DE102012014225B4 - Adhesive for highly porous ceramics - Google Patents

Abstract

Adhesive for highly porous ceramics, suitable for the production of diesel particle filters from SiC segments, to glue together a honeycomb body made of at least 2 segments, to encase it and to close the channels in opposite directions on at least one side, • with a thermal conductivity of the adhesive between 40 and 80 W. / m × K in the temperature range used, • a specific heat capacity of the adhesive of 0.6 kJ / kg × K at 20 ° C and over 1.0 kJ × K at temperatures of 1000 ° C, whereby the heat capacity of the adhesive per unit volume 40 to 60% greater than the heat capacity of the ceramic and thus represents a heat sink during regeneration, • with a thermal expansion coefficient of the adhesive in the area of application that does not deviate by more than 5% from that of the honeycomb ceramic, the adhesive a) being fiber-free b) as a temperature-resistant binder, a phosphate binder selected from monoaluminium phosphate, phosphoric acid, monomagnesium phosph at, alkali metal phosphate, boron monophosphate or sodium phosphate, c) has 0.1 to 20% by weight of mullite with a maximum grain size of 200 μ as a reaction partner for the phosphate binder, d) has a clay as a further reaction partner for the phosphate binder, selected from Bentonite, kaolinite, Fireclay or a mixture thereof, e) contains 0.01 to 5% by weight of hydroxyethyl cellulose (HEC) for water retention, f) 5 to 50% by weight of coarse-grained SiC powder with a maximum grain size of 1 mm has g) 5 to 60 wt .-% fine-grained SiC powder with a maximum grain size of 100 microns.

Description

The invention relates to a highly porous adhesive, which is suitable in the production of diesel particulate filters made of SiC segments, honeycomb body of at least two segments to glue together, to encase and to close the channels in opposite directions at least on one side.

For the filtration of liquid or gaseous substances more and more ceramic parts are used. Since these parts can not be made crack-free in such large dimensions, because shrinkage occurs during sintering, the parts are made of individual segments and glued together. The ceramics can come from the field of oxidic substances or from the area of non-oxidic substances, such as SiC or Si ₃ N ₄ or mixtures thereof. The segments have a certain porosity, so that the finest molecules can go through the wall such. As air or exhaust gas. This filtration may be diesel soot filters composed of individual honeycomb segments and bonded together. The invention is an adhesive which can also be used to seal individual channels and even provide advantages for wrapping the parts.

So far, many patents coming from Japan are described so that the adhesive for DPF consists of SiC powder as a filler, a silica sol as an inorganic binder, inorganic fibers and an organic binder. The adhesive is to connect the segments together and thermally isolate.

In EP 0816065 A1 is described a material for bonding and clogging the channels of diesel particulate filter, which should be elastic. This is very difficult for ceramic materials. The binder used is a colloidal liquid (colloidal sol). However, this only provides insufficient inherent strength and low adhesion to the ceramic. Because the binder consists of nanoparticles, only after the sintering a strength is achieved, which is necessary for such adhesives. Nevertheless, it is at the lower limit. In order to obtain a required minimum tensile strength, fibers or whiskers must still be added to the compound. When processing for the environment not dangerous. Even in later use, fibers could also be released from the mass. SiC is used as the filler, which seems to be suitable and necessary when using diesel particulate filters made of SiC components. The use of cellulose as an organic adjuvant is processing technology help, because a so-called green strength is achieved. The fiber length is given as 1 to 100 mm, which is difficult to control in practice.

In EP 1 142 619 B1 becomes one of the EP 0 816 065 A1 corresponding mass described with SiC powder, SiO ₂ -based colloidal sol, organic binder and inorganic fibers. The inorganic fibers are derived from the system SiO ₂ -Al ₂ O ₃ . These fibers have far lower thermal conductivity than z. B. SiC particles and thus act as a thermal insulator. For a sealing and sealing layer that may not be wrong, but for a glue this is the wrong choice. Thus, the WLF for the adhesive is given with values of 0.1 to 10 W / m × K, whereas the WLF of the SiC ceramic of the sealing layer is given as 20 to 80 W / m × K.

In EP 1 270 202 B1 the filler of the adhesive is considered and the α-type SiC is highlighted as appropriate. The use of α-SiC is necessary because otherwise at the application temperatures up to 1200 ° C conversions would occur.

In EP 1 306 358 B1 will again be the glue type EP 0 816 065 A1 described with SiC as a filler, an inorganic binder based on a colloidal sol based on SiO ₂ , an organic binder based on polysaccharide and / or cellulose and an inorganic fiber. In this fiber, it is emphasized that the shot content should be only 1 to 10%. The fibers come from the range SiO ₂ and Al ₂ O ₃ . The indication of the amount of molten glass indicates that they are vitreous / amorphous fibers which will undergo transformation at the operating temperature and thus lose their elasticity. They crystallize. The silica sol brings only after sintering a ceramic bond and thus a necessary strength.

In EP 1 382 445 A2 again the same type of glue is described. Again whiskers with a size of 0.1 to 10 or 15 microns and a proportion of 20 to 40% are addressed, alternatively inorganic powder. Whiskers are suspected of causing cancer. For the SiC-based fillers it is stated that if they are greater than 100 μm, the bond strength would be reduced. With gaps of 1 to 3 mm between the individual SiC elements, coarser powder must be present to ensure structural strength.

• • Geringe spezifische Wärmekapazität mit Werten von nur 20 bis 90% der Wärmekapazität der Keramik
• • Eine gute Wärmeisolation, hervorgerufen durch Zugabe von Porenbildner zur Masse, um im gesinterten Zustand viele Poren zu erzeugen. Dieser Gedankengang ist falsch, weil bei dem Abbrand des Rußes punktuell hohe Temperaturen auftreten können. Eine hohe Wärmeleitfähigkeit verteilt die Energie schnell und lässt erst gar nicht hohe Temperaturspitzen zu. Wenn zudem noch eine hohe Wärmekapazität gegeben ist, unterstützt dies den Weg der „Wärmesenke bzw. Wärmeabfuhr”.
• • Zudem soll die Wärmedehnung des Klebers unterschiedlich sein zu der Wärmedehnung der Keramik. Wären sie identisch oder fast identisch, nähme die Stabilität der Verklebungsschicht durch Thermoschockreaktionen stark ab. Der Einwand dagegen lautet, dass bei Verbindungen mit unterschiedlicher Wärmedehnung an der Übergangsstelle Spannungen auftreten, die zur Rissbildung führen können. Dies wird noch verstärkt, wenn beim Abbinden des Klebers Schwindungen auftreten sollten. Das Gegenteil ist der Fall: Die Wärmedehnung sollte so ähnlich wie möglich sein, um mechanische Spannungen zu minimieren.

In EP 1 479 881 A1 Again, the adhesive of SiC powder, colloidal sol, inorganic fibers and organic binder is discussed. The adhesive should have the following properties:

• • Low specific heat capacity with values of only 20 to 90% of the heat capacity of the ceramic
• • A good thermal insulation, caused by the addition of pore-forming agent to the mass to produce many pores in the sintered state. This train of thought is wrong, because at the burnup of the soot punctually high temperatures can occur. A high thermal conductivity distributes the energy quickly and does not even allow high temperature peaks. If, in addition, a high heat capacity is given, this supports the path of the "heat sink or heat removal".
• • In addition, the thermal expansion of the adhesive should be different to the thermal expansion of the ceramic. If they were identical or almost identical, the stability of the bond layer would be severely reduced by thermal shock reactions. The objection, on the other hand, is that, in the case of compounds with different thermal expansion at the transition point, stresses occur which can lead to the formation of cracks. This is exacerbated when shrinkage should occur when the adhesive sets. The opposite is the case: the thermal expansion should be as similar as possible to minimize mechanical stress.

In EP 1 719 881 A2 is again addressed to a necessary porosity of the adhesive in order to keep the heat capacity low and the thermal conductivity low. This approach is wrong in our eyes.

Out DE 41 09 916 A1 For example, a composition containing a refractory material in the form of SiC and phosphate binder emerges. As further mixing components Bentonite and Töpfertone and methylcellulose are indicated. As an addition, a conductive metal such as copper or copper alloy should be added. The addition is intended to increase the thermal conductivity of the mortar to fill the gaps between tube blocks / shields and steam and water pipes in waste incinerators or commercial energy recovery boilers.

In all other mentioned patent applications reference is made to inorganic fibers or whiskers. Since the asbestos fibers have already made very bad experiences, one would like to get away from such materials in the future. They pollute the environment during processing and endanger the employees who have to handle the fibers. But they are a necessity when using silica sols as an inorganic binder to obtain sufficient strength. Since gas flows occur in the filters, there must be some tensile strength and this is achieved by the fibers. The colloidal silica sol consists of a liquid and finest amorphous particles. These particles have a very large specific surface area and can sinter early. The disadvantage is that shrinkage already occurs due to the large amount of water during drying. The next point is that the strength is first generated by sintering these finest SiO ₂ particles. This goes along with the sintering shrinkage. The prefabricated segments are held together by a shrinking adhesive, which leads to tensions even in thermal equilibrium.

• • Die Masse soll keine Fasern enthalten
• • Sie soll temperaturbeständig bis 1200°C sein.
• • Sie soll ähnliche Wärmedehnung wie die SiC-Keramik haben
• • Sie soll eine hohe Wärmeleitfähigkeit haben.
• • Sie soll eine große spezifische Wärmekapazität aufweisen.
• • Sie soll eine feste Anhaftung an die SiC-Keramik gewährleisten.
• • Sie soll eine gute Temperaturwechselbeständigkeit haben.

The object of the invention is to provide a material composition for an adhesive which has the following properties:

• • The mass should not contain fibers
• • It should be temperature resistant up to 1200 ° C.
• • It should have similar thermal expansion as the SiC ceramic
• • It should have a high thermal conductivity.
• • It should have a high specific heat capacity.
• • It should ensure a firm adhesion to the SiC ceramic.
• • It should have a good thermal shock resistance.

• • Kolloidale Sole haben, wie bereits besprochen, eine zu geringe Festigkeit.
• • Hydraulische Bindemittel, z. B. auf Zementbasis, verlieren zu früh die Festigkeit, bevor die keramische Bindung einsetzt. Zudem müsste zu viel artfremdes Material auf Basis Bindeton verwendet werden, was eine Verschlechterung bzgl. Wärmedehnung und die Gefahr von Kristallumwandlungen ergäbe.
• • Wasserglas bringt die Gefahr, dass die vorhandenen Alkalien eine frühzeitige Bildung einer Schmelze begünstigen könnten.
• • Phosphatbinder sind geeignet.

For temperature-resistant adhesives up to 1200 ° C, the following products are available as stable binders:

• • Colloidal sols, as already discussed, have too low a strength.
• • Hydraulic binders, eg. As cement-based, lose too early the strength before the ceramic bond begins. In addition, too much alien material would have to be used based on binding clay, which would result in a deterioration in terms of thermal expansion and the risk of crystal transformations.
• • Water glass puts the danger that the existing alkalis could favor an early formation of a melt.
• • Phosphate binders are suitable.

With a phosphate binder there is a suitable binder for this temperature application. If a suitable reactant is added, excellent strength can be achieved even at very low temperature. This reaction partner can come from the field of oxide ceramics as MgO, Al ₂ O ₃ , mullite, spinel or other oxides. As the filler, SiC, Si ₃ N ₄ or a mixture thereof is used because of thermal expansion and thermal conductivity. In addition, water is added to adjust the viscosity, and in order to have sufficient water retention, a cellulose is added.

SiC is added in different grain sizes. Coarser SiC forms a scaffold and causes a high thermal conductivity. Fine-grained SiC fills in the interstices, reduces the porosity of the adhesive and thus increases the specific heat capacity per volume. This again increases the thermal conductivity. The phosphate binder reacts in the lower temperature range due to its acidic properties with oxidic raw materials (reactants) by a neutralization reaction.

• • Die Wärmeleitfähigkeit liegt bei ca. 40 bis 80 W/m × K im verwendeten Temperaturbereich.
• • Die spezifische Wärmekapazität liegt bei ca. 0,6 kJ/kg × K bei 20°C und steigt bis auf über 1,0 kJ/kg × K. Pro Volumeneinheit ist die Wärmekapazität des Klebers um 40 bis 60% größer als die Wärmekapazität der Wabenkeramik.
• • Der thermische Ausdehnungskoeffizient liegt im Anwendungsbereich sehr nah an der Wabenkeramik und weicht nicht mehr als 5% davon ab.

The adhesive described in the invention has the following properties:

• • The thermal conductivity is about 40 to 80 W / m × K in the temperature range used.
• • The specific heat capacity is approx. 0.6 kJ / kg × K at 20 ° C and rises above 1.0 kJ / kg × K. Per volume unit, the heat capacity of the adhesive is 40 to 60% greater than the heat capacity the honeycomb ceramic.
• • The coefficient of thermal expansion in the field of application is very close to the honeycomb ceramic and does not deviate more than 5% from it.

These properties of the adhesive are achieved by mixing inorganic powders, preferably SiC, with a phosphate binder and a reactant, e.g. As mullite achieved. It is followed by a heat treatment, wherein the phosphate binder cures with the mullite by a neutralization reaction.

The individual components are shown:

SiC coarse grained:

This SiC builds up the main structure. The large grains make it possible to set the highest possible thermal conductivity. Instead of SiC, Si ₃ N ₄ can also be used. Grain size range in general: -500 μm Flow range: 10-50%

SiC fine grained:

The fine grain is intended to fill up the spaces between the large grains and thus to keep the porosity of the connecting layer as low as possible. Instead of SiC, Si ₃ N ₄ can also be used. Grain size range in general: -100 μm Flow range: 10-40%

mullite:

The mullite is the reaction partner for the phosphate binder. Flow range: 0.1-20%

Volume:

Additional reaction partner for the phosphate binder. The term clay can be used to refer to bentonite, kaolinite, fireclay, montmorillonite and illite. Flow range: 0.01 to 5%

Phosphate binders:

It is a monoaluminum phosphate used whose reaction with the mullite begins only at higher temperatures. Flow range: 1 to 40%

Water:

With the addition of water, the viscosity of the adhesive is adjusted. Flow range: 1 to 40%

cellulose:

The cellulose retains water in the adhesive and prevents it from being removed from the adhesive too quickly when the adhesive is applied to a highly porous honeycomb segment. Flow range: 0.01 to 10%

For the bonding of porous SiC filter elements, a mixture of the following substances was composed: component properties Quantity in g SiC large grain <350 μ 28 SiC fine-grained (1) FEPA No. F 400 21 SiC fine grained (2) FEPA No. F 500 42 mullite <100 μm 8th volume <50 μm 2 Mono aluminum phosphate 50% strength 20 cellulose hydroxyethyl 1 H ₂ O 20

Claims (7)

Adhesive for highly porous ceramics, suitable for the production of diesel particle filters made of SiC segments, in order to glue together a honeycomb body of at least 2 segments, encase it and close the channels in opposite directions at least on one side, • with a thermal conductivity of the adhesive between 40 and 80 W / m × K in the temperature range used, • a specific heat capacity of the adhesive of 0.6 kJ / kg × K at 20 ° C and over 1.0 kJ × K at temperatures of 1000 ° C, where the heat capacity of the adhesive per unit volume is 40-60% greater than that Heat capacity of the ceramic and thus represents a heat sink during regeneration, With a coefficient of thermal expansion of the adhesive in the application range which does not deviate more than 5% from that of the honeycomb ceramic, the glue a) is fiber-free b) as a temperature-resistant binder, a phosphate binder selected from monoaluminum phosphate, phosphoric acid, monomagnesium phosphate, alkali metal phosphate, boronophosphate or sodium phosphate, c) as reaction partner for the phosphate binder 0.1 to 20 wt .-% mullite having a maximum particle size of 200 μ, d) has as a further reactant for the phosphate binder a clay selected from bentonite, kaolinite, fireclay or a mixture thereof, e) for the water retention comprises 0.01 to 5 wt .-% hydroxyethyl cellulose (HEC), f) 5 to 50 wt .-% coarse SiC powder having a maximum grain size of 1 mm, g) 5 to 60 wt .-% fine-grained SiC powder having a maximum particle size of 100 microns. Adhesive according to claim 1, characterized in that the temperature-resistant binder is a monoaluminum phosphate. Adhesive according to one of the preceding claims, characterized in that the coarse-grained SiC powder has a maximum particle size of 500 microns. Adhesive according to one of the preceding claims, characterized in that the adhesive contains 30% by weight of coarse-grained SiC powder. Adhesive according to one of the preceding claims, characterized in that the adhesive contains 35 wt .-% fine-grained SiC powder. Adhesive according to one of the preceding claims, characterized in that the adhesive contains 8% by weight of mullite. Adhesive according to one of the preceding claims, characterized in that mullite has a maximum particle size of 100 microns.