. The method according to any one of claims 1 to 4, wherein the melt flow index of the polypropylene is between 5 and 15. The method according to any one of claims 1 to 4, wherein the melt flow index of the polypropylene is between 7 and 12. The method according to any one of claims 1 to 7, wherein casting further includes imparting a textured surface to the polypropylene release sheet. The method according to claim 8, wherein the textured surface of the polypropylene release sheet is applied directly to the top layer of the decorative laminate sheet assembly.

Inventors: OLIVER JAY T, PALMER DOROTHY TEPERA, OLIVER, JAY T., PALMER, DOROTHY TEPERA
Applicants: PREMARK RWP HOLDINGS INC, PREMARK RWP HOLDINGS, INC.
Application year: 2000
Main classification: B32B27
IPC codes: B32B27, B44F1, B32B37