Coating of composite wood panels with aminoplast resin films fitted with an abrasion-resistant, easy-clean and hydrophobic surface

Abstract

Finishing of decorative and/or overlay papers impregnated with aminoplast resin which are used for coating composite wood panels and form an abrasion-resistant, easy clean and hydrophobic surface, wherein after resin impregnation, the impregnated papers are coated in a second application step with a sol-gel preparation containing dissolved metal alkoxides and fullerene-like nanostructures and nanotubes made of metal disulfides of the metals molybdenum and/or tungsten and, after drying and final condensation, the surfaces are formed in a hydraulic heating press.

Claims

1. A method comprising: providing decorative and/or overlay papers, impregnating the decorative and/or overlay papers with aminoplast resin to form impregnated papers, after the impregnation, coating the impregnated papers with a sol-gel preparation comprising: dissolved metal alkoxides, fullerene-like nanostructures made of metal disulfides of molybdenum and/or tungsten, and fullerene-like nanotubes made of metal disulfides of molybdenum and/or tungsten such that coated papers are formed, drying the coated papers to form dried papers, and pressing the dried papers in a hydraulic heating press such that the dried papers are condensed and such that an abrasion-resistant, hydrophobic surface is formed on the dried papers so that finished papers are formed.

2. The method according to claim 1, wherein the metal alkoxides are selected from the group consisting of aluminum, titanium, silicon, and zirconium.

3. The method according to claim 1, further comprising: dispersing nanoscale metal oxides in the sol-gel preparation.

4. The method according to claim 3, wherein the nanoscale metal oxides are selected from the group consisting of SiO.sub.2, Al.sub.2O.sub.3, TiO.sub.2, and ZrO.sub.2.

5. The method according to claim 1, further comprising: dispersing the fullerene-like nanostructures and nanotubes with cetyltrimethylammonium bromide or with benzyl-C12-14-alkyldimethylammonium chloride.

6. The method according to claim 1, further comprising: preparing the sol-gel preparation by using 3-glycidoxypropyltrimethoxysilane, wherein the sol-gel preparation comprises an ormosil.

7. The method according to claim 1, further comprising: preparing the sol-gel preparation by using Al(OCH(CH.sub.3)C.sub.2H.sub.5).sub.3, wherein the sol-gel preparation comprises a Yoldas sol.

8. The method according to claim 1, further comprising providing a composite wood panel such that the finished papers are a coating on the composite wood panel.

Description

(1) The underlying objective of the invention is to finish decorative papers and overlays impregnated with aminoplast resin in such a way that premature wear of the chromed press plate surfaces is prevented, the aminoplast resin surfaces are resistant to abrasion after final condensation and an easy clean and hydrophobic surface is created in addition.

(2) This objective is achieved as proposed by the invention due to the fact that after being impregnated with an aminoplast resin, the decorative and/or overlay papers are provided with an additional sol-gel coating incorporating nanostructured tungsten disulfide or molybdenum disulfide and the nanostructures of the metal disulfides are provided in the form of fullerene-like nanoparticles and nanotubes.

(3) Additionally incorporating the fullerene-like nanoparticles of, for example, tungsten disulfide in the sol-gel mixture results in surfaces with an extra high separation and sliding effect once the films have hardened. This prevents wear on the press plate surfaces and improves the tribological properties of the composite layers. The surface layers of Al.sub.2O.sub.3, TiO.sub.2 or SiO.sub.2, for example, obtained by the sol-gel process create the hardness of the surfaces of the subsequently coated composite wood panels and additionally create an easy clean and hydrophobic surface.

(4) The sol-gel process is a wet-chemical process for producing ceramic as well as hybrid organic-inorganic materials. Thin layers but also small particles and fibers, aerogels and xerogels and also monolithic materials can be produced using the sol-gel process. In principle, the sol-gel process involves two typical stages through which each product passes. First of all a sol is produced, which consists of finely dispersed colloidal particles of approximately 1 nm to 100 nm in size in a liquid or of dispersed oligomers consisting of branched macromolecules. To create the sol, a so-called precursor is used, namely metal alkoxides or metal salts which are dissolved in water or in some other liquid. When, for example, the hydrolyzable alcoholates of multivalent metal ions (M=Ti, Si, or Al) from an alcohol solution are applied to a surface, a metal hydroxide network forms in the presence of H.sub.2O as soon as the solvent evaporates, even at low temperatures. This then contains numerous MOH groups and is therefore hydrophilic and antistatic.

(5) The hydrolysis and condensation reactions result in a growth of particles and increased polymerization until a solid network is finally formed within the liquid phase, which is then referred to as a gel. Due to evaporation of the solvent, a xerogel is formed from the gel which then turns into a solid and compact form when exposed to higher temperatures. Splitting of the H.sub.2O causes metal oxide groups to form and the surfaces become hard and scratch-resistant.

(6) Thin layers can be deposited on or applied to the impregnated decorative or overlay papers, also referred to as substrate in the description below, by means of various coating processes. For example, dip-coating and spin-coating may be used, whilst coating by means of a blade or application using a distributor roller have also proved suitable for applying a one-sided surface coating. The substrate is usually coated with the liquid sol. After application, the solvent then evaporates so that the concentration of particles becomes significantly higher, which then causes the particles to bond with one another to create a gel and hence a solid but still porous layer. This layer, also known as a xerogel, still contains small amounts of the solvent. It is not until the heating process in the adjoining drying zone of the impregnating channel that the solid phase of the metal oxide starts to form from the sol-gel process, after which the total hard layer of metal oxides forms on final condensation of the impregnating resins in the heating press when subjected to pressure and temperature.

(7) The choice of metal oxides is made depending on the desired surface hardness. Metals which have proved to be suitable metal compounds are aluminum, zirconium, titanium and silicon. Very good sol-gel layers can be produced by means of their oxide compounds. Two sol-gel layers will be described below, namely Al.sub.2O.sub.3 and SiO.sub.2.

(8) The starting material for preparing Al.sub.2O.sub.3 may be a Yoldas sol, for example. The first step of the method is the hydrolysis of aluminum alkoxide, whereby, for example, aluminum-tri-sec-butoxide Al(OCH(CH.sub.3)C.sub.2H.sub.5).sub.3 is hydrolyzed in a large surplus of water at ca. 85 C. This results in an aluminum hydroxide suspension to which a small quantity of nitric acid HNO.sub.3 is then added to obtain a clear sol and/or a colloidal solution.

(9) If it is desirable to operate the Yoldas process with a lower stoichiometric water content, aluminum-tri-sec-butoxide is mixed with absolute ethanol and acetic acid in a ratio of Al-tri-sec-butoxide:C.sub.2H.sub.5OH:H.sub.2O (DI)=1:16:0.6 and heated to ca. 65 C. whilst continually stirring for ca. 50 minutes, for example. The originally cloudy mixture becomes slowly clear after the onset of hydrolysis. The clear sol mixture is cooled to room temperature. In the second step, a gel is produced by adding methanol CH.sub.3 and H.sub.2O, the weight ratios of which may be approximately sol:CH.sub.3:H.sub.2O=1 g:0.2 g:0.003 g, to which a small quantity of acetic acid is then added.

(10) Another sol might be prepared from a commercially available boehmite powder.

(11) To prepare SiO.sub.2 gels, silicic acid esters may be used, for example a tetraethyl orthosilicate TEOS is used as a precursor. Such hybrid organic-inorganic layers of organically modified silicates are often referred to as ormosils. Various materials may be used to prepare ormosil layers, in particular starting with different silanes. The subsequent mechanical behavior of the sol-gel layers will depend on the chemical structure and their concentration in the sol. For example, a silane of formula (OC.sub.2H.sub.5).sub.3Si(CH.sub.2).sub.3CH(O)CH.sub.2 3-glycidoxypropyltrimethoxysilane may be used, also known under the brand names Dynasyl-An or GLYMO. If the intention is to increase the hardness of the layers, nanoparticles of SiO.sub.2 or Al.sub.2O.sub.3 may also be added in addition. OH groups on the surface of the colloidal, amorphous SiO.sub.2 particles can then react with the silane used and thus incorporate the particles in the layer matrix. Another option would be to prepare layers of organically modified SiO.sub.2 with a hydrophilic or hydrophobic or dirt-repellent effect.

(12) After producing the sol, which is provided for the coating, followed by the preparation for the inorganic fullerene-like tungsten disulfide particles, the two preparations are brought together.

(13) Inorganic fullerene-like nanoparticles and nanotubes have particle diameters of 10 to 25 nm. Nanotubes have diameters of 10 to 25 nm and lengths of 200 to 300 nm. The first inorganic fullerene-like particles of tungsten disulfide or molybdenum disulfide were observed in thin layers produced by sulfidizing WO.sub.3 layers and/or MoO.sub.3 layers in a reducing atmosphere. Inorganic fullerenes were produced for the first time in Israel in 1990 and the materials used were tungsten disulfide (WS.sub.2) and molybdenum disulfide (MoS.sub.2). Since then, numerous other inorganic fullerene-like materials have been produced, such as TiS.sub.2, selenides, bromides and chlorides such as NiBr.sub.2, NiCl.sub.2, as well as various oxides such as V.sub.2O.sub.5 and boron nitride. For the application on which the invention is based, tungsten disulfide and molybdenum disulfide were chosen. Tungsten disulfides in the form of fullerene-like nanoparticles and nanotubes are extremely suitable for a number of uses and applications due to their physical properties and crystallographic morphology. Fullerene-like tungsten disulfide has proved to be outstandingly effective in the sol-gel preparation process and prevents subsequent wear on the coated composite wood panels during subsequent use. Wear on the press plates during the pressing operation is likewise prevented. The lubricating effect of tungsten disulfide and molybdenum disulfide in tribological contact is primarily based on the formation of a thin film of WS.sub.2 and/or MoS.sub.2, which forms in the contact zone on the surfaces of rubbing bodies. This so-called tribofilm enables surfaces to slide on one another with little friction and this reduces wear of the rubbing bodies. This effect is therefore totally positive with respect to frictional forces acting on the chromed press plate surfaces.

(14) Most inorganic fullerene-like tungsten disulfide particles are commercially available in the form of dry powder. Due to the production process, however, the particles are quite tightly packed (aggregated) and agglomerated and thus form secondary particles with a diameter of a few micrometers. If the powder is added directly to water and ethanol or to an aqueous sol in this form, the tungsten disulfide precipitates due to the high mass. The tungsten disulfide particles must therefore be de-agglomerated before being used to produce the preparation in the sol-gel process and stabilized in the sol as individual particles. The use of dispersing agents has proved to be of advantage. For example, the WS.sub.2 powder is dispersed with cetyltrimethylammonium bromide sold by Sigma-Aldrich or with Pretoctol (BASF) by means of ultrasound technology.

(15) The proportion of fullerene-like tungsten disulfide WS.sub.2 in the sol will depend on the desired surface finish of the coated composite wood panels and the mold releasing properties of the press plates used and may be 1 to 50%, relative to the proportion of solids.

(16) Having been prepared in this manner, the sol-gel preparations incorporating the dispersed WS.sub.2 particles are then applied to the surfaces of the resin-impregnated decorative or overlay papers, as described above.

(17) The sol-gel coating may be applied in impregnating-drying systems of the type known from the prior art used for impregnation with thermosetting resins. This being the case, the overlay paper is firstly impregnated with the appropriate liquid aqueous aminoplast resin and dried in the heated drying zone to the point where it has a specific moisture content, for example, undergoing a pre-condensation at the same time. In a second application zone, the prepared sol incorporating the fullerene-like WS.sub.2 particles is applied before the transfer to the heated drying channel. The speed and channel temperature will depend on the respective resin parameters, which will be set by the user beforehand.