MONOLITHIC SILICA MOLDED BODY AND ITS PRODUCTION

Description

[0001] The invention relates to a method for producing a monolithic silica molded body and to a silica molded body produced by this method. The silica molded body is particularly suitable for thermal insulation of buildings.

[0002] Reducing the energy consumption of buildings throughout their entire life cycle represents one of the major challenges facing the construction industry. Effective thermal insulation is known to play a significant role in reducing the energy demand of buildings. Porous materials, particularly nanoporous ones with a very high pore content, have become particularly important in this context. In addition to high thermal insulation properties, the materials used as thermal insulation in construction must meet a number of additional requirements – including mechanical stability, hydrophobicity, water vapor permeability, and non-combustibility.

[0003] Synthetic organic thermal insulation materials, such as expanded polystyrene or polyurethane, exhibit thermal conductivities in the range of 0.03 Wm -1< K -1< to 0.04 Wm -1< K -1<; while natural organic thermal insulation materials such as hemp, wood wool, natural wool, cork, and cellulose exhibit excellent carbon dioxide balance values, their thermal conductivities are above 0.04 Wm -1< K -1<. Organic thermal insulation materials also often have the disadvantage of being flammable.

[0004] Most inorganic thermal insulation materials, such as mineral wool (glass and rock wool), foam glass, perlite, ceramic foam and expanded clay, are not combustible, but regularly have thermal conductivities of more than 0.03 Wm -1< K -1< , and in most cases even thermal conductivities of more than 0.04 Wm -1< K 1<.

[0005] Inorganic thermal insulation materials also include porous silicon oxide-based materials, such as silica aerogels, as well as precipitated or fumed silicon dioxide. These thermal insulation materials can achieve thermal conductivities of 0.02 Wm -1 < K -1 < and less. Silica aerogels, known since the 1930s, possess particularly excellent thermal insulation properties, achieving a porosity in the range of 95% to 99.8% and exhibiting pore sizes in the nanopore range with an extension of less than 100 nm. This leads to the particularly low thermal conductivity of silica aerogels below 0.02 Wm -1 < K 1 < .

[0006] Silica aerogels have been the most effective inorganic thermal insulation materials for years due to their structural composition and the associated thermal insulation properties. However, two of the main disadvantages of silica aerogels are, firstly, the frequently used but complex supercritical drying process and, secondly, the low mechanical stability of monolithic thermal insulation bodies made of silica aerogels.

[0007] One way to increase mechanical stability is to strengthen the silicate walls between the individual nanopores; however, this usually leads to a reduction in porosity and a significant increase in thermal conductivity in the range above 0.02 Wm -1< K -1< .

[0008] EP 2 743 243 A2 and EP 2 743 243 B1 describe a process for increasing the mechanical stability of silica thermal insulation bodies. Accordingly, a starting material containing a silica precursor (also referred to as a synthesis mixture) is hydrolyzed, and phase separation is triggered by the addition of a polymer. The hydrolyzed starting material gels in a mold at a gelling temperature to form a silica gel molded body. The silica gel molded body is then heated and dried by hydrothermal heating in a gas-tight reactor chamber at a temperature of at least 100°C, ultimately producing the silica thermal insulation body. These silica thermal insulation bodies have a bimodal, hierarchical pore structure with a porosity ranging from 80% to almost 95%.

[0009] Silica thermal insulation bodies and their production are also known from DE 694 07 295 T2 or WO 95 / 03256 A1, DE 10 2009 053 782 A1, US 2011 / 0237692 A1 or WO 2010 / 080238 A2, EP 0 363 697 A1 or DE 697 16 126 T2.

[0010] WO 2010 / 080238 A2 describes a process for the formation of hybrid aerogels from a metal oxide precursor and a branched telechelic copolymer, for example by co-hydrolysis and co-condensation of the metal oxide precursor and the branched telechelic copolymer.

[0011] EP 0 363 697 A1 discloses a process for producing porous glass, which comprises preparing a reaction solution containing a metal alkoxide or its oligomer and an organic polymer, hydrolyzing and polymerizing the metal alkoxide or its oligomer in the solution to obtain a gel, and calcining the gel to obtain porous glass.

[0012] WO 95 / 03256 A1 describes a porous inorganic material and a process for producing the porous inorganic material, which has interconnected continuous macropores with an average diameter of more than 0.1 µm and additional mesopores in the walls of these macropores, these mesopores having an average diameter between 2 nm and 100 nm.

[0013] However, the excellent mechanical strength of the thermal insulation bodies produced using this process is offset by their high thermal conductivity, in the range above approximately 0.05 Wm -1< K 1< compared to silica aerogels. Another disadvantage is the complex process step of hydrothermal heating. However, the particularly high porosity in the range of over 90% has so far only been achievable with such specific process steps.

[0014] The object of the invention is to provide a method for producing a monolithic silica molded body with a hierarchical pore structure, which exhibits a thermal conductivity in the range of 0.015 to 0.030 Wm -1< K 1< while maintaining high mechanical and chemical stability. By means of this method, large-format silica molded bodies with freely selectable geometry can be produced without cracks in a less complex manner than the known prior art, particularly with regard to drying.

[0015] This object is achieved by a method for producing a monolithic silica molded body, ie a one-piece molded body formed from a salt or ester of orthosilicic acid and its condensates, with the features of patent claim 1 and a silica molded body with the features of patent claim 5. Advantageous developments of the inventions are described in the subclaims.

[0016] According to the proposed method for producing a monolithic silica molded body, a starting material containing at least one silica precursor is first provided and hydrolyzed according to the generally known procedure of a sol-gel process using water. As a result of the hydrolysis reaction, a reaction solvent is formed in addition to the hydrolyzed silica precursors, i.e., silicic acids. Consequently, a silica-solvent mixture containing this reaction solvent is formed as a result of the hydrolysis.

[0017] In addition to one or more silica precursors, the starting material may contain other components in a homogeneous mixture. Various alkoxysilanes and / or metal-silica solutions can act as silica precursors; according to the invention, tetraethylorthosilicate (TEOS) or tetramethylorthosilicate (TMOS) is used as the silica precursor.

[0018] The silica-solvent mixture contains the reaction solvent formed during hydrolysis, depending on the silica precursor used, in an amount determined by the amounts of reactants in the hydrolysis reaction and the choice of hydrolysis conditions. When tetraethyl orthosilicate (TEOS) is used as the silica precursor, ethanol is the reaction solvent; when tetramethyl orthosilicate (TMOS) is used as the silica precursor, methanol is formed as the reaction solvent. The reaction solvent is therefore an organic solvent.

[0019] Hydrolysis can be further assisted by the addition of an acid, whereby the acid can be an inorganic acid, for example a mineral acid (such as sulfuric, nitric or hydrochloric acid), or an organic acid (such as acetic or tartaric acid). Furthermore, the starting material can contain a base precursor, i.e. a material or reactant that decomposes above a material-specific activation temperature and releases a basic decomposition product. The base precursor can contain or consist of urea and / or hartshorn salt, for example. The addition of the base precursor also increases the average pore diameters of the primary and secondary pores.

[0020] Hydrolysis triggers polycondensation or crosslinking of the hydrolyzed silica precursors, converting them into silica gel. Gelation is accompanied by the formation of pores of a first pore species in the silica gel, referred to herein as primary pores.

[0021] The addition of a polymer triggers polymer-induced phase separation in the hydrolyzed starting material or in the silica-solvent mixture. This phase separation is accompanied by the formation of pores of a second pore species, referred to as secondary pores. The secondary pores are larger than the primary pores and have a larger average pore diameter than the primary pores. According to the invention, the primary pores have an average pore diameter in the range of 1 nm to 100 nm, whereas the average pore diameter of the secondary pores is in the range of 500 nm to 20 µm. Polyethylene oxide (PEO) acts as the phase-separating polymer.

[0022] Both the primary pore network formed by the primary pores and the secondary pore network formed by the secondary pores are open-cell pore networks, with the primary pore network permeating the entire silica gel, and the silica gel permeated by the primary pore network being, in turn, permeated by the secondary pore network, so that the walls between two secondary pores are permeated with the primary pores. The resulting silica gel thus exhibits a bimodal pore structure (i.e., pores of two pore species with different, but essentially constant, pore sizes within the species). This pore structure is also hierarchically arranged, i.e., the larger secondary pores are formed in or by the silica gel permeated by the smaller primary pores.

[0023] According to the invention, the proportion of the reaction solvent in the silica-solvent mixture is increased by adding a predetermined amount of an additional solvent that is materially identical to the reaction solvent. The additional solvent can be added before, during, or after the hydrolysis. This means that after hydrolysis, the silica-solvent mixture contains, in addition to the reaction solvent, the additional solvent that is materially identical to the reaction solvent. The amount of reaction solvent and the amount of additional solvent together constitute the total solvent amount. By adding the additional solvent, the total solvent amount in the silica-solvent mixture is therefore increased compared to the total solvent amount, which is otherwise determined solely by the amount of reaction solvent formed due to hydrolysis.

[0024] According to the invention, the molar ratio of additional solvent to silica precursor used is in a range from 0.5 to 6, particularly preferably in the range from 0.8 to 3. When these ranges are used, the silica molded body exhibits a particularly balanced ratio of low thermal conductivity and high mechanical stability after completion of the process. In the narrower preferred range, a particularly stable equilibrium in this regard is achieved.

[0025] The polymer can also be added before, during, or after hydrolysis. According to one embodiment, the polymer is added to the starting material before hydrolysis. This enables, in particular, complete homogenization of the starting materials, i.e., thorough mixing of the starting material with the polymer before hydrolysis. It can also be provided to first mix the polymer and, if appropriate, the other starting materials with water and then add the silica precursor to this mixture, thereby hydrolyzing it. The polymer can also already be present in the starting material. In any case, phase separation necessarily only occurs in the hydrolyzed starting material. Preferably, the polymer is first mixed with the additional solvent and water, optionally also with the acid and / or the base precursor, and homogenized; the silica precursor is then added to the homogenized liquid.

[0026] According to the process, the hydrolyzed starting material, to which the polymer has been added, is introduced into a mold, wherein the mold defines the geometry or shape of the silica molding to be produced (i.e., the geometry of the silica molding essentially corresponds to the geometry of the mold). The hydrolyzed starting material, i.e., the silica-solvent mixture, gels in the mold at a predetermined temperature (hereinafter also referred to as the "gelling temperature") to form a silica gel molding, i.e., a molding that essentially has the geometry of the silica molding to be produced, but consists of (a) silica gel.

[0027] The silica gel comprises a solid phase or component and a liquid phase or component, with both the primary pores and the secondary pores formed by the solid phase and initially filled with the liquid phase. In addition to water, the liquid phase contains the solvent formed from the reaction solvent and the co-solvent. The liquid phase may also contain dissolved polymer components and (depending on the starting materials used) base precursor components and acid components.

[0028] The final step of the process is drying the silica gel molding; drying transforms the silica gel molding into a rigid silica molding. During drying, the liquid phase is removed from the open-cell pore network of the silica gel, particularly by evaporation. Drying is preferably carried out by storing the silica gel molding under common ambient conditions (1 atm and 10 - 30 °C), i.e., by air drying. Alternatively, drying can be carried out at slightly reduced pressure. To assist the drying process, the silica gel molding can be heated to a temperature of 30 °C to 80 °C instead of the economically preferred drying at room temperature.

[0029] It has been shown that the addition of the additional solvent increases the overall porosity of the silica gel or the silica molding - with almost unchanged mechanical stability - i.e. it has a pore-forming or porogenic effect, whereby in particular the average pore diameters of the secondary pores are larger compared to a silica gel or silica molding produced without additional solvent.

[0030] The process according to the invention makes it possible to produce silica moldings which have a porosity in the range between 96% and 97%, a thermal conductivity in the range of 0.01 to 0.02 Wm -1< K -1< , high mechanical stability with a compressive strength of 0.05 to 2 MPa and high chemical stability (including non-flammability).

[0031] Due to the hierarchical pore structure, the drying process of the silica gel is significantly simplified compared to the prior art because drying can be carried out under normal atmospheric conditions at room temperature, i.e., drying with subcritical parameters, without the need for a pressure autoclave. This is due in particular to the enlarged secondary pores, in which the liquid phase contained in the primary pores aggregates after gelation and is subsequently effectively transported to the surface of the silica molded body. The capillary forces that occur when the liquid phase is removed from the silica gel are thus kept low. A ratio of average secondary pore diameter to average primary pore diameter in the range of 300 to 800 has been found to be particularly advantageous for the drying process.

[0032] The effect described above, combined with the higher mechanical stability of the silica molded body produced by the process according to the invention, makes it possible to avoid the complex supercritical drying process commonly used for homogeneously porous silica aerogels. During drying, the temperature can thus be kept below the critical temperature and / or the pressure below the critical pressure of the liquid phase of the silica gel (or of water as its main component) without significantly impairing the structure of the solid phase of the silica gel molded body through drying. Thus, a crack-free transition from the silica gel molded body to the silica molded body is achievable during drying with subcritical parameters.

[0033] The possibility of converting the silica gel moldings into silica moldings by air drying without causing damage not only reduces the equipment required (by eliminating the need for autoclave drying), but also results in a significant reduction in the process time required to produce the silica moldings.

[0034] In addition, the process time in the production of silica molded bodies is reduced because the hydrolysis time is shortened by the addition of the additional solvent according to the invention.

[0035] The dried monolithic silica molding is particularly suitable for thermal insulation of buildings, where the silica molding itself is used as a thermal insulation body or is previously divided into several thermal insulation bodies (e.g., thermal insulation panels). The silica molding can also be formed into a thermal insulation panel itself. Such plate-shaped silica moldings have, for example, a dimension of more than 20 cm, preferably more than 50 cm, in at least one spatial dimension.

[0036] According to one embodiment of the process for producing the silica molded body, the liquid phase is removed from the silica gel molded body after gelation by means of at least one washing process, i.e., largely replaced by a replacement liquid. The hierarchical pore structure achieved by the process promotes the washing process, just as it does during the drying process.

[0037] After drying, the dried monolithic silica molding can also be subjected to a thermal post-treatment (calcination) to remove the polymer added to the starting material. For this purpose, the silica molding is exposed to a temperature of 200°C to 700°C for a period of 30 minutes to 24 hours, for example. It has been found that such a thermal post-treatment further reduces the thermal conductivity of the silica molding and further increases its mechanical stability, which can be attributed in particular to the removal of residual liquid and / or thermally induced rearrangements.

[0038] Gelation takes place in the casting mold at a setting temperature of below 100°C, according to the invention at a temperature between 40°C and 70°C. Gelation can be carried out, for example, in a gas-tight reactor chamber. Gelation can essentially be carried out at normal pressure, i.e. at approximately 1 bar; however, it can also be provided to carry out gelation at a slightly elevated ambient pressure of, for example, up to 2 bar (the pressure increase can be achieved by placing the casting mold with the silica gel in the gas-tight reactor chamber and pressurizing the reactor chamber with a gas, preferably an inert or noble gas). It has been shown that such an elevated ambient or air pressure can have a positive effect on gelation. The gelation time can be between 3 hours and 3 days.

[0039] According to the disclosure, a silica molded body is further provided, wherein the molded body can be produced in particular by means of a method according to one of the embodiments described above.

[0040] The pore size of the primary pores and the secondary pores, as well as the ratio of the volume occupied by the primary pores and the secondary pores, can be specifically influenced or adjusted by the choice of reactants and the setting of the respective reaction parameters (especially pressure, temperature, and reaction time). For example, the pore size of the primary pores can be influenced by the selected silica precursor, and the pore size of the secondary pores can be influenced by the type and proportion of the polymer. The primary pores and the secondary pores form an open-cell, bimodal, hierarchical pore structure, with the primary pores having a smaller pore diameter or pore size than the secondary pores, and the walls between adjacent secondary pores are interspersed with the primary pores.

[0041] According to the invention, the primary pores have an average pore diameter in the range of 1 nm to 100 nm (preferably 10 nm to 20 nm), whereas the secondary pores have an average pore diameter in the range of 500 nm to 20 µm (preferably 5 µm to 15 µm).

[0042] According to one embodiment, the silica molded body is designed (by appropriately selecting the manufacturing parameters) such that the secondary pores constitute at least half of the cavity volume of the thermal insulation molded body. The high proportion of secondary pores results in a pore structure that is particularly conducive to crack-free removal of the liquid phase from the silica gel while ensuring low thermal conductivity and good thermal insulation.

[0043] The invention is illustrated below by means of exemplary embodiments and comparative examples with reference to the attached figure.

[0044] The figure schematically illustrates, as a sectional view, the pore structure of a silica molded body 1 produced according to the manufacturing method described above. The silica molded body 1 has an open-cell pore network with primary pores 2 and secondary pores 3, wherein the primary pores 2 have a significantly smaller pore diameter (in the range of 10 nm to 20 nm) than the secondary pores 3 (pore diameter in the range of 5 µm to 15 µm). The silica phase 4 is permeated by the smaller primary pores 2, with the larger secondary pores 3 in turn being formed by the silica phase 4 permeated by the primary pores 2. The pore network formed by the primary pores 2 and the secondary pores 3 thus has a bimodal, hierarchical structure.

[0045] Two examples of the production of such a silica molded body using the disclosed method are given below. Example 1

[0046] A mixture of 160 g of polyethylene oxide (as polymer), 1440 g of water, 100 g of sulfuric acid, 240 g of ethanol, and 260 g of urea (as base precursor) is homogenized. The hydrolyzed reaction mixture, together with 1166 g of tetraethyl orthosilicate as silica precursor, is introduced into a mold and allowed to gel in a reactor chamber under the exclusion of air at a gelation temperature of 50 °C for 24 hours, forming a silica gel molded body that conforms to the geometry of the mold. The silica gel molded body is then removed from the reactor chamber, washed with demineralized water, and finally dried at room temperature and atmospheric pressure of 1 bar to form a monolithic silica molded body. The dried, monolithic silica molded body is then subjected to a thermal treatment (calcination) at 500 °C for 10 hours. Example 2

[0047] A mixture of 160 g of polyethylene oxide (as polymer), 1440 g of water, 200 g of tartaric acid, 480 g of ethanol, and 260 g of urea (as base precursor) is homogenized. The hydrolyzed reaction mixture, together with 1166 g of tetraethyl orthosilicate (as silica precursor), is introduced into a mold and gelled in a reactor chamber under exclusion of air at a gelation temperature of 50 °C for 18 hours, forming a silica gel mold. The silica gel mold is then washed or rinsed first with water and subsequently with a (essentially) nonpolar exchange liquid (e.g., ethanol), whereby the liquid phase of the silica gel is largely replaced first by water and then by the exchange liquid. The silica gel mold is then dried, forming a monolithic silica mold.The dried silica molded body is finally subjected to a thermal treatment (calcination) at 400 °C for 6 hours.

Claims

1. A method for producing a monolithic silica molded body (1) with a hierarchical pore structure of primary pores (2) and secondary pores (3), the average pore diameter of which is larger than the average pore diameter of the primary pores (2), wherein the primary pores (2) have an average pore diameter between 1 nm and 100 nm and the secondary pores (3) have an average pore diameter between 500 nm and 20 µm, comprising the following steps: - providing a starting material containing at least a silica precursor, wherein the silica precursor is tetraethylorthosilicate or tetramethylorthosilicate; - hydrolyzing the starting material to form a silica-solvent mixture, wherein the silica-solvent mixture contains as solvent a reaction solvent resulting from the hydrolysis, wherein the reaction solvent is ethanol when using tetraethyl orthosilicate as silica precursor and methanol when using tetramethyl orthosilicate as silica precursor, wherein the amount of solvent in the silica-solvent mixture is increased by adding a predetermined amount of an additional solvent which is materially identical to the reaction solvent, wherein the amount of substance of the additional solvent added is in the range of 0.5 to 6 times the amount of substance of the silica precursor contained in the starting material, - initiating a polymer-induced phase separation in the silica-solvent mixture by adding a polymer, wherein the polymer is polyethylene oxide; - gelling the silica-solvent mixture in a mold containing the starting material at a gelling temperature to form a silica gel molded body, wherein the hierarchical pore structure is formed from primary pores (2) as a result of hydrolysis and from secondary pores (3) as a result of polymer-induced phase separation, wherein the walls of the silica gel molded body are penetrated with the primary pores (2) between adjacent secondary pores (3), wherein the gelling temperature is greater than 40 °C and less than 70 °C; and - drying the silica gel molded body to form the silica molded body (1).

2. Method according to claim 1, characterized in that the drying is carried out at room temperature and atmospheric pressure.

3. Method according to claim 1 or 2, characterized in that the amount of substance of the additional solvent added is in the range of 0.8 to 3 times the amount of substance of the silica precursor contained in the starting material.

4. Method according to one of claims 1 to 3, characterized in that the additional solvent is added to the starting material before hydrolysis.

5. Silica molded body (1) produced according to one of claims 1 to 4, wherein the silica molded body (1) has an open-cell pore structure with primary pores (2) and secondary pores (3), the average pore diameter of which is larger than the average pore diameter of the primary pores (2), and wherein the walls between adjacent secondary pores (3) are penetrated with the primary pores (2), wherein the primary pores (2) have an average pore diameter between 1 nm and 100 nm and the secondary pores (3) have an average pore diameter between 500 nm and 20 µm, characterized in that the silica molded body (1) has a porosity in the range between 96% and 97%.

6. Silica molded body (1) according to claim 5, wherein the primary pores (2) have an average pore diameter between 10 nm and 20 nm and the secondary pores (3) have an average pore diameter between 5 µm and 15 µm.

7. Silica molded body (1) according to claim 6, wherein the secondary pores (3) form at least half of the cavity volume of the silica molded body (1).