Description A subcritical method of drying lyogels to produce aerogels. The invention relates to a method of drying lyogels to produce aerogels. The liogels are gels that contain a liquid, the dispersing agent. In the particular case when the liquid of the gel is water, they are also called hydrogels. In the present Application, the term "lyogel" further includes hydrogels. Aerogels in the broad sense that is to say in the sense of "gel with air as dispersant", is a suitable gel that is produced by drying. The term "aerogels" in this sense includes aerogels in the strict sense, xerogels and cryogels. In this regard, a dry gel is termed airgel in the strict sense if the gel liquid is removed at temperatures above the critical temperature and starts with pressures that are higher than the critical pressure. On the other hand, if the liquid in the gel is removed subcritically, for example, with the formation of a vapor-liquid interconnection, then the resulting gel is often also called xerogel. As regards the use of the term aerogels in the present Application, there are gels which are
subcritically dried. Aerogels have a very low density and high porosity for solid substances. Therefore, because of the minimum pore size, aerogels, particularly those with porosities above 60% and densities of 0.6 g / cu.cm, demonstrate an extremely low thermal conductivity and therefore are used as materials heat insulating materials such as those described for example in EP-AO 171 722. For industrial applications, aerpiles are predominantly used as granules. In what refers to the application, it is essential that the airgel granulate to be used consist of particles of a suitable shape, preferably spherical and size distribution. However, by virtue of their low density, aerogels also exhibit low mechanical stability, particularly in relation to total loads and against abrasion. Because of the capillary forces that occur in subcritical drying and the reduction involved, not all lyogels are suitable for subcritical drying to produce an airgel. During drying, the gel is considerably reduced if the meniscus of the
liquid leaves the interior of the gel in order to, from a certain point affecting the drying process, to retreat more or less completely in its initial form again. Consequently, depending on the qualities of the inner surface of the gel, some minimum stability of the gel network is essential, modification of the inner surface of the gel that is often required in order to avoid a reaction of adjacent pore walls in the reduced condition and a collapse of the gel that this would cause. The corresponding processes where the inner surface of a SiO2 lyogel is modified organically and the resulting gel dried subcritically to produce an airgel, are disclosed for example in US-A-5 565 142, DE-A-43 42 548 and in FIG. German Patent Application 19648798 unpublished. Gels that are not suitable for subcritical drying disintegrate under subcritical drying, with a loss of porous structure and therefore no longer demonstrate the favorable properties of aerogels. According to the gel, the modification of the surface, the granular shape and the size and drying conditions, so that the gel particles can, during drying, be destroyed on a macroscopic scale, that is to say while retaining the nanoporous structure. Is
true that the airgel retains its properties but due to the granular size now smaller than an irregular or indefinite grain shape, it can no longer be used satisfactorily. The drying methods that at first glance seem obvious for such drying problems are not satisfactorily resolved for the major industrial production of airgel granules in a definitive manner. Due to the low density of aerogels, it has been discovered that a fluidized bed drying system is unsuitable for major industrial production5. In order to remove the airgel particles from the bed, - >- it would be necessary to work below the fluidization point; the necessary gas flow velocities would then be so slow that there would be no guarantee of heat supply and vapor dispersion in an acceptable drying time. Where higher gas flow rates need to be used, the airgel will not dry completely because it is transported out of the dryer. Further, in the fluidized phase, the gel particles collide with each other so that there is considerable abrasion and grain fracture. The contact drying is not sufficiently effective due to the high thermal insulation capacity of the aerogels, a transfer of
Heat in the most remote layers of the contact surface is not done fast enough so that it is only applied to the bottom layer and therefore, in view of the quantities required, excessively large surface areas are required. According to DE-A-43 16 540, aerogels are dried by means of dielectric processes. However, due to the necessary electrical power and relatively high investment for an appropriate drying apparatus, these methods are not economical enough. Therefore, the object of the present invention is to provide a method for subcritical drying of lyogels which are suitable for subcritical drying to produce aerogels and to minimize the destruction of airgel particles and abrasion of airgel particles during the dried and can be used on a large industrial scale. Surprisingly, the problem is solved by a method for subcritical drying of lyogels 20 to produce aerogels and characterized in that the lyogel particles are placed as a fixed bed and
* applies a flow of dry gas through them. < * A Performed in this way, drying does not cause grain breakage or abrasion since the particles have a stationary mass. Surprisingly, drying can
performed in relatively short time and at layer heights that are considerable for fixed bed drying. In principle, any lyogel that can be dried subcritically is suitable for the method, that is, its gel structure must be sufficiently ñus. stable to avoid the collision of the structure by the action of capillary forces and if necessary are modified appropriately on the surface in order to by. example avoid a reaction between the walls of the
pores in the reduced condition. As for the lyogels, according to the type of gel structure these can be organic or inorganic lyogels. For example, they can be produced on a base of metal oxides that are suitable for the technique
Sol-gel (GJ Bnnker, GW Scherer, Sol-gel Science, 1990, chapters 2 and 3), such as Si- or Al compound examples or on the basis of organic substances which are suitable for the sol-gel technique as per example
• condensates of melamine formaldehyde (US-A-5 086 085)
or condensates of resorcin formaldehyde (US-A-4 873 218) or even on the basis of mixtures of the aforementioned materials, preferably Si02 gels and in particular are preferably Si02 gels of organically modified surface such as those described by
Example in the German Patent Application unpublished N °. 19648798. In the case of gel fluid, which can in principle be pure substances or mixtures, the gel liquid preferably contains more than 50% by weight of organic solvents, preferably acetone or hexamethyl disiloxane. Naturally, the gel liquid may also contain small amounts of other substances such as hydrochloric acid or water residues. The lyogel particles can in principle have any shape and size but preferably
_. substantially spherical particles with diameters between
*. 100 μm and 5 cm and particularly preferably with diameters between 0.5 mm and 5 mm. It is also possible to dry the mixtures of particles of different shape and / or different size. In order to prevent the airgel particles that are already dry and therefore light to disintegrate, among other things it might be necessary that the gas flow velocity be low so that the dried airgel particles do not break. However, this results in low gas flow and therefore very limited energy input. To dry aerogels, a quick energy input is useful. Therefore, to use relatively high gas flow rates, the volume, in those places where the gas flow emerges from the fixed bed, it should be limited to at least one composition that is permeable to the drying gas but not to the particles. Surprisingly, the fixed bed is preferably and in the manner known per se, traversed by the flow of drying gas passing therethrough from below, so that the composition, which is permeable to drying gas but not the particles, becomes the carrier of the fixed bed. The drying apparatus that is then to be used is not only simpler and less expensive, but also the fracture of the grains is reduced, since the top layers pressing on the bottom layers are the first to dry. Dry and mechanically sensitive aerogels are then found in the heavier and less dry gel particles with mechanical loading capacity and are not loaded mechanically with more weight. For continuous operation, the fixed bed can also be transported with the carrier, for example in a suitable band. Preferably, the gas flow then passes through the fixed bed transversely to the direction of movement. The thickness of the fixed bed in the direction of flow of the drying gas stream can be
surprisingly bigger Preferably, it is between 20 cm and 100 cm and particularly preferably between 20 cm and 60 cm. Any gas suitable for drying can be used as the drying gas; If the gel liquid contains large fractions of organic solvents, then an inert gas such as nitrogen may have to be used. The incoming gas flow can also contain solvent gas that allows the process, in other words a
circulatory pattern of gas flow, by which, as usual, the solvent gas is constantly removed from the circuit, for example by condensation. To accelerate the drying, in its entirety, it may be useful to first dry the gel in a circulatory manner but use new drying gas
to dry the last residues of the gel liquid. The method according to the invention can be carried out by means of an apparatus known to the person skilled in the art, for example, stationary bed dryers, suitable containers with sieve bottoms.
or suitable drying bands, etc. which may possibly be modified in a manner obvious to the person skilled in the art. Preferably, the method for drying - modified surface SiO2 aerogels will be used as disclosed
in the example of DE-A-43 42 548 or in the German Patent Application without publication N °. 196 487 98. In this case, if the gel liquid contains more than 50% by weight of hexamethyl disiloxane or acetone, then the temperature of the drying gas is preferably between 100 ° C and 200 ° C and with particular preference between 140 ° C and 180 ° C. In the case of modified surface SiO2 aerogels, the incident flow rates vary preferably between 1 and 40 cm / s and particularly preferably between 5 and 30 cm / s. & Although the method is particularly suitable for producing aerogels with densities below 300 g / cu.cm, it is also possible to use it to dry higher density xerogels. The method according to the invention will be described hereinafter with reference to an example of the embodiment but nevertheless not limited thereto. Example: A modified surface SiO2 lyogel that was produced according to examples 1 to 4 in the German Patent Application No. 1964877798 and having an exact grain size distribution of about 1 mm in diameter is dried in a fixed bed with a height of 50 cm with a stream of nitrogen flowing through the fixed bed from top to bottom at an incident flow rate 20 cm / s and with a gas inlet temperature of 160 ° C. Initially, the gel is dried for 2 hours in a circulatory manner, leaving the drying gas the fixed bed saturated with moisture and then with total condensation with a load of 10 g / kg of inert gas, is again introduced to the fixed bed. Then, drying is continued for half an hour with fresh drying gas. The dry airgel shows virtually no grain fracture and almost no abrasion.