EP1852197A1 - Material for foundry core with aerogel sand comprising water swellable clay - Google Patents

Abstract

Core material, for sand mold casting containing aerogel sand, comprises clayey sand with a content of phyllosilicate (3-20%). An independent claim is included for a method for preparing core material comprising preparing a sol, mixing the sol with clayey sand, gelling the sol and drying the gel.

Description

The invention relates to a core material of clay-containing sand containing Aerogelsand and a process for its preparation.

Casting in bonded sand molds is a standard casting technique used to make workpieces of various alloys, especially aluminum, magnesium, titanium or cast iron alloys. Models of castings are made of wood, styrofoam and other materials, fixed in suitable mold boxes and filled with sand, the sands are compacted. The sands are usually chemically or thermally bound by organic binders (plastics), giving the sand mold sufficient strength. ( J. Sprunk, W. Blank, W. Grossmann, E. Hauschild, H. Rieksmeier, HG Rosselnbruch; Investment casting for all industrial sectors, 2nd edition, headquarters for casting use, Düsseldorf 1987 ; KA Krekeler, Feingießen, in: Handbuch der Fertigungstechnik Bd. 1, publisher: G. Speer, Hanser Verlag, Munich 1981 . W. Tilch, E. Fleming, molding materials and molding process, German publishing house for primary industry, Leipzig / Stuttgart 1993 ).

Cavities within the mold must be preformed stable by means of a core. Such cores are usually produced because of the prevailing high thermal and mechanical stress from plastic-bonded sands. Disadvantage of today's conventional method for core production is that the removal of the cores from the casting, especially in light metal alloys is possible only with great effort (eg mechanical destruction, shaking, thermal decomposition), the distribution of the sands in the core is inhomogeneous and / or cracking germs Among other things, the break under thermal-mechanical Can lead to stress. In particular, the thermal decomposition of the organic binder is problematic in light metal casting and not solved. As a rule, core fragments remain in the casting, which must be removed mechanically.

Aerogels are highly porous, open-pore solids, which are usually obtained via sol-gel processes via the gelation of colloidally disperse solutions and subsequent supercritical drying. For some years, it has also been possible to gel plastics using sol-gel processes and to convert them by supercritical drying into a highly porous organic solid (see, for example, US Pat DE 195 23 382 A1 . DE 694 09 161 T2 and US Patent No. 5,086,085 ). Pyrolysis of such plastic aerogels under inert gas or in vacuo at temperatures above 1000 ° C converts them into carbon aerogels. Like the oxidic aerogels, plastic and carbon aerogels have extremely low effective thermal conductivities (of the order of a few mW / K / m) and are considerably lighter. The physical and mechanical properties of plastic and carbon aerogels are documented in the literature ( RW Pekala, CT Alviso, FM Kong, SS Hulsey; J. Non-Cryst. Solids 145 (1992) 90 ; RW Pekala, CT Alviso, Mat. Res. Soc. Symp. Proc. 270 (1992) 3 ; R. Petricevic, G. Reichenauer, V. Bock, A. Emmerling, J. Fricke; J. Non-Cryst. Solids (1998 )). They can be varied within a wide range by the starting materials, their mixture and the production process.

EP 1 077 097 A1 describes the preparation and use of highly porous, open-pored plastic and / or carbon aerogels obtainable by sol-gel polymerization of organic plastic materials. The described aerogelsands can also be used as core materials, but the aerogelsands can be removed by oxidation at a temperature of less than 500 ° C.

Aerogelsands arise from the combination of conventional foundry sands and aerogels. If, in particular, RF aerogelsands are pyrolyzed, carbon aerogelsands are formed. Typically, carbon and carbonaceous products only burn at a noticeable rate at temperatures above 800 ° C. This means that carbon-bonded aerogelsands would be out of the question as a core material for aluminum casting. Carbon aerogelsands oxidize noticeably from 480 ° C ( DE 10200427382.0 ).

RF aerogelsands are made from foundry sands, resorcinol and formaldehyde. Resorcinol and formaldehyde are mixed as major constituents of the binder in a molar ratio of about 1.3: 1, and Na ₂ Co _{3 is added} as a catalyst and deionized water. The sol is mixed with the sand and gelled under exclusion of air. Drying the mixture of wet gel and sand at 20-40 ° C produces the airgel sands.

Sande can be introduced at levels between 50 and 90 wt .-% in so-called Aerogelsande. Their shape and size as well as their chemical composition are the factors influencing the properties of a core. To prepare the Aerogelsande quartz, Alodur were ^® - (96% Al ₂ O _3), and Siliziumcarbidsand (SiC) are used. Aerogelsande have sufficient strengths that can be changed with the grain size of the sands but also the binder content. The thermal conductivities are in the range of conventional mold and core material systems.

The RF Aerogelsands are the necessary prerequisite to produce carbon aerogelsands. Pyrolysis can be used to produce carbon aerogelsands from them. In this case, bodies of RF airgel-bound sands are used in an oven; the furnace evacuated and flooded with argon. Heating the oven to above 1000 ° C and holding at this temperature for a few hours, depending on the size of the shaped body, converts the RF aerogels to carbon aerogels. After a cooling process, the C-aerogels or carbon-Aerogelsande be taken. The shaped bodies of sand and carbon airgel thus obtained do not lose their shape and are mechanically stable, comparable to the starting material - the plastic Aerogelsand.

DE 102 16 464 A1 describes core materials of inorganic airgel and inorganic fillers, such as foundry sand, a process for their preparation and their use for the production of molds. The claimed core materials consist in addition to the fillers of Silicaaerogel.

1. a) Alterung eines Sols im Verlauf von 1 bis 14 Tagen,
2. b) Vermischung von 1 bis 6 Gewichtsanteilen des Sols mit 25 Gewichtsanteilen des Füllstoffs, der eine Temperatur im Bereich von 40 °C bis 80 °C aufweist und
3. c) Gelierung und Trocknung der Mischung.

DE 103 52 574 A1 relates to a process for the preparation of a filler-containing airgel, which is characterized in that it comprises the following steps:

1. a) Aging of a sol over 1 to 14 days,
2. b) mixing 1 to 6 parts by weight of the sol with 25 parts by weight of the filler having a temperature in the range of 40 ° C to 80 ° C and
3. c) gelation and drying of the mixture.

Instead of the chemical binders customary today, clay has hitherto been added to the sand or natural sand types have been selected in which clay has already been present in sufficient quantity (molding sand). The clay-bound sands are generally bound with water. However, this has the disadvantage that these molding materials have no high strength and are therefore unsuitable for core production in principle. To date, cores are being made for molding with polymeric binders such as phenolic or furan resins. However, these can not be used in combination with natural sands, since they must necessarily have a high water content for swelling the layer material. This water would lead to a dilution of the resins and thus to a significant reduction in the binding capacity. Therefore were with Introduction of polymeric binders in molding so far only pure special and therefore expensive foundry sands are used, which consist either at least 99 wt .-% of SiO ₂ (quartz sands) or at least 97 wt .-% of Al ₂ O ₃ (ALODUR ^® ). Even with the recently discovered core materials from aerogels and molding sands only special foundry sands are used. For example, in the EP 1 077 097 A1 ALODUR ^® sand used.

The swelling ability of clay materials depends on external circumstances, such as the water vapor pressure or electrolyte-containing pore waters. From 90 ° C, the interlayer water begins to escape, from about 250 ° C it is completely expelled. Upon cooling, rehydration already takes place again. The swelling also depends on the location of the cargoes. Charges in the O-layers are preferred over the T-layers.

• Vermiculite, Quellfähigkeit bis 15 Å, Ladung 0,6 bis 0,9
• Smectite, Quellfähigkeit ab 15 Å, Ladung 0,2 bis 0,6.

In addition, the swelling also depends on the hydration energy of the Z-layer cations. The swellability ranges from a slight expansion to complete dissolution. It is distinguished in:

• Vermiculite, swelling capacity up to 15 Å, charge 0.6 to 0.9
• Smectite, swelling capacity from 15 Å, charge 0.2 to 0.6.

The swelling depends on the charge of the elemental layers and the Z cations. K + as Z-cation can lower the charge to 0.6 to 0.7 / O ₁₀ (OH) ₂ . Underneath, mica passes into smectitic structures, with other cations then from 0.9 onwards into vermiculite.

• (K,Na,Ca,Mg)ⁱ⁺[Si_4-xAl_xO₁₀][(Al,Fe³⁺)_2-y(Mg,Fe²⁺)_y+z(OH)₂] mit i = z + y - 2z und 1 > i > 0,2 gehören:
• dioktaedrischer Vermiculit mit i = 0,6 bis 0,9. Sie stellen den Übergang von degradierten Glimmern zu den Smectiten dar. Die Ladungen befinden sich hauptsächlich in den T-Schichten. Die Kationen der Z-Schichten sind nicht fixiert in ihren Positionen.

To the dioctahedral three-layer minerals with swelling capacity of the composition:

• (K, Na, Ca, Mg) ^{i +} [Si _4-x Al _x O ₁₀ ] [(Al, Fe ³⁺ ) _2-y (Mg, Fe ²⁺ ) _{y + z} (OH) ₂ ] with i = z + y - 2z and 1>i> 0.2 include:
• dioctahedral vermiculite with i = 0.6 to 0.9. They represent the transition from degraded mica to smectites. The charges are located mainly in the T-layers. The cations of the Z layers are not fixed in their positions.

Smectite with i = 0.2 to 0.7, such as montmorillonite and beidellite. Montmorillonite has a low total charge, 0.25 to 0.5 / O ₁₀ (OH) ₂ . The charge is predominantly generated by the Mg ²⁺ in the O-layers. Beidellit has the charge lying mostly in the T-layers. He is the Al-richest smectite.

Tri-octahedral three-layer minerals with swelling capacity of the composition: (K, Na, Ca, Mg) [Si _4-x Al _x O ₁₀ ] [(Mg, Fe ²⁺ ) _3-y (Al, Fe ³⁺ ) _yz (OH) ₂ ] with i = x - y + 3z include: trioctahedral vermiculite i = 0.6 to 0.9. They are known much longer than the dioctahedral vermiculites. When heated, leaf up to 30 times their volume. Use as insulation and packaging material.

Trioctahedral smectite i = 0.2 to 0.7. They rarely occur as weathering products under surface conditions. Mainly under marine conditions, for example saponite, a trioctahedral smectite with a high Mg content.

Bentonite is the term used to describe clayey rocks that have been formed by the weathering of volcanic ash. Bentonite was named after the first site of Fort Benton, Wyoming (USA). Its unusual properties are determined by the clay mineral montmorillonite. The name montmorillonite derives from the southern French town of Montmorillon, where clay also occurs.

Montmorillonite is an aluminum hydrosilicate belonging to the group of phyllosilicates (foliar structure silicates). Montmorillonite is the major representative in the group of three-layer silicates, also referred to as smectites. In practice, bentonite, smectite and montmorillonite are synonyms for Swellable multi-layer silicates used. Bentonite may also contain accompanying minerals such as quartz, feldspar, mica. Bentonite deposits are found throughout the world. However, due to the different genes, the mineralogical compositions and thus also the technical usability are very different. In general, a distinction is made between primary and secondary deposits. Primary deposits are the result of the local weathering of volcanic rocks (for example in the Westerwald). In secondary deposits (for example in Bavaria), volcanic ash was first transported, for example by wind, deposition and subsequent weathering.

The most important two-layer material is the kaolinite. Its elemental layer is composed of an SiO ₂ tetrahedral layer and an Al ₂ O ₃ octahedron layer. In three-layer minerals, the elemental layer consists of two outer tetrahedral layers and one inner octahedral layer. This group includes the swellable montmorillonite or bentonite.

The swelling process is caused by the fact that water penetrates between the elemental layers and can change their distance. A distinction is made between "intracrystalline swelling", ie the widening of the distance of the elemental layers by the entry of excess water, and the "osmotic swelling", which results from concentration differences between "inner solution" and "external solution". If the swelling of a montmorillonite occurs within a limited volume (for example in a sealing layer), a swelling pressure is built up, which can reach several bar depending on the density. The swelling pressure prevents further penetration of water.

A montmorillonite crystal is composed of about 15 to 20 elemental layers. Between these layers are next to the crystal water exchangeable cations that compensate for the negative excess charges of the grid. These are not particularly tightly bound and can be replaced by other cations or by positively charged organic molecules. Bentonite or montmorillonite has a special ability for ion exchange and for the addition of positively charged particles. Because adsorption processes are surface reactions, the required adsorption capacity is significantly dependent on the specific surface area of the clay mineral. The specific surface area of montmorillonite can be up to 800 sqm / g.

Object of the present invention is therefore to provide a core material of airgel and sand, which is much easier and cheaper to manufacture.

The problem underlying the invention is achieved in a first embodiment by a core material for the casting of sand-containing airgel, which is characterized in that the sand contains clay-containing 3 to 10% by weight layer silicates.

Clay sand in the sense of the invention is a sand (molding sand), which is moist on the one hand and on the other hand contains a substantial amount of clay. Typical clay-containing sands contain, for example, between 5 and 15% by weight of clay. Clay-containing sand in the sense of the invention is thus a clay mineral sand. This contains so-called clay minerals, in particular phyllosilicates in an amount of 3 to 10 wt.%, Such as illite, kaolinite or montmorillonite.

Especially from the viewpoint that clay-containing molding sands are less and less used in foundries, it is now all the more surprising that the core material of airgel and clay-containing sand compared to conventional core materials of airgel and foundry sand has a particularly high strength and cures quickly. This is possible This may be explained by the fact that polymer-based aerogels in particular naturally have a high water content and thus combine the high binding power of the clay mineral with the good binding properties of the airgel. Particularly advantageous in the subject matter of the present invention is also the special low price of clay-containing sand, which allows the production of cores for molding at a particularly low cost. Due to the high strength of the core material according to the invention, the core material can be manufactured with a significantly reduced binder content.

It is essential to the molding sands used according to the invention that they contain swellable phyllosilicates. This directly gives the cores an initial strength that is not feasible with the aerogels binder alone. Not all silicates and not all phyllosilicates are swellable and certainly not every sound is swellable (the clay must contain the right amount of swellable phyllosilicates).

Form sands themselves are never used as cores in light metal casting because they just have moisture that leads to disasters in response to the aluminum. Using sands as a core sands alone is an innovation. Furthermore, no foundry would try to bind mold sand with polymeric binders (ie with polyurethane, furan resins etc) and he would take just as little phenolic resins, since he knows from experience that moisture is released (polycondensation with elimination of water) and moisture and aluminum do not agree. Therefore, the idea underlying the invention to use aerogels and natural sands with high swellable phyllosilicate content is just inventive. In aerogels, water is an important component, otherwise gelation occurs. The water formation in the polycondensation reaction does not disturb here, but rather causes the gel to be formed better. According to the invention can get the moisture out of the core body, because that is in the aerogels as opposed to the commercial binders prior art. Therefore, the use of sands with a high proportion of swellable phyllosilicates (vermiculite, montmorillonite, etc.) is a wonderful possibility that is not obvious to the skilled person and not even if he has knowledge about aerogels binder.

The airgel is advantageously a resorcinol-formaldehyde airgel. These aerogels are obtained by drying a sol of resorcinol and formaldehyde. This material is therefore particularly advantageous because it is already used in the previous foundry technology as a non-porous and solid polymer-based binder resorcinol-formaldehyde resin.

Advantageously, the airgel is present as binder in a quantity range of 6 to 15 wt .-% in the core material. If the amount of the airgel is well below this range, increased abrasion and low strength of the core material are observed. If the proportion of airgel as a binder is well above 15% by weight, the gelling time in the production of the core material of a few minutes for the core material according to the invention is extended up to several hours for a core material containing more airgel.

The clay-containing sand is advantageously contained in a quantity range of 85 to 94 wt .-% in the core material. A higher content of clay-containing sand leads to lower stability of the core material. A lower content of clay-containing sand leads to a significantly longer gel time.

The strength of the core material is advantageously at least 100 N / cm ² (1 MPa).

The mean grain diameter of the clay-containing sand is advantageously at least 100 microns.

The clay content of the clay-containing sand is advantageously in a range of 3 to 20 wt .-%, in particular 8 to 15 wt .-%. The water content of the natural sand used is advantageously in a range of 3 to 10 wt .-%.

The thermal conductivity of the core material according to the invention is advantageously in a range of 0.1 to 0.5 W / mK. As a result, the core material according to the invention can be used for applications such as aluminum casting and cast steel.

1. a. Herstellung eines Sols,
2. b. Mischung des Sols mit tonhaltigen Sand,
3. c. Gelierung des Sols zu einem Gel, und
4. d. Trocknung des Gels.

In a further embodiment, the object underlying the invention is achieved by a method for producing the core material according to the invention, comprising the following steps:

1. a. Producing a sol,
2. b. Mixture of sol with clayey sand,
3. c. Gelation of the sol to a gel, and
4. d. Drying the gel.

In comparison to the known processes for the production of core materials based on airgel, the process according to the invention differs above all in that a considerable shortening of the gelation time can be achieved with this process. While in the known method, the gelling was often several hours, can be achieved with the inventive method, surprisingly, a gelling time of only a few minutes. Due to the higher resulting strength of the core material, in contrast to the known methods, a significantly lower binding fraction can also be used.

Advantageously, clay-containing sand having a water content in a range from 3 to 10% by weight is used in the process according to the invention. At this water content it could be observed that a particularly high strength of the resulting core material could be achieved.

In a further embodiment, the object underlying the invention is achieved by the use of the core material according to the invention as a core material for molding.

• 1.Herstellung der Aerogellösung:
  • 22 g Resorcinol + 20 ml Formaldehydlösung (37 %ig) + 0,013 g Na₂CO₃ + 82 ml H₂O und Rühren bei Raumtemperatur.
• 2.Mischen der Aerogellösung mit tonhaltigem Sand:
  • Beispiel: 100 cm³ tonhaltiger Sand (Fa. Klophaus, Solingen, Sandklasse SGII) mit einer mittleren Korngröße 120 µm nahm 45 ml Lösung auf. Die Aerogellösung wurde unter Rühren in einem Hobart-Mischer dem Sand zugefügt.
• 3.Befüllung der Kernform:
  • Befüllung der Kernform unter Rüttel- und Klopfverdichtung.
• 4.Abbinden des Sandes zu ausreichender Nassfestigkeit innerhalb von 5 Minuten.
• 5.Trocknen:
  • Trocknen der offenen Form in einer Stunde bei 120 °C im Trockenschrank.
• 6.Einbau des aerogelgebundenen Sandkernes in eine Standardgussform

Embodiment:

• 1.Manufacture of the aerosol solution:
  • 22 g of resorcinol + 20 ml of formaldehyde solution (37%) + 0.013 g of Na ₂ CO ₃ + 82 ml of H ₂ O and stirring at room temperature.
• 2. Mixing the aerogell solution with clay-containing sand:
  • Example: 100 cm ^{3 of} clay-containing sand (from Klophaus, Solingen, SGII class) with a mean particle size of 120 μm took up 45 ml of solution. The aerosol solution was added to the sand while stirring in a Hobart mixer.
• 3.filling the core form:
  • Filling of the core mold under vibration and knock compaction.
• 4. Tie the sand to sufficient wet strength within 5 minutes.
• 5.Trocknen:
  • Dry the open mold in one hour at 120 ° C in a drying oven.
• 6.Installation of the airgel-bonded sand core into a standard casting mold

According to this regulation, different specimens were produced, which were produced with a different proportion of aerosol solution. The curing data are given in Table 1 again. <U> Table </ u> Proportion of aerosol solution (% by weight) characteristics 20% Hard after 2 to 3 h 15% Hard after 1 h; highest strength, high wet strength 10% At 80 ° C after 1 h solid; solid at 120 ° C after 20 to 30 minutes; 8th % At 80 ° C solid after 1.5 to 2 h; at 120 ° C solid after 20 to 30 min 4% Low strength; no curing at RT 2% Strong abrasion, no hardening at RT, very porous

The final strength was determined for selected combinations of clay-containing sand and the aerosol solution. The data are given in Table 2. <u> Table 2: </ u> Proportion of aerosol solution Bending strength [N / cm ² ] 6% 30 ± 5 10% 100 ± 17 15% 210 ± 30

Claims (9)

Core material for molding from sand-containing aerogelsand, characterized in that the core material clay-containing sand having a content of layered silicates of 3 to 20%. Core material according to claim 1, characterized in that the airgel is a resorcinol-formaldehyde airgel. Core material according to claim 1, characterized in that the airgel is contained in a quantity range of 6 to 15 wt.% In the core material. Core material according to claim 1, characterized in that the clay-containing sand is contained in a quantity range of 85 to 94 wt.% In the core material. Core material according to claim 1, characterized in that the strength is at least 100 N / cm ² . Core material according to claim 1, characterized in that the average grain diameter of the clay-containing sand is at least 100 microns. Process for producing the core material according to claim 1, comprising the following steps: a. Producing a sol, b. Mixture of sol with clayey sand, c. Gelation of the sol to a gel, and d. Drying the gel. A method according to claim 7, characterized in that one uses clay-containing sand with a water content in a range of 3 to 10 wt.%. Use of the core material according to claim 1 as a core material for molding.