Brief description of the drawings
FIG. 1 is a graphical representation of green density of examples of the tested surface coating formulations compared to commercial ceramic paste surface coating products.
FIG. 2 is a graphical representation of the viscosity of the tested surface coating formulation examples compared to a commercial ceramic paste surface coating product.
FIG. 3 is a graphical representation of the modulus of rupture under various conditions for the examples of tested surface coating formulations compared to commercial ceramic paste surface coating products.
Detailed Description
We have now shown that the addition of secondary fibres during the drying and sintering stages of the manufacture of ceramic honeycomb structures minimises cracking of the surface coating. These secondary fibres are not necessarily high temperature resistant fibres, and fibres which do not have the function of resisting particularly high temperatures also very well prevent cracking of the surface coating during the drying and sintering stages. The term "honeycomb structure" includes any honeycomb structure used in exhaust gas treatment devices, such as catalytic converters, diesel particulate filters, selective catalytic reduction devices, NOxPorous ceramic structures in traps and similar devices.
Further, we have shown that during the drying stage, a non-adsorbed, hard, dense, eggshell-like surface is formed on the surface of the surface coating as described herein. While not being limited in theory, it is believed that the surface is formed by migration of silica species during the drying stage. The surface prevents the acidic catalyst coating from being adsorbed into the surface coating. It is preferable to prevent absorption because, as described above, the surface coating can be degraded by exposure to an acidic catalyst coating. Preventing absorption also allows the use of a smaller amount of catalyst coating, reducing the cost of the overall product.
Provided herein are ceramic honeycomb structure surface coatings, and methods of producing ceramic honeycomb structure surface coatings, which provide a hardshelled, acid and base resistant, cut resistant, high strength ceramic honeycomb structure surface coating, and which can block contamination control catalysts from being absorbed into the surface coating.
In one embodiment, a ceramic facecoat material for a porous ceramic (e.g., honeycomb) substrate comprises refractory ceramic fibers or biosoluble inorganic fibers; viscosity modifiers, colloidal inorganic oxides; optionally, an inorganic binder; optionally, inorganic particulates; and optionally, secondary inorganic fibers.
The refractory ceramic fibers or biosoluble inorganic fibers may include at least one of aluminosilicate fibers, alkaline earth silicate fibers, or calcium aluminate fibers. The Refractory Ceramic Fibers (RCF) may include, but are not limited to, aluminosilicate fibers. The alkaline earth metal silicate fibers may include, but are not limited to, magnesium silicate fibers or calcium magnesium silicate fibers.
These primary fibers (RCF or biosoluble inorganic fibers) can be applied at varying degrees of particulate content, from "as is" (as produced) to high coefficient and air classified fibers, in which substantially all of the particulates are removed. In certain embodiments, the primary fibers may be ball milled.
The viscosity modifier may include, but is not limited to: alkylcellulose polymers, such as Methylcellulose (MC) and/or derivatives thereof, for example Hydroxypropylmethylcellulose (HPMC), Hydroxyethylmethylcellulose (HEMC), Hydroxyethylcellulose (HEC), carboxymethylcellulose (CMC), hydroxyethylcarboxymethylcellulose (hecc), or carboxymethylhydroxyethylcellulose (CMHEC), or mixtures thereof. In certain embodiments, the viscosity modifier has a viscosity in the range of about 20cps to about 2000 cps.
Other non-limiting examples of viscosity modifiers include: polyalkylene oxides, certain polysaccharides, polyacrylic acids, polyacrylamides, and mixtures thereof. The polyalkylene oxide may include, but is not limited to: polyethylene oxide having a molecular weight in the range of about 1 million to 4 million g/mol. Illustrative examples of suitable polysaccharides include welan gum, diutan gum, xanthan gum, and mixtures of the foregoing. The polyacrylic acid may have a molecular weight of about 500,000g/mol or greater.
The colloidal inorganic oxide may be colloidal silica, colloidal alumina, colloidal zirconia, or mixtures of the foregoing. Colloidal silica, such as those from Nalco Chemical Company, is a stable dispersion of nanosized silica particles in water or other liquid medium. The colloidal silica particles may be sized to have diameters in the range of about 4 to about 100 nanometers. The colloidal silica may be stabilized, for example with sodium or ammonia ions, and may have a pH in the range of about 2 to about 12.
The inorganic particulate may include, but is not limited to, at least one of alumina, cordierite (e.g., cordierite clinker), mullite, titania, aluminum titanate, or silicon carbide. The inorganic particles optionally include at least one component having a coefficient of thermal expansion that is compatible with the coefficient of thermal expansion of the ceramic honeycomb substrate to which the facecoat is applied. The inorganic particulates may have a particle size of about 300 microns or less, and in certain embodiments less than about 100 microns.
The inorganic binder may include clay. The clay may be calcined or uncalcined and may include, but is not limited to, attapulgite, ball clay, bentonite, hectorite, kaolin, kyanite, montmorillonite, palygorskite, saponite, sepiolite, silimanite, or combinations thereof. The inorganic binder particle size may be about 150 microns or less, and in certain embodiments less than about 45 microns.
The secondary inorganic fibers may include, but are not limited to, glass fibers, leached silica, high alumina fibers, mullite fibers, magnesium aluminosilicate fibers, S-2 glass fibers, E-glass fibers, or fine (sub-micron) diameter aluminosilicate fibers (HSA), and mixtures thereof.
In addition to the secondary inorganic fibers, organic binder fibers may optionally be included in the topcoat formulation. Suitable examples of binder fibers include polyvinyl alcohol fibers, polyolefin fibers such as polyethylene and polypropylene, acrylic fibers, polyester fibers, vinyl acetate fibers, nylon fibers, and combinations of the foregoing. Suitable amounts of these fibers range from 0 to about 10 weight percent, based on 100% by weight of the total composition.
Other organic binders or resins may optionally be included in the topcoat formulation. Examples of suitable organic binders or resins include, but are not limited to, water-based acrylic emulsions, styrene-butadiene, vinyl pyridine, acrylonitrile, vinyl chloride, polyurethane, and the like. Silicone emulsions are also suitable. Other resins include low temperature, flexible thermosetting resins such as unsaturated polyesters, epoxy resins, and polyvinyl esters (e.g., polyvinyl acetate or polyvinyl butyral latex). Up to 10% by weight of an organic binder or resin may be used. The solvent for the binder may, if desired, comprise water or a suitable organic solvent for the binder used, for example acetone. The solution concentration of the binder in the solvent (if used) can be determined by conventional methods based on the desired binder loading and the processability of the binder system (viscosity, solids content, etc.).
The refractory ceramic fibers typically comprise substantially alumina and silica, and typically comprise about 45 to about 60 weight percent alumina and about 40 to about 55 weight percent silica. RCF fibers are typically less than 5mm in length, and their average fiber diameter may range from about 0.5 μm to about 10.5 μm. FiberFraX is available from Unifrax I LLC, Niagara Falls, New YorkRefractory aluminosilicate ceramic fibers (RCF).
The term "biosoluble inorganic fibers" refers to fibers that are substantially decomposable in a physiological medium or simulated physiological medium such as simulated lung fluid, physiological saline, buffered saline solution, or the like. The solubility of the fiber can be determined by measuring the solubility of the fiber in a simulated physiological medium as a function of time. Biosoluble properties can also be evaluated by observing the effect of direct implantation of fibers in experimental animals or by examination of animals or humans that have been exposed to fibers, i.e. biological resistance. U.S. patent No. 5,874,375 to Unifrax I LLC discloses a method of measuring the biosoluble properties of fibers in physiological media.
Another method of assessing the biosoluble properties of fibers is based on the composition of the fiber. For example, germany grades respirable inorganic oxide fibers based on compositional coefficients (KI values). The KI value can be calculated by adding the weight percent of alkali and alkaline earth metal oxides in the inorganic oxide fibers and subtracting twice the weight percent of alumina in the inorganic oxide fibers. Biosoluble inorganic fibers typically have a KI value of about 40 or greater.
Without limitation, suitable examples of biosoluble inorganic fibers that can be used to prepare the surface coating materials described herein include those described in U.S. patent nos. 6,953,757; 6,030,910, respectively; 6,025,288, respectively; 5,874,375, respectively; 5,585,312, respectively; 5,332,699, respectively; 5,714,421, respectively; 7,259,118, respectively; 7,153,796, respectively; 6,861,381, respectively; 5,955,389, respectively; 5,928,975, respectively; 5,821,183 and 5,811,360, each of which is incorporated herein by reference.
The biosoluble alkaline earth silicate fibers may comprise the fiberization product of a mixture of magnesium oxide and silica, commonly referred to as magnesium silicate fibers. The magnesium silicate fiber typically comprises the fiberization product of about 60 to about 90 weight percent silica, from greater than 0 to about 35 weight percent magnesia, and 5 weight percent or less impurities. According to certain embodiments, the alkaline earth silicate fibers comprise the fiberization product of about 65 to about 86 weight percent silica, about 14 to about 35 weight percent magnesia, 0 to about 7 weight percent zirconia, and 5 weight percent or less impurities. According to other embodiments, the alkaline earth silicate fibers comprise the fiberization product of about 70 to about 86 weight percent silica, about 14 to about 30 weight percent magnesia, and 5 weight percent or less impurities.
Illustrative examples of such biosoluble inorganic fibers include, but are not limited to, ISOFRAXAlkaline earth silicate fibers having an average diameter of between about 0.6 microns and about 2.6 microns, available from UnifraxI LLC, Niagara Falls, New York. Is commercially availableISOFRAX ofThe fibers generally comprise the fiberization product of about 70 to about 80 weight percent silica, about 18 to about 27 weight percent magnesia, and 4 weight percent or less impurities.
Alternatively or additionally, the biosoluble alkaline earth silicate fibers may comprise the fiberization product of a mixture of oxides of calcium, magnesium and silicon. These fibers are commonly referred to as calcia-magnesia-silicate fibers. The calcia-magnesia-silicate fiber typically comprises the fiberization product of about 45 to about 90 weight percent silica, from greater than 0 to about 45 weight percent calcia, from greater than 0 to about 35 weight percent magnesia, and 10 weight percent or less impurities.
Commercially available calcia-magnesia-silicate fibres are available under the registered trademark inulfax from Unifrax I LLC (Niagara Falls, New York). INSULFRAXThe fibers generally comprise the fiberization product of about 61 to about 67 weight percent silica, from about 27 to about 33 weight percent calcia, and from about 2 to about 7 weight percent magnesia. Other commercial calcia-magnesia-silicate fibers include from about 60 to about 70 weight percent silica, from about 25 to about 35 weight percent calcia, from about 4 to about 7 weight percent magnesia, and optionally trace amounts of alumina; or, from about 60 to about 70 weight percent silica, from about 16 to about 22 weight percent calcia, from about 12 to about 19 weight percent magnesia, and optionally, trace amounts of alumina.
Biosoluble calcium aluminate fibers are disclosed in U.S. patent No. 5,346,868, U.S. patent publication No. 2007-0020454 a1, and international patent publication No. WO/2007/005836, which are incorporated herein by reference.
With respect to the secondary fibers, other alumina/silica ceramic fibers,such as high alumina or mullite ceramic fibers, may be prepared by sol gel methods and typically contain greater than 50 percent alumina. One example is FIBERMAXFibers, available from Unifrax I LLC of Niagara Falls, New York. Commercially available magnesia/alumina/silicate fibers such as S2-GLASS are available from Owens Corning, Toledo, Ohio. The S2-GLASS fibers typically comprise from about 64 to about 66 percent silica, from about 24 to about 25 percent alumina, and from about 9 to about 10 percent magnesia.
Leached silica may be leached in any manner and may utilize any technique known in the art. Generally, leaching can be accomplished by subjecting the glass fibers to an acidic solution or other solution suitable for leaching the non-siliceous oxides and other components in the fibers. Detailed descriptions and methods of making leached glass fibers rich in silicon content are disclosed in U.S. Pat. No. 2,624,658, the entire disclosure of which is incorporated herein by reference. Another method for making leached glass fibers rich in silicon content is disclosed in european patent application publication No. 0973697.
Leached glass fibers are available from BelChem Fiber Materials GmbH, Germany under the trademark BELCOTEX, HitcoCarbon Composites, Inc. of Gardena California under the registered trademark REFASIL, and Polotsk-Steklooklokno, Reublic of Belarus under the trademark PS-23 (R).
In another embodiment, a method of making a surface coating for a porous ceramic (honeycomb) structure is provided, the method comprising forming a mixture of: ceramic fibers or biosoluble inorganic fibers; a viscosity modifier; a colloidal inorganic oxide; optionally, an inorganic binder; optionally, inorganic particulates; and optionally secondary inorganic fibers.
In one embodiment, the dry components are mixed in one part and the wet components (colloidal inorganic oxide and water) are mixed separately in a second part, after which the two parts are mixed together. In another embodiment, the dry components can be added to the wet components in any order and mixed. The topcoat material may be dried, for example, at about 50 ° to about 100 ℃ for about two hours, or until completely dried. The dried facecoat material can be sintered at about 500-.
In the production of exhaust gas treatment devices, after sintering the surface-coated ceramic honeycomb structure, the honeycomb body may be soaked in an acidic or basic solution or dispersion containing the catalyst and then dried and re-sintered.
In certain embodiments, a facecoat material for a porous ceramic (honeycomb) substrate is provided, comprising refractory ceramic fibers or biosoluble inorganic fibers; a viscosity modifier; a colloidal inorganic oxide; an inorganic binder; inorganic fine particles; and, a secondary inorganic fiber.
Examples
Various examples of tested surface coating formulations (examples A, B and C) are set forth in Table 1 below. These coatings were tested and compared to commercial ceramic paste products used as DPF washcoat formulations.
TABLE 1
FIG. 1 shows the green density test results of tested facecoat formulations examples A, B and C compared to a commercial ceramic paste facecoat product. Each surface coating material was prepared as a flat plate to a thickness of several millimeters. The volume and weight of the plates were measured and their density was calculated. Each tested topcoat formulation exhibited a higher green density than the commercial material control sample. The higher density provides strength and improved resistance to the adsorbed catalyst coating material.
FIG. 2 shows the results of viscosity testing of tested topcoat formulations example A, B and C compared to a commercial ceramic paste topcoat product. The viscosity was measured using a standard Brookfield viscometer using a spindle speed of 1rpm No. 7. As shown, the viscosity test for this material may have a variance of about +/-15%. However, each of the exemplary tested topcoat formulations exhibited a lower viscosity than the commercial material control samples. The lower relative viscosity allows for easier pumping of the surface coating formulation and application to a substrate.
FIG. 3 shows the results of a test of modulus of rupture (MOR) of tested topcoat formulations examples A, B and C after treatment under various conditions, as compared to a commercial ceramic paste topcoat product.
The samples of example A, B and C were heat treated to simulate the top coating application conditions and to simulate the treatment conditions (acid-base treatment and heat treatment) during the catalytic coating step. The 4 point MOR test was performed according to ASTM C880. In particular, as shown in fig. 3, the first column of each sample individually shows the results of the MOR test tested when each sample was compacted, the second column of each sample shows the results of the MOR test tested after each sample was heat treated, and the third column of each sample shows the results of the MOR test after each sample was heat treated, acid/base (alkaline) washed, and second sintered.
When tested after compaction, heat treatment, and after acid-base (alkaline) treatment and a second heat treatment, each tested surface coating formulation exhibited a higher modulus of rupture than the commercial material control sample.
Even after heat treatment, the overall MOR strength was higher for examples A, B and C relative to the comparative product. The formulations of examples B and C did not show a significant decrease in MOR strength after heat treatment. Even when there was a decrease after heat treatment, the percent decrease in MOR for examples A, B and C was quite low relative to the comparative product.
The overall MOR strength was higher for examples A, B and C relative to the comparative product after the heat treatment followed by the acid and base soak treatments. The percent reduction in MOR for examples A, B and C was quite low relative to the comparative product after the heat treatment followed by the acid and base soak treatments.
For examples B and C, the thermal expansion coefficients between 20 and 900 ℃ were tested and were 36X 10, respectively-7And 40X 10-7Consistent with commercial ceramic honeycomb substrates.
The surface coating formula comprises the following components in parts by weight: refractory ceramic fibers or biosoluble inorganic fibers, from about 15 to 50%; a viscosity modifier, from about 0.15 to about 0.5%; colloidal inorganic oxide from about 2 to about 20%; inorganic particulates from 0 to about 40%, inorganic binders (clays) from 0 to about 10%; secondary inorganic fibers from 0 to about 10% and, water from about 25 to about 50%. In certain embodiments, the component may have a weight content of: refractory ceramic fibers or biosoluble inorganic fibers, from about 20 to 40%; a viscosity modifier, from about 0.25 to about 0.4%; colloidal inorganic oxide, from about 5 to about 10.5%; inorganic particulates from about 25 to about 37%, inorganic binders (clays) from about 1.5 to about 5%; secondary inorganic fibers from about 1.15 to about 5% and, water from about 29 to about 47%.
Other surface coating material formulations were successfully prepared and are reported in tables 2-5 below.
TABLE 2
Tested topcoat formulations examples D and E, as described above, were tested for 4 point MOR according to ASTM C880. The MOR of the compact of example D was 603psi and the MOR after acid/heat treatment was 606.5. The MOR of the compact of example E was 1147.9psi and the MOR after acid/heat treatment was 479.8.
TABLE 3
It is understood that the embodiments described herein are merely illustrative and that variations and modifications may be effected by one skilled in the art without departing from the spirit and scope of the invention. All such variations and modifications are intended to be within the scope of the present invention as described above. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to achieve the desired results.