Method for forming a decor sheet for decorative laminates

Abstract: A decor sheet is useful in a decorative laminate includes cellulosic fibres, a sulfur dye and a precipitate of a basic salt. The salt and the dye are introduced to a papermaking furnish during the manufacture of the decor sheet. A decor sheet thus produced exhibits reduced deterioration after it is impregnated with a laminating resin. Alkaline earth metal hydroxides are the preferred basic-salt precipitates.

Description: The present invention relates to a method of forming a decor sheet for a decorative laminate. Decorative laminates are well known in the art. They are typically formed from a book of core sheets of kraft paper generally brown in colour and impregnated with a phenolic resin and at least one resin-impregnated decor sheet which is pigmented or coloured and disposed on at least one face of the book of core sheets. A resin-impregnated overlay may also be superimposed on the decor sheet for scratch resistance. These sheets are consolidated under heat and pressure into a laminate useful in covering walls, furniture, countertops, and the like. In the past, decor sheets have been coloured using a class of dyes known as sulfur dyes. Decor sheets coloured with sulfur dyes have suffered from the drawback that the sulfur dye appears to accelerate the rate of cure of the resin to such an extent that the resin becomes highly crosslinked, brittle and exhibits poor postformability. The invention provides a method for forming a decor sheet which comprises preparing a papermaking furnish of a cellulosic pulp, a sulfur dye, a water soluble alkaline earth metal salt and sodium hydroxide, said water soluble alkaline earth metal salt and said sodium hydroxide interacting to form a precipitate comprising a water insoluble alkaline earth metal hydroxide; and forming a sheet from said furnish. The invention extends to a method for preparing decorative laminates incorporating such decor sheets. The term "water soluble" as used with respect to salts herein means that the salt is soluble in water in the amounts disclosed herein. For example, in the case of magnesium sulfate, the salt is sufficiently soluble in water to be added to the furnish for the decor sheet in an amount of 20-85% based on oven dry pulp. The term "water insoluble" when used with respect to the precipitated salt herein means that the solubility of the precipitated salt in water is sufficiently low that at least a majority of the salt precipitates from solution under the conditions taught herein. As explained in more detail below, we introduce certain water insoluble alkaline earth metal hydroxides to the papermaking furnish for the decor sheet. While not desiring to be bound by this as an explanation, these hydroxides appear to precipitate on to the fibres in the furnish and to neutralize acidic groups in the sulfur dyes and thereby prevent the dyes from catalyzing curing of the resin. The furnish for the decor sheet can be prepared from any cellulosic pulp including hardwood, softwood or mixtures of hardwood and softwood pulps. The pulp may be a chemical pulp such as a kraft pulp or a sulfite pulp, a chemi-mechanical pulp or a mechanical pulp. Alpha-cellulose such as cotton linter may be added to the furnish to further enhance postformability in a known manner. Additives such as wet strength resins, retention aids, pH stabilizers, salting out agents, and alum may be used in a known manner to control end use characteristics. A combination of water soluble alkaline earth metal salts is added to the furnish, which salts interact by ion exchange to precipitate a water insoluble alkaline earth metal hydroxide in the furnish. In one arrangement, an alkaline earth metal salt such as magnesium sulfate is reacted with sodium hydroxide to precipitate magnesium hydroxide in the furnish. An alkaline earth metal salt such as an alkaline earth metal sulfate or chloride (e.g. magnesium sulfate or magnesium chloride) is added to the furnish in an amount of about 20 to 85% by weight, all percents by weight herein being based on the oven dry weight of pulp in the furnish. It is anticipated that the amount of alkaline earth metal salt will typically range from about 20 to 50% by weight. The amount of precipitated hydroxide such as Mg(OH) 2 in the furnish is typically about 5 to 20% and more typically 5 to 12%. To precipitate the alkaline earth metal hydroxide in the furnish, sodium hydroxide is added to the furnish. Preferably, this hydroxide is added in an amount in excess of the stoichiometric amount required to precipitate the alkaline earth metal hydroxide in the furnish. Sodium hydroxide is generally added to the furnish in an amount of about 5 to 40% and preferably 15 to 25% by weight based upon the oven dry weight of the pulp in the furnish. Sulfur dyes are well known in the art and commercially available. Some typical examples include C.I. Leuco Sulfur Black 1 Dye. The following dyes which are commercially available from Sandoz Corporation are useful herein: Sodyesul Black 4GCF, Sodyesul Black PLCF, Sodyesul Black 2RCF, Sodyesul Red 2B, and Hoechst Duasyn Thioblack SR, Duasyn Thiocarbon LP (C.I. Sulfur Black 1), etc. The sulfur dye is preferably added to the furnish in its water soluble leuco form. The dye is typically added in an amount of about 10-30% based upon oven dry pulp. The amount will vary depending upon the decorative effect which is desired. Dyes added for tinting may be used in much lower amounts. To convert the dye from its soluble leuco form to its insoluble form and deposit the dye onto the cellulosic fibres, an agent such as ferrous sulfate is used in a manner well known in the art. As is known, sulfur dyes are often supplied commercially in the leuco form in the presence of sodium sulfide which maintains them in that form. The ferrous sulfate interacts with the sulfide and causes the leuco dye to convert to its oxidized form. The sulfur dye can also be added in its oxidized form, converted to its leuco form by the addition of a reducing agent and reconverted to the oxidized form to deposit it on the fibres. Because there is a tendency for sulfur dyes to oxidize over time and generate acidic groups which can degrade the paper, a pH stabilizer such as calcium carbonate may be added to the furnish in a known manner. Any conventional laminating resin may be used with the decor sheet to form decorative laminates. Suitable laminating conditions including the selection of the resin, the amount of the resin, the temperature and pressure under which the laminate is consolidated, are all known in the art. While the decor sheet is normally incorporated into a laminate such that it overlays the core sheets, the present decor sheet can be used at any location in the laminate in which its decorative effect is desired.For optimum postformability, we produce a decor sheet which exhibits a disc cure time, as defined in the following examples, of about 7 to 10 minutes. The invention is illustrated in more detail by the following non-limiting examples: Example 1 To a slurry of bleached hardwood kraft, bleached softwood sulfite, and second cut cotton linters at 4% consistency in water, 2% sodium carbonate, 4% calcium carbonate, 20% C.I. Leuco Sulfur Black 1 dye, 12% ferrous sulfate heptahydrate, 50% magnesium sulfate heptahydrate, 20% sodium hydroxide and 1% polyamide epichlorohydrin wet strength resin were added based upon pulp weight. This furnish was then diluted to 0.5% with water, adjusted to pH 10.0 with papermakers alum and formed into a fibrous mat by draining the suspension through a wire screen. The fibrous mat was then removed from the screen, placed between felts, pressed between steel rollers at 20 psi (1.379 x 10 5 N/m 2 ) and dried against a heated metal surface. The paper thus produced was evaluated for its catalytic activity toward the crosslinking reaction of melamine formaldehyde resin as follows: Discs of 1.625 inch (4.1275 cm) diameter are die cut from the paper and weighed to the nearest milligram. The discs are individually placed with the wire side down between a 6 inch (15.24 cm) by 12 inch (30.48 cm) by 0.0015 inch (0.00381 cm) thick piece of foil which has been folded in half lengthwise polished side in. Powdered melamine formaldehyde resin of equal weight to the paper disc is spread evenly over the felt side of the disc using a spatula. This "sandwich" is then placed into a preheated (291°F + 0.5°) (143.89°C + 0.28°C) platen press. A pressure of 1000 psi (6.895 x 10 6 N/m 2 ) is applied for a preset time and then released. The "sandwich" is then removed from the press and placed between two metal blocks to rapidly cool the disc. The disc is then removed from the foil and placed into a boiling solution of 0.1% Rhodamine B dye in distilled water for 3 minutes. The disc is removed from the solution, blotted and allowed to air dry. Examination of the disc surface will indicate the state of cure of the resin. A distinct reddish colour is indicative of resin undercure, whereas a fully cured disc is not discoloured by the Rhodamine dye. This test is repeated at incremental pressing times to determine the time at which full resin cure is achieved. This time is designated as the disc cure time. The disc cure time for this example was found to be eight minutes. Example 2 To a slurry of bleached hardwood kraft, bleached softwood sulfite, and second cut cotton linters at 4% consistency in water, 2% sodium carbonate, 4% calcium carbonate, 20% C.I. Leuco Sulfur Black 1 Dye, 12% ferrous sulfate heptahydrate, 85% magnesium sulfate heptahydrate, 20% sodium hydroxide and 1% melamine formaldehyde wet strength resin were added based upon pulp weight. This furnish was then diluted to 0.5% with water, adjusted to pH 8.0 with papermakers alum and formed into a fibrous mat by draining the suspension through a wire screen. The fibrous mat was then removed from the screen, placed between linter blotters, pressed between steel rollers at 20 psi (1.379 x 10 5 N/m 2 ), and dried against a heated metal surface. Disc cure time, six minutes. Example 3 To a slurry of bleached hardwood kraft, bleached softwood sulfite, and second cut cotton linters at 4% consistency in water, 2% sodium carbonate, 4% calcium carbonate, 20% C.I. Leuco Sulfur Black 1 Dye, 12% ferrous sulfate heptahydrate, 85% magnesium sulfate heptahydrate, 15% sodium hydroxide and 1% melamine formaldehyde wet strength resin were added based upon pulp weight. This furnish was then diluted to 0.5% with water, adjusted to pH 8.0 with papermakers alum and formed into a fibrous mat by draining the suspension through a wire screen. The fibrous mat was then removed from the screen, placed between linter blotters, pressed between steel rollers at 20 psi (1.379 x 10 5 N/m 2 ) dried against a heated metal surface. Disc cure time, five minutes. Comparative Example 1 To a slurry of bleached hardwood kraft, bleached softwood sulfite, and second cut cotton linters at 4% consistency in water, 2% sodium carbonate, 4% calcium carbonate, 20% C.I. Leuco Sulfur Black 1 Dye, 12% ferrous sulfate heptahydrate, 15% sodium hydroxide and 1% melamine formaldehyde wet strength resin were added based upon pulp weight. This furnish was then diluted to 0.5% with water, adjusted to pH 8.0 with papermakers alum and formed into a fibrous mat by draining the suspension through a wire screen. The fibrous mat was then removed from the screen, placed between linter blotters, pressed between steel rollers at 20 psi (1.379 x 10 5 N/m 2 ) and dried against a heated metal surface. Disc cure time, three minutes. Comparative Example 2 To a slurry of bleached hardwood kraft, bleached softwood sulfite, and second cut cotton linters at 4% consistency in water, 2% sodium carbonate, 4% calcium carbonate, 20% C.I. Leuco Sulfur Black 1 Dye, 12% ferrous sulfate heptahydrate, 20% magnesium hydroxide, 15% sodium hydroxide and 1% melamine formaldehyde wet strength resin were added based upon pulp weight. This furnish was then diluted to 0.5% with water, adjusted to pH 8.0 with papermakers alum and formed into a fibrous mat by draining the suspension through a wire screen. The fibrous mat was then removed from the screen, placed between linter blotters, pressed between steel roller at 20 psi (1.379 x 10 5 N/m 2 ) and dried against a heated metal surface. Disc cure time, two minutes, thirty seconds.

Claims: A method for forming a decor sheet which comprises preparing a papermaking furnish of a cellulosic pulp, a sulfur dye, a water soluble alkaline earth metal salt and sodium hydroxide, said water soluble alkaline earth metal salt and said sodium hydroxide interacting to form a precipitate comprising a water insoluble alkaline earth metal hydroxide; and forming a sheet from said furnish. A method according to Claim 1, wherein said sulfur dye is added to said furnish as a leuco dye and said leuco dye is converted to its oxidized form. A method according to Claim 1, wherein said sulfur dye is added to said furnish in its oxidized form, converted to its soluble leuco form by addition of a reducing agent and reconverted to its oxidized form to deposit the dye on the fibres in the furnish. A method according to any one of the preceding claims, wherein said precipitated hydroxide is magnesium hydroxide. A method according to any one of the preceding claims, wherein said hydroxide is precipitated by ion exchange interaction of a water soluble alkaline earth metal salt and sodium hydroxide. A method according to any one of the preceding claims wherein said water soluble alkaline earth metal salt is an alkaline earth metal sulfate or chloride. A method according to any one of the preceding claims wherein said water soluble alkaline earth metal salt is magnesium sulfate or magnesium chloride. A method according to any one of the preceding claims, wherein said precipitated hydroxide is present in said sheet in an amount of about 5 to 20% based on oven dry pulp. A method according to any one of the preceding claims, wherein said sheet exhibits a disc cure time of about 7 to 10 minutes. A method for preparing a decorative laminate which comprises forming a decor sheet using a method according to any one of the preceding claims and consolidating the decor sheet so formed and a book of core sheets impregnated with a phenolic resin under heat and pressure.

Inventors: CANARY STEPHEN A, CANARY, STEPHEN A.
Applicants: MEAD CORP, THE MEAD CORPORATION
Application year: 1992
Main classification: B44C5
IPC codes: B44C5, D21H27