Long-lasting, thermally stable coating material for non-absorbent substrates

Abstract

Long-lasting, thermally stable coating material for non-absorbent substrates that may be exposed to high heat loads, especially for surfaces of heating elements, containing: a) potassium silicate in the range of 1.16 kg / dm³ to 1.38 kg / dm³, b) aluminum hydroxide in the range of 50 wt.% to 80 wt.% based on the potassium silicate, c) water in the range of 10 wt.% to 50 wt.% based on the potassium silicate, and d) titanium dioxide in the range of 20 wt.% to 50 wt.% based on the potassium silicate.

Description

The invention relates to a long-lasting, thermally stable coating material, which is particularly suitable for non-absorbent substrates that may also be exposed to high heat loads, such as the surfaces of heating elements. The properties of the coating material are achieved by the inventive mixture of potassium silicate, aluminum hydroxide, titanium dioxide, and water. The coating materials are selected according to the properties of the substrates. A distinction can be made between absorbent and non-absorbent substrates. Absorbent substrates include, for example, concrete, tile substrates, cement screed, plaster, and mortar, while non-absorbent substrates include, among others, glass, metal, and plastic. Regardless of their suitability for specific substrates, they must, depending on the application conditions, exhibit, among other things, suitable mechanical stability, thermal stability, chemical resistance, and optical quality. A titanium dioxide-reduced dispersion paint is known from German utility model DE 20 2017 104 645 U1, containing 3-15 wt.% polymer dispersion, 10-50 wt.% filler, 1-5 wt.% water glass, 2-15 wt.% calcium silicate hydroxide mineral, 2-15 wt.% precipitated calcium carbonate, and 0.1-7.0 wt.- % titanium dioxide and / or a pigment substance from the group of oxides ZnO, ZrO2, SnO2, or a mixture thereof, whereby all the aforementioned components are counted as solids, and water added to 100 wt.%. The dispersion paint is intended for use on interior walls and ceilings and is designed to be characterized by high coverage and high wet scrub resistance. Interior walls and ceilings consist of absorbent materials, such as concrete or gypsum, and are not exposed to temperatures exceeding normal room temperature. It is not designed for temperature resistance far above room temperature. German patent application DE 10 2014 013 455 A1 further describes a can-preservative-free, siliconate-based dispersion paint. This dispersion paint contains 2-30% polymer dispersion (calculated as solids), 10-60% pigment and / or filler, 0.5-5% siliconate as an additive, and 100% added water. The pigments in the dispersion paint include, in particular, titanium dioxide, iron oxide, chromium oxide, cobalt blue, phthalocyanine pigments, spinel pigments, and nickel and chromium titanates, or a mixture thereof. The dispersion paint is intended for coating mineral substrates and is said to exhibit improved storage and processing properties. Mineral substrates include, for example, concrete or gypsum and are absorbent materials.Since no information is given here regarding the temperature resistance of the dispersion paint, an expert assumes that the dispersion paint is stable over the long term at ambient temperature, but not at extremely high temperatures. DE 4313820 A1 discloses a thermally curable coating compound, in particular for the water-resistant coating of building and mineral wool insulation boards, based on water glass and mineral aggregates, which, on a dry weight basis, contains 10 to 30% water glass with a molar ratio SiO2 / Me2O ≥ 3.0, where Me represents an alkali metal, 10 to 40% aluminum hydroxide, 10 to 60% inert fillers, 3 to 15% additives, and water. German patent DE 19818545 A1 discloses a coating compound, adhesive / sealant, and bonding agent / bridge for use not only in preventive fire protection. This compound, acting as a bonding agent / bridge (primer), enables the bonding of water-repellent materials or the fastening of water-based adhesives. The products in question are characterized, among other things, by their temperature and water resistance. German patent DE 19838946 A1 discloses a water-based adhesive and / or coating compound and its use. This relates to adhesives and / or coating compounds based on aqueous alkali silicate solutions and calcium carbonate. The products are characterized by a low alkali silicate content and a high calcium carbonate content. Despite their high solids content, they exhibit low viscosity and can be easily applied by spraying. European patent EP 1 297 079 B1 also describes a dispersion paint containing potassium silicate, intended for use on mineral substrates. It is said to differ from existing dispersion paints, which are often unsuitable for allergy sufferers due to their preservative content, and to be preservative-free. The dispersion paint contains, in particular, titanium dioxide, iron oxide, chromium oxide, cobalt blue, phthalocyanine pigments, spinel pigment, and nickel and chromium titanate as pigments. The patent discloses dispersion paints containing 8-12 wt% polymer dispersion, 10-35 wt% pigments, 10-35 wt% fillers, max. 1.5 wt% potassium silicate, 0.05-0.15 wt% alkali hydroxide, and water supplemented to 100 wt%.No information is provided regarding the temperature resistance or suitability of the dispersion paint disclosed here for surfaces exposed to high heat input, which is why long-term stability is only assumed at room or ambient temperature. All coatings based on water glass known from the prior art are intended for absorbent substrates and for use under room or ambient temperature conditions and are therefore not suitable for use on non-absorbent substrates and under high heat loads. The object of the invention is to develop a long-lasting, thermally stable coating material for non-absorbent substrates. This problem is solved with a coating material according to the main claim. Advantageous embodiments are specified in the dependent claims. The invention is described below using exemplary embodiments. For each embodiment, the coating material intended for non-absorbent substrates consists of potassium silicate in the range of 1.24 kg / dm3 to 1.26 kg / dm3, which corresponds to 28° Bé to 30° Bé, aluminium hydroxide in the range of 58 wt.% to 66 wt.% based on the potassium silicate and water in the range of 18 wt.% to 25 wt.% based on the potassium silicate. Non-absorbent substrates include, for example, plastic or metal surfaces. Compared to absorbent substrates, non-absorbent substrates pose a challenge for the application of thermally stable coating materials because their adhesion to these surfaces is very low. The demands placed on a coating material become even higher when the substrates, and therefore the coating material itself, are exposed to high heat input, e.g., on heating elements. Depending on the coating system, discoloration and mechanical deformations, such as stress cracks, can occur. In a first group of embodiments, potassium silicate is used in a density range of 1.24 kg / dm³ to 1.26 kg / dm³, corresponding to 28° Bé to 30° Bé. Potassium silicate, together with sodium silicate and lithium silicate, forms the group of water glasses; chemically speaking, these are water-soluble potassium silicates, sodium silicates, and lithium silicates. Water glasses are used, for example, as adhesives for tiles on walls and tiled stoves, as a consolidant and sealant for plaster and masonry in construction, and for hardening sand molds and cores in foundries. In paints and varnishes, they primarily serve as inorganic binders. For the subject matter of the present invention, potassium silicate is used as a film-forming agent. The evaporation of the solvent water after application leads to silicification, which is irreversible. This transforms an aqueous dispersion into a solid coating.The potassium silicate it contains gives the coating solvent resistance. In a second group of embodiments, aluminum hydroxide is present in the coating material in a range of 62 wt.% to 66 wt.% based on potassium silicate. According to the invention, aluminum hydroxide serves to increase mechanical strength and abrasion resistance, as well as to reduce cracking. Aluminum hydroxide occurs in the minerals gibbsite (hydrargillite), bayerite, nordstrandite, diaspore, and boehmite. It is the world's most important mineral flame retardant. For example, DE 10 248 174 C1 claims a flame-retardant polymer composition and its use, as well as a process for producing a flame retardant, wherein the flame retardant contains 40-80 wt.% of aluminum hydroxide. DE 19 812 279 C1 comprises a flame-retardant polymer mixture and a process for producing a filler, wherein the flame-retardant polymer mixture contains 55-75% boehmite. In a third group of embodiments, water is present in a range of 20 wt.% to 25 wt.% based on potassium silicate. According to the invention, it is used to stabilize the coating and to reduce blistering at high temperatures on plastic surfaces. Further groups of embodiments result from the combination of the three groups of embodiments mentioned above. To give the coating material a white hue, further groups of embodiments contain titanium dioxide in a range of 30% to 50% based on water glass. Titanium dioxide (TiO2), also known as titanium(IV) oxide or titanium white, belongs to the group of so-called white pigments and, within that group, to the subgroup of oxides. White pigments exhibit a high refractive index (greater than 1.8), very low light absorption, and strong non-selective light scattering. The higher the proportion of white pigments, the higher the degree of whiteness and luminosity, and consequently, the opacity. The white pigments include the subgroup of oxides such as zinc oxide ZnO (zinc white), zirconium(IV) oxide ZrO2 and tin(IV) oxide SnO2, the subgroup of carbonates such as lead white 2PbCO3·Pb(OH)2, the subgroup of sulfates such as barium sulfate BaSO4 and the subgroup of sulfides such as zinc sulfide ZnS.Titanium dioxide is considered the most important white pigment with the highest refractive index; it has the highest opacity of all white pigments and is therefore particularly suitable for white coatings. Advantageously, the titanium dioxide is one or more modifications known as rutile, anatase, brookite, akaogiite, and riesite. The latter groups of embodiments are advantageously modified to give the coating material a different color by adding alkali-resistant inorganic pigments in the coating material in amounts ranging from greater than 0 wt.% to 100 wt.% based on the amount of titanium dioxide, depending on the desired color. Pigments can be classified according to their chemical structure (inorganic or organic), their technical properties (corrosion protection, magnetism), and their optical properties (color and, if applicable, optical interference effects) (Pigments, Fillers, Dyes: DIN Standards, 7th ed., German Institute for Standardization (ed.), Berlin; Vienna; Zurich: Beuth, 2012). A further classification distinguishes between natural and synthetically produced pigments.Natural pigments are earths and minerals, which are mostly processed mechanically by drying and grinding. Synthetically produced pigments include, for example, metallic effect pigments, carbon black, white pigments, and iron oxide pigments. Industrial production allows for better stability and higher purity of synthetic inorganic pigments (G. Pfaff: Industrial Inorganic Pigments. Wiley-VCH). A specific example of implementation is presented below in tabular form as a recipe. Potassium water glass 1.24 kg / dm 3 to 1.26 kg / dm 3 (28 - 30 Bé) 100 parts Aluminium hydroxide 58 parts Titanium dioxide 24 pieces Water 18 pieces A coating material according to the invention is characterized in particular by high and lasting color stability at high temperatures. A further essential property of the coating is its resistance to solvents.

Claims

Long-lasting, thermally stable coating material for non-absorbent substrates that may be exposed to high heat loads, especially for surfaces of heating elements, containing: a) potassium silicate in the range of 1.16 kg / dm³ to 1.38 kg / dm³, b) aluminum hydroxide in the range of 50 wt.% to 80 wt.% based on the potassium silicate, c) water in the range of 10 wt.% to 50 wt.% based on the potassium silicate, and d) titanium dioxide in the range of 20 wt.% to 50 wt.% based on the potassium silicate. Long-lasting thermally stable coating material according to claim 1, characterized in that it is a coating material for heating elements for converting electrical energy into heat energy. Long-lasting thermally stable coating material according to claim 1 or 2, characterized in that the potassium silicate is present in the range of 1.24 kg / dm3 to 1.26 kg / dm3. Long-lasting thermally stable coating material according to one of the preceding claims, characterized in that the aluminium hydroxide is present in the range of 58 wt.% to 66 wt.% based on the potassium silicate. Long-lasting thermally stable coating material according to one of the preceding claims, characterized in that the water is present in a range of 18 wt.% to 25 wt.% based on the potassium silicate. Long-lasting thermally stable coating material according to one of the preceding claims, characterized in that the titanium dioxide is present in the range of 30 wt.% to 35 wt.% based on the potassium silicate. Long-term resistant thermally stable coating material according to one of the preceding claims, characterized in that the titanium dioxide is one or more titanium dioxide modifications, namely rutile, anatase, brookite, akaogiite and / or riesite. Long-lasting thermally stable coating material according to one of the preceding claims, characterized in that the coating material additionally contains one or more alkali-resistant inorganic pigments in the range of greater than 0 wt.% to 100 wt.% based on the amount of titanium dioxide.