WO2020014828A1 - Methods for forming aerogels - Google Patents

Methods for forming aerogels Download PDF

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Publication number
WO2020014828A1
WO2020014828A1 PCT/CN2018/095817 CN2018095817W WO2020014828A1 WO 2020014828 A1 WO2020014828 A1 WO 2020014828A1 CN 2018095817 W CN2018095817 W CN 2018095817W WO 2020014828 A1 WO2020014828 A1 WO 2020014828A1
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Prior art keywords
alcogel
pore
aged
gel
aerogel
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PCT/CN2018/095817
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French (fr)
Inventor
Hui Wang
Qi Zhang
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Honeywell International Inc.
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Priority to PCT/CN2018/095817 priority Critical patent/WO2020014828A1/en
Publication of WO2020014828A1 publication Critical patent/WO2020014828A1/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/157—After-treatment of gels
    • C01B33/158—Purification; Drying; Dehydrating
    • C01B33/1585—Dehydration into aerogels
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/152—Preparation of hydrogels
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/10—Solid density
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/14—Pore volume
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/16—Pore diameter

Definitions

  • the present invention relates to methods forming aerogels, the aerogels formed thereby, and articles containing the aerogels.
  • Aerogels in general, are materials that are chemically intert and highly porous ceramics.
  • aerogels are produced by forming a gel.
  • a gel is a nanostructured network of interconnected particles that spans the volume of a liquid medium.
  • Gels have some properties like liquids, such as density, and some properties like solids, such as a fixed shape. Because the pores in a gel are nano-sized, the capillary forces exerted by the liquid are strong enough to hold it inside the gel and prevent the liquid from simply flowing out.
  • WO92/20623 notes that sodium silicate-hydrate was used as starting material for the preparation of silica aerogel, and that the process involved the formation of a gel in which the liquid was water (known as an aquagel or hydrogel) as a result of using hydrogen chloride to catalyze the reaction between the silicate and water.
  • WO92/20623 explains that such a route involves chemical reactions that generally leave impurities in the water portion of the gel that must be removed in order to achieve the advantageous properties of the finished aerogel material.
  • Removal of the impurities is typically done by soaking the gel in r a solvent, such as water, acetone and acetonitrile, allowing impurities to diffuse out and allowing the chosen solvent to diffuse into the pores.
  • a solvent such as water, acetone and acetonitrile
  • the solvent in which the gel is soaked is typically exchanged with fresh solvent multiple times over the course several days in order to effect the removal of the impurities. This is typically called a wash process.
  • WO92/20623 also describes another process for forming an aerogel in which tetramethoxysilane, Si (OCH 3 ) 4 (also sometimes referred to herein for convenience as , TMOS) , has also been used as starting material for the preparation of silica aerogel.
  • TMOS tetramethoxysilane
  • Si (OCH 3 ) 4 also sometimes referred to herein for convenience as , TMOS
  • TMOS tetramethoxysilane
  • TMOS is described as being a suitable starting material for an aerogel process since it is easy to handle, easy to prepare in pure form and easy to hydrolyse.
  • a gel is formed in which the liquid is alchohol (i.e., methanol) since formation of the solid network takes place by a direct acid-and/or base-catalysed hydrolysis of TMOS.
  • the solvent that makes up the liquid portion of the gel must be removed from the solid skeleton or framework to obtain an aerogel in which the pores are filed with gas, usually air but also in some cases other gases such as nitrogen, and in which the solid framework portion of the liquid-containing gel remains substantially intact, that is, without substantial collapse or shrinkage.
  • gas usually air but also in some cases other gases such as nitrogen
  • the solid framework portion of the liquid-containing gel remains substantially intact, that is, without substantial collapse or shrinkage.
  • Removal of the solvent while preserving the porous solid structure has presented problems because the solid portion of the gel structure has a tendency to shrink upon removal of the solvent. This shrinkage, which in turn causes the porous solid structure to crack, and/or break and/or densify, is believed by applicants to be caused by capillary forces of the liquid solvent that act upon the solid network structure as the liquid solvent evaporates.
  • An undesirable result can be a finished product in which the porosity is less than desired, and in some cases less than 50%.
  • Figure 1 is a schematic illustration of process according to one embodiment of the present invention.
  • step (c) before, during or after said providing step (a) and/or said preparing step (b) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage;
  • Applicants invention is directed in the broad sense to all aerogels, and this includes aerogels in which the solid portion is formed from a variety of known materials, including silica, cellulous, polyimide and TiO2 to produce silica aerogel, cellulous aerogel, polyimide aerogel and TiO2 aerogel, among others.
  • aerogels in which the solid portion is formed from a variety of known materials, including silica, cellulous, polyimide and TiO2 to produce silica aerogel, cellulous aerogel, polyimide aerogel and TiO2 aerogel, among others.
  • silica aerogels and methods of forming silica aerogels but it will be understood that the descriptions contained herein and the teaching provided herein apply also to aerogels generally and to cellulous aerogel, polyimide aerogel and TiO2 aerogel and like materials specifically.
  • step (c) before, during or after said providing step (a) and/or said preparing step (b) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage;
  • the present invention includes method of making aerogels in numerous and varied forms and sizes
  • the methods of forming an aerogel according to the present invention provide methods of forming monolithic aerogel, such methods comprising:
  • step (c) before, during or after said preparing step (b) and/or said providing step (a) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage;
  • the methods of forming an aerogel according to the present invention provide methods of forming relatively large, monolithic aerogels, such methods comprising:
  • step (c) before, during or after said solvent preparing step (b) and/or said providing step (a) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage;
  • the present invention also provides methods of forming a silica aerogel comprising:
  • step (c) before, during or after said providing step (a) and/or said preparing step (b) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage;
  • Aerogel What remains when the liquid part of a wet-gel like material (including materials formed by sol-gel processes) is removed without substantially damaging the solid part, including any solid part formed from silica, cellulous, polyimide, TiO2 and the like. Aerogels as produced herein generally retain the original shape of the wet-gel like material and at least 50% (typically great than about 85%) of the wet-gel like material's volume.
  • a condensation reaction occurs when two metal hydroxides (M-OH+HO-M) combine to give a metal oxide species (M-O-M) .
  • M-OH+HO-M metal hydroxides
  • M-O-M metal oxide species
  • a cross-linking agent can be an organic or an inorganic compound that forms a bond with a reactive side group accessible on a sol-gel like material to form a cross-linked sol-gel like material that can be dried to form a cross-linked aerogel.
  • Cross-linked aerogel an aerogel having at least two side groups that are linked by a cross-linking agent that forms a bond with the side groups.
  • Gel Point The point in time at which the network of linked oxide particles spans the container holding the liquid part of the wet-gel material. In a sol-gel process, the sol becomes a gel or gel-like material at the gel point.
  • Specific Surface Area The specific surface area as measured using a Micromeritics ASAP2420 in which the degassing temperature is about 150°C and in which the time is 4 about hours.
  • Substantial proportion As used in connection with the volume of fluid means at least 50%by volume.
  • Supercritical fluid A substance that is above its critical pressure and critical temperature. A supercritical fluid possesses some properties in common with liquids (density, thermal conductivity) and some in common with gases (fills its container) .
  • the present invention relates to processes for the preparation of aerogels.
  • a “wet gel” is provided that has been aged.
  • the term “wet gel” refers to a gel consisting essentially of a solid matrix or network, preferably of metal oxide particles, preferably comprising, consisting essentially of or consisting of silica particles, and a liquid material that is contained in the matrix or network in a gel structure.
  • wet gels are frequently and most commonly formed by sol-gel type processes.
  • a liquid continuous phase is formed to contain a solid disperse phase, and such sols are frequently formed by either (i) growing the disperse solid phase in situ; or (ii) synthesizing solid nano-sized particles which are then dispersed into the liquid phase.
  • sols are frequently formed by either (i) growing the disperse solid phase in situ; or (ii) synthesizing solid nano-sized particles which are then dispersed into the liquid phase.
  • Sols can be processed to form the wet, aged gels of the present invention.
  • a sol is converted to a gel when the solid nanoparticles dispersed in the liquid potion of the sol join together to form a network of particles that spans the liquid.
  • this forming of the network occurs as a result of reactive surface groups on the nanoparticles that condense together to form bonds, and in other cases a binding agent may be added to help bond the particles to form the network.
  • the sol transitions to a gel in which the solid is the continuous phase and the liquid is held within the solid network structure.
  • the viscosity of the sol increases and eventually approaches infinity (i.e., a “wet gel” is formed) .
  • the point in time when the particle network extends across essentially the entire volume of the liquid causing it to immobilize is called the gel point.
  • the gel-point does not necessarily define the final desired properties of the solid network being formed, such as the strength and porosity of the solid network.
  • the formation of a wet gel in which the solid network has the final desired properties, such as network strength and pore size, may include additional steps, such as adding additional catalyst and/or water and/or other ingredients intend to achieve the desired properties, together with adjustments to other sol conditions such as temperature and the time required to allow the required properties to develop.
  • Such post-gel-point operations and processing is referred to herein broadly as an ageing process, and the end product thereof is referred to as a “wet, aged gel.
  • aged gels in which the liquid is predominantly water are referred to as an aged hydrogel
  • aged gels in which the liquid is predominantly alcohol (s) are referred to as an aged alcogel.
  • wet gels which have large proportions (i.e., greater than 20%) of each of water and alcohols are referred to herein for convenience as hydro/alcogels.
  • the present invention includes processes for the preparation of aerogels.
  • aerogels have been formed as relatively small, discrete particles (including particles embedded in or coated onto fiberous materials, such as blankets and the like) , as thin films, and as relatively large, monolithic aerogels of various shapes and sizes, and all such forms can be made in accordance with the present methods.
  • the term “monolithic” means the aerogel is in substantially the shape in which it was converted from a wet-gel to aerogel, such as would occur for example by forming the aerogel by placing the wet-gel or sol-gel in a mold and then forming the aerogel in the mold, and to cut or sliced pieces for blocks thereof.
  • aerogels that are formed as plurality of small particles and then deposited and adhered to a blanket would not be within the meaning of a monolithic aerogel.
  • the present invention produces surprising and unexpected results, especially in connection the formation of monolithic aerogels, relating to the ability to form high porosity, monolithic articles and shapes that retain sufficient strength to avoid fracturing, breaking or substantial densification during the step of removing the pore-liquid of the present invention from the wet-gel of the present invention.
  • This surprising and unexpected result of the present methods permits the formation of important monolithic articles having a hard-to-achieve combination of desirable properties, including very high thermal and/or electrical insulating value, high light transmittance and relatively high strength, using a process that does not require supercritical drying and has numerous and beneficial processing advantages compared to prior super-and sub-critical processes.
  • One article that can advantageously formed according to the present methods which heretofore has not been practically formed in large-scale commercial production, is insulating, monolithic aerogel with high levels of light transmittance for use as window panes in residential windows, office business windows and the like.
  • the present invention includes processes for the preparation of aerogels having a porosity of at least about 85%, more preferably at least about 90%, and even more preferably at least about 95%.
  • the present invention includes processes for the preparation aerogels, and preferably of monolithic aerogels having a volume of at least about 80 cm 3 , having; (i) a density of not greater than about 2.5 g/cm3; (ii) specific surface area of from about 450 m2/g to about 1000 m2/g as measured by BET; (iii) a porosity of at least about 50%; and (iv) a mean pore size of from about 1 to about 20 nm.
  • the present invention includes processes for the preparation monolithic aerogels having a volume of at least about 80 cm 3 , a specific surface area of from about 450 m2/g to about 1000 m2/g as measured by BET, a porosity of at least about 50%, more preferably at least about 55%, and even more preferably at least about 60%.
  • the processes of the present invention generally includes the steps of:
  • wet silica gels have been prepared, for example, by the aqueous condensation of sodium (or potassium) silicate in an acid medium (see for example S. S. Kistler, J. Phys Chem., 256 (1932) , pp. 52-64) .
  • This is one efficient method of forming a wet gel, but the salts formed as by-products inside the gel have to be removed by several washing cycles, which can be a time-consuming operation and therefore can be disadvantageous.
  • sol-gel processes use a sol-gel approach that helps to avoid the formation of undesirable by-products, and such processes are generally preferred herein, as disclosed in US Patent No. 3,672,833, which is incorporated herein by reference.
  • a colloidal solution i.e. a sol
  • the inorganic polymerization reaction is carried out to obtain a gel, as described above.
  • any molecule that can undergo hydrolysis and polycondensation and so form reactive “inorganic” monomers or oligomers can be used as a precursor in the sol-gel process (see for example R. K. Iller: “The Chemistry of Silica” , published by Wiley in New York in 1979 (given here as one of the references) ) .
  • Typical precursors are metal alkoxides with the general formula M (OR) n, which suitably act as a source of “inorganic” monomers, with the advantage of being soluble in the usual organic solvents.
  • alkoxides are both hydrolysed and condensed to form polymeric species with metal-oxygen-metal bonds [see for example H.J. Schmidt, Non-Cryst. Solids, No.
  • R is an alkyl group, and in certain preferred embodiments R is C2H5.
  • the sol forms a porous solid matrix that remains in the residual liquid reaction mixture.
  • One advantage of this process is that it is readily adaptable to form the the wet gel is a monolithic body by placing the sol in a mold having the desired shape of aerosol to be produced, and then producing a wet gel having basically the same shape and size as the mold used.
  • the gel should undergo an ageing process in which the wet gel is left for a period of time period, preferably at a temperature above room temperature, to help grow the solid portion of the gel to its final strength in which the backbone structure is reinforced relative to its condition at the gel-point.
  • the aging temperature includes temperatures in the range of from about 50°C to about 100°C, and more preferably in 60°C to 85°C, and the aging time is from 0.1 hours to 48 hours, and more specifically, 0.5 hours to 24 hours.
  • the product formed has relatively low levels of chemically impurities and is and is highly homogeneous, the composition can be chosen from a wide range, relatively low temperatures are needed, and monolithic pieces with more or less the required shape can be obtained.
  • one difficulty that it has been generally found in connection with such processes is the extremely difficulty of converting such wet gels, even after aging, to relatively large monolithic pieces that have a high porosity and which substantially retain the size and shape of the mold with no cracks and no substantial shrinkage. This because in processes previously used cracks are generally formed during the process of removing the liquid of the aged wet gel at sub-critical conditions of the liquid solvent in the gel.
  • the sub-critical cracking and shrinking associated with prior processes is advantageously and unexpectedly avoided while using a process that is much less costly and time-consuming than super-critical processing and at the same time safer than prior solvent drying techniques.
  • aerogel preparation methods which include the step of preparing the aged wet gel for sub-critical drying by replacing a substantial portion by volume of the liquid component of wet gel, and in preferred embodiments substantially all of the liquid component of wet gel, with a pore-replacing fluid that avoids the difficulties of those used according to prior processes.
  • the present invention thus includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise- height method of less than 20 dynes per centimeter while at the same time having normal boiling point of less than 50C.
  • the present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 20 dynes per centimeter while at the same time having normal boiling point of less than about 40C.
  • the present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 20 dynes per centimeter while at the same time having normal boiling point of less than about 20C.
  • the present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 50C.
  • the present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 40C.
  • the present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 30C.
  • the present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 30C.
  • the present invention also includes processes in which the pore-replacing fluid has a vapor thermal conductivity as measured at 20C by of less than about 15 W/mK, more preferably of about 10 W/mK or less.
  • Examples of preferred pore-replacing fluids of the present invention thus include fluids comprise at least 50%by weight, or at least 60%by weight, or at least 70%by weight, or at least 80%by weight, or at least 90%by weight, or consists essentially of, or consists of one or more of the following components, which have the surface tension, boings point and thermal conductivity properties as indicated:
  • the pore-replacing fluid of the present invention comprises at least 50%by weight, or at least 60%by weight, or at least 70%by weight, or at least 80%by weight, or at least 90%by weight, or consists essentially of, or consists of one HCFO-1233zd (E) .
  • the pore-replacing fluid of the present invention comprises at least 50%by weight, or at least 60%by weight, or at least 70%by weight, or at least 80%by weight, or at least 90%by weight, or consists essentially of, or consists of one HCFO-1233zd (E) .
  • the pore-replacing fluid of the present invention comprises, consists essentially of or consists of a combination of HCFO-1233zd (E) and isopropanol.
  • the pore-replacing fluid of the present invention comprises, consists essentially of or consists of a combination of from about 50 percent to about 90 percent by volume of HCFO-1233zd (E) and from about 10 percent to about 50 percent by volume of isopropanol.
  • the replacement step is easily and relatively quickly achieved, without negatively effecting the strength or structure of the solid silica network or backbone, by immersing the wet gel in the replacement fluid under ambient temperatures (or in some cases moderately elevated temperatures) and ambient pressures, preferably and especially in embodiments in which the wet gel comprises a liquid component that comprises an alcohol (i.e., the wet gel is an alcogel) or an alchol/water mixture (i.e., the wet gel is an hydro/alcogel) .
  • the wet gel comprises a liquid component that comprises an alcohol (i.e., the wet gel is an alcogel) or an alchol/water mixture (i.e., the wet gel is an hydro/alcogel) .
  • a main reactor or mold 10 is provided with an aged, wet aerogel contained therein and as such the pores of the aged aerogel contain a liquid solvent, preferably an alcohol or an alcohol/water solvent.
  • the temperature/pressure condition of the reactor/mold is brought to within the range preferred for the replacement step of the present invention, and this can be achieved, for example, by simply pumping the selected pore-filling fluid of the present invention into the reactor/mold. More specifically, the reactor/mold is at a pressure of less than about 150 psia, or preferably at a pressure of less than about 90 psi, or preferably less than about 60 psia and even more preferably less than about less than about 30 psia.
  • the reactor/mold is at a pressure of less than about 150 psia and a temperature of less than about 100°C, or preferably at a pressure of less than about 90 psi and a temperature less than about 80°C, or preferably less than about 60 psia and at a temperature less than about 60°C, or preferably less than about less than about 30 psia and a temperature less than about 50°.
  • the reactor/mold is at a temperature of less than about 100°C, or preferably at a temperature less than about 80°C, or preferably at a temperature less than about 60°C, or preferably at a temperature less than about 50° or preferably at a temperature less than about 50°.
  • the reactor/mold is at about room temperature, and in such cases the pressure in the mold/reactor will correspond preferably to the saturation pressure of the pore-filling fluid chosen in accordance with the present invention.
  • the reactor/mold will be only slightly above ambient pressure (less than about 0.3 MPa) .
  • the preferred methods of the present invention utilize pore-fillin fluids as described herein to achieve reactor/mold pressures that are only slightly above atmospheric pressure preferably, less than about 2 atmospheres.
  • the steps of the present methods can be carried out either continuously, batch –wise, or in a combination of continuous and batch wise processes.
  • the pore-filling fluid is pumped continuously into the reactor/mold under conditions which allow a continuous withdrawal of a waste liquid comprising a mixture of the pore-filling fluid and the solvent material which had been in the pores of the aerogel, and in such a case of the continuous operation is maintained for a period sufficient to achieve and aerogel in which a substantial proportion of the fluid in the pores, and in preferred embodiments substantially all of the fluid in the pores, is the pore-filling fluid of the present invention.
  • the step of replacing the solvent can occur in a one or more (preferably a series) of batch–wise operations, for example, by adding the reactor/mold with a predetermined batch of the pore–filling fluid and allowing it to soak for a period of time.
  • a proportion of the solvent contained in the pores of the aerogel will be replaced by the pore-filing fluid according to the present invention, and as a consequence the fluid surrounding the aerogel will contain a mixture of pore-filling fluid and the solvent material, which is referred to hereinafter for the purposes of convenience as “waste fluid. ” .
  • the waste fluid is withdrawn from the reactor/mold, and in those embodiments in which a series of batch operations are used to conduct the replacement step a second batch of pore-replacement fluid is added to the reactor/mold once the waste fluid from the previous batch has been removed.
  • the waste fluid is subjected to a separation step 11 in which the pore–filling fluid of the present invention is separated from the solvent that has been removed from the aerogel.
  • the separation step produces a recycle stream of pore-filling fluid according to the present invention in which less than 10%by weight, even more preferably less than 5%by weight, and even more preferably less than 2%by weight of the recycle stream is the solvent that has been removed from the pores of the aerogel.
  • One advantageous feature of the present invention is that the pore-filing fluids of the present invention are easily separated from solvents that are contained in the pores of the aerogel and thus permits a highly efficient and effective recycle stream which can be returned to the source of the pore-filling fluid, such as tank 12.
  • the separation step can be achieved by simply heating under relatively low pressure, for example atmospheric pressure or slightly above, the waste liquid to a temperature of about 30 –35°C to form a vapor that will be relatively rich in the pore-filing fluid, and then condensing such enriched stream to produce a liquid stream having a relatively higher concentration of pore-filling fluid.
  • a step can be repeated over a series of temperatures and pressures, (for example as would occur in a distillation column) to achieve a relatively purified stream of pore-filling fluid for recycle for use again in the process.
  • the waste-fluid that has been removed can be sent to a waste tank 13 and/or for further processing, such as possible further separation of solvent and pore-fluid to generate additional recycle streams and/or incineration, and the like.
  • the replacement step is conducted for a time sufficient to ensure that a substantial proportion, and preferably at least about 75 volume percent, or at least about 85 volume percent, or at least about 95 volume percent of the pore volume of the aerogel contains the pore-filling fluid of the present invention.
  • a substantial proportion and preferably at least about 75 volume percent, or at least about 85 volume percent, or at least about 95 volume percent of the pore volume of the aerogel contains the pore-filling fluid of the present invention.
  • the extent of replacement is monitored by monitoring the relative concentration of the pore-filling fluid in waste liquid leaving the reactor/mold.
  • the concentration of the pore-filling fluid in the stream is believed to be an approximate representation of the volume concentration of the pore volume occupied by the pore-filling fluid in the aerogel, and accordingly in preferred embodiments this concentration is monitored and the removal step is continued until the concentration of pore-filling fluid in the waste stream is at or above a predetermined level, preferably at or above 50%by weight, were preferably at or above 75%by weight, or preferably at or above 85%by weight, or preferably at or above 95%by weight based on the total weight of the waste liquid stream.
  • the replacing step of the present invention can be discontinued and the removing step can be initiated.
  • the aerogel preferably in a reactor/mold
  • the removal step comprises evaporating the pore-filling fluid, preferably by raising the temperature of the aerogel.
  • One preferred embodiment of evaporating the pore-filling fluid comprises introducing a relatively hot, inert gas (such as nitrogen) into the reactor/mold containing the aerogel so as to vaporize a substantial proportion of the pore-filling fluid of the present invention under subcritical conditions.
  • a relatively hot, inert gas such as nitrogen
  • preferred embodiments are conducted such that during the drying step the temperature/pressure of the drying gas is higher than the saturation temperature pressure of the pore-filling fluid so as to cause evaporation of the pore-filling fluid while at the same time being below the critical conditions of the pore-containing fluid.
  • the aerogel remains at a pressure of less than about 150 psia, or preferably at a pressure of less than about 90 psi, or preferably less than about 60 psia and even more preferably less than about less than about 30 psia.
  • the aero during the drying step is at a pressure of less than about 150 psia and a temperature of less than about 100°C, or preferably at a pressure of less than about 90 psi and a temperature less than about 80°C, or preferably less than about 60 psia and at a temperature less than about 60°C, or preferably less than about less than about 30 psia and a temperature less than about 50°.
  • the aerogel during the drying step is at a temperature of less than about 100°C, or preferably at a temperature less than about 80°C, or preferably at a temperature less than about 60°C, or preferably at a temperature less than about 50° or preferably at a temperature less than about 50°.
  • a wet gel consisting of tetraethyl orthosilicate (TEOS -Si (OC2H5) ) as the source of silica and tetrabutylammonium hydroxide as a base catalyst in a solvent comprising isopropanol and water is formed.
  • TEOS -Si tetraethyl orthosilicate
  • O2H5 tetraethyl orthosilicate
  • HMDS hexamethyldisilazane
  • the washed, aged gel having ethanol substantially filling the pores, which defined a starting wet volume, is then dried by placing the wet gel in an oven at 100C and ambient pressure for a period of approximately one (1) hour and the drying was done at ambient pressure in air.
  • This subcritical drying operation resulted in substantial fracturing of the gel as it dried into many pieces much smaller than the starting volume of wet gel.
  • Comparative Example 1 was repeated except that the washed, aged gel having ethanol substantially filling the pores was provided for use in a replacement step according to the present invention. Specifically, the aged, wet gel produced in Comparative Example 1 was soaked for about 24 hours in liquid transHFCO-1233zd at room temperature and pressure, and then the waste liquid containing a mixture of transHFCO-1233zd and ethanol was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel. The wet gel was soaked in this second batch for 24 hours and the waste liquid was again removed and a third batch of transHFCO-1233zd was used to soak the wet gel for three days.
  • the waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the third soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for four hours.
  • the waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the fourth soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for final soaking period of 17 hours.
  • the waste liquid that from this fifth batch was removed and found to have the following relative volume percentages of transHFCO-1233zd of 96.88%and ethanol of 2.34%and IPA of 0.78%. As a result of this replacement process, a substantial proportion of the pore volume of the gel contains transHFCO-1233zd.
  • the gel that was subject to the replacement step was then dried by placing the gel in a heated oven at a temperature of approximately 35C for about 2 hrs. After the replacing and drying step the size and shape of the dried gel substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step.
  • the aerogel thus produced was tested and found to have the following properties:
  • Comparative Example 1 was repeated except in two respects. First, the gel was not silyated with hexamethyldisilazane (HMDS) , and second the washed, aged gel having ethanol substantially filling the pores was provided for use in a replacement step according to the present invention. Specifically, the aged, wet gel produced in as in Comparative Example 1, but without the strengthening step, was soaked for about 24 hours in liquid transHFCO-1233zd at room temperature and pressure, and then the waste liquid containing a mixture of transHFCO-1233zd and ethanol was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel.
  • HMDS hexamethyldisilazane
  • the wet gel was soaked in this second batch for 24 hours and the waste liquid was again removed and a third batch of transHFCO-1233zd was used to soak the wet gel for three days.
  • the waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the third soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for four hours.
  • the waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the fourth soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for final soaking period of 17 hours.
  • the waste liquid that from this fifth batch was removed and found to have approximately the same relative percentages of transHFCO-1233zd as reported in Example 1.
  • the pore volume of the gel contains transHFCO-1233zd.
  • the gel that was subject to the replacement step was then dried by placing the gel in in a closed container a heated maintained at about 35C for 2hrs and then increased to 60C for 2hrs and finally increased to 180C for 1hr. After the replacing and drying step the size and shape of the dried gel substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step.
  • the aerogel thus produced was tested and found to have the following properties:
  • the replacement step of the present invention involves a series of six (6) one hour soakings of the alcogel in a liquid bath of transHFCO-1233zd at room temperature and pressure. After each one hour soaking in the liquid bath of transHFCO-1233zd, the bath liquid (containing a mixture of transHFCO-1233zd and materials to be removed such as ethanol and IPA) is removed and a fresh batch of transHFCO-1233zd liquid was used to soak the wet gel. As a result of this replacement process, a substantial proportion of the pore volume of the gel contains transHFCO-1233zd.
  • the gel after the above-described replacement step has a liquid with the following approximate concentrations by volume of components in the cells:
  • the gel was then dried by first placing the gel in a closed container overnight at about room temperature. A pressure of about 1.2 atmospheres results in the container based on the saturation pressure of transHFCO-1233zd. The container is then placed in an oven maintained at a temperature of about 35°C for 1hr. After the replacing and drying step the size and shape of the dried gel has substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step.
  • the aerogel thus produced was tested and found to have the following properties:
  • a prepared, transparent, aged alcogel (with ethanol in the pores) was provided.
  • the gel is placed in a vessel which has a continuous flow of fresh liquid comprising about 70%by volume of transHFCO-1233zd and 30%by volume of IPA is provided to the vessel to maintain a liquid level above the height of the gel in the vessel while continuously removing a liquid stream containing transHFCO-1233zd and dissolved/carried impurities (including ethanol) .
  • the output liquid stream from the vessel was processed to remove the impurities and to recycle transHFCO-1233zd/IPA to use as fresh transHFCO-1233zd/IPA. This continuous replacement process is continued for about 3 hours.
  • the pore volume of the gel contains the transHFCO-1233zd/IPA mixture.
  • the gel after the above-described replacement step has a liquid that is estimated to be approximate 98 –99%by volume of 1233zd/IPA.
  • the gel was then dried by first placing the gel in a closed container overnight at about room temperature. A pressure of about 1.2 atmospheres results in the container based on the saturation pressure of transHFCO-1233zd. The container is then placed in an oven maintained at a temperature of about 35°C for 1hr. After the replacing and drying step the size and shape of the dried gel has substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step.
  • the aerogel thus produced was tested and found to have the following properties:

Abstract

Provided is a method for forming an aerogel comprising steps of: a) providing an aged hydrogel or an aged alcogel and/or an aged hydro/alcogel; (b) replacing a substantial portion of the solvent of the hydrogel, alcogel and/or hydro/alcogel with a pore-replacement fluid comprising trans-1-chloro-3,3,3-trifluoro-propene (HCFO-1233zd(E)), cis-1-chloro-3,3,3-trifluoro-propene (HCFO-1233zd(Z)), cis-1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz(Z)), 1-methoxyheptafluoropropane, and combinations of these; and (c) vaporizing the pore-replacement fluid under a pressure of about 150 psia or less to remove a substantial portion of the pore-replacement fluid from the gel and produce the aerogel.

Description

METHODS FOR FORMING AEROGELS BACKGROUND
Field of Invention
The present invention relates to methods forming aerogels, the aerogels formed thereby, and articles containing the aerogels.
Background
Aerogels, in general, are materials that are chemically intert and highly porous ceramics.
Generally, aerogels are produced by forming a gel. As is known, a gel is a nanostructured network of interconnected particles that spans the volume of a liquid medium. Gels have some properties like liquids, such as density, and some properties like solids, such as a fixed shape. Because the pores in a gel are nano-sized, the capillary forces exerted by the liquid are strong enough to hold it inside the gel and prevent the liquid from simply flowing out.
In order to form the solid framework of a gel, one or more chemical reactions are required. For example, WO92/20623 notes that sodium silicate-hydrate was used as starting material for the preparation of silica aerogel, and that the process involved the formation of a gel in which the liquid was water (known as an aquagel or hydrogel) as a result of using hydrogen chloride to catalyze the reaction between the silicate and water. WO92/20623 explains that such a route involves chemical reactions that generally leave impurities in the water portion of the gel that must be removed in order to achieve the advantageous properties of the finished aerogel material. Removal of the impurities is  typically done by soaking the gel in r a solvent, such as water, acetone and acetonitrile, allowing impurities to diffuse out and allowing the chosen solvent to diffuse into the pores. The solvent in which the gel is soaked is typically exchanged with fresh solvent multiple times over the course several days in order to effect the removal of the impurities. This is typically called a wash process.
WO92/20623 also describes another process for forming an aerogel in which tetramethoxysilane, Si (OCH 3)  4 (also sometimes referred to herein for convenience as , TMOS) , has also been used as starting material for the preparation of silica aerogel. TMOS is described as being a suitable starting material for an aerogel process since it is easy to handle, easy to prepare in pure form and easy to hydrolyse. A gel is formed in which the liquid is alchohol (i.e., methanol) since formation of the solid network takes place by a direct acid-and/or base-catalysed hydrolysis of TMOS. This is known as a sol-gel process and produces an “alcogel, ” that is, a silica skeleton surrounded by aqueous methanol. This approach has been extended to prepare aerogels from a wide variety of metal oxide aerogels.
In either of the processes described above, the solvent that makes up the liquid portion of the gel must be removed from the solid skeleton or framework to obtain an aerogel in which the pores are filed with gas, usually air but also in some cases other gases such as nitrogen, and in which the solid framework portion of the liquid-containing gel remains substantially intact, that is, without substantial collapse or shrinkage. Removal of the solvent while preserving the porous solid structure has presented problems because the solid portion of the gel structure has a tendency to shrink upon removal of the solvent. This shrinkage, which in turn causes the porous solid structure to  crack, and/or break and/or densify, is believed by applicants to be caused by capillary forces of the liquid solvent that act upon the solid network structure as the liquid solvent evaporates. An undesirable result can be a finished product in which the porosity is less than desired, and in some cases less than 50%.
One approach to overcoming this problem has been to transform a solvent within the gel into a vapor above its critical point and then allowing the supercritical vapor to escape and leave the intact porous solid structure. Such a process has been used to produce an aerogel that was transparent, low density, and highly porous. In some cases, such a process involved exchanging the water (in the case of hydrogels) with a hydrocarbon or alcohol or other material that was more suitable for supercritical removal from the gel. US Patent 3,672,833 describes such a process. Applicants have come to appreciate that such a supercritical process has substantial drawbacks. For example, higher pressures and potentially high temperatures must be used to achieve supercritical removal of the liquid form the pores of the gel, and such operations are costly from an equipment stand-point, expensive from an operations standpoint (due to the need to achieve high temperatures and/or high pressures in the process) , and potentially dangerous in view of the use of such high temperature and pressure environments, especially when flammable materials are used as the solvents.
An alternative to supercritical drying has been proposed, for example, in US 5,966,832. In the process disclosed in the US 5,966,832, subcritical drying is said to take place in a relatively complex arrangement that maintains the drying solvent, which can comprise ethyl alcohol (i.e., ethanol) , iso-propanol, iso-butanol, 2-pentanol, 2, 2, 4-trimethylpentane, water, and mixtures thereof, at elevated temperatures and pressures,  although the pressures is said to be maintained below the critical pressure of the solvent. However, applicants have found that this process and similar subcritical process are still lacking in that complex equipment is still used and/or high pressures are still used and/or that a vapor liquid interface is still formed in the drying process that causes substantial detrimental shrinkage of the gel upon drying.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic illustration of process according to one embodiment of the present invention.
SUMMARY OF THE INVENTION
Applicants have found that one or more of the above-noted disadvantages and failings of the prior techniques can be overcome by methods of forming an aerogel comprising:
(a) providing an aged hydrogel or an aged alcogel and/or an aged hydro/alcogel;
(b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing a substantial portion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a pore-replacement fluid comprising, consisting essentially of, or consisting of a fluid selected from the group consisting of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) , cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) , cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) , 1-methoxyheptafluoropropane, and combinations of these, to produce an aged gel in which the liquid portion thereof comprises in substantial  proportion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume of said pore-replacement fluid;
(c) before, during or after said providing step (a) and/or said preparing step (b) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
(d) vaporizing said pore-replacement fluid under a pressure of about 150 psia or less to remove a substantial portion of said pore-replacement fluid from said gel and produce an aerogel.
Applicants invention is directed in the broad sense to all aerogels, and this includes aerogels in which the solid portion is formed from a variety of known materials, including silica, cellulous, polyimide and TiO2 to produce silica aerogel, cellulous aerogel, polyimide aerogel and TiO2 aerogel, among others. For the purposes of convenience, the remaining description hereof refers in specific to silica aerogels and methods of forming silica aerogels, but it will be understood that the descriptions contained herein and the teaching provided herein apply also to aerogels generally and to cellulous aerogel, polyimide aerogel and TiO2 aerogel and like materials specifically.
Applicants have found that one or more of the above-noted disadvantages and failings of the prior techniques can be overcome by methods of forming a silica aerogel comprising:
(a) providing an aged hydrogel or an aged alcogel and/or an aged hydro/alcogel;
(b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing a substantial portion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95% by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a pore-replacement fluid comprising, consisting essentially of, or consisting of a fluid selected from the group consisting of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) , cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) , cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) , 1-methoxyheptafluoropropane, and combinations of these, to produce an aged gel in which the liquid portion thereof comprises in substantial proportion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume said pore-replacement fluid;
(c) before, during or after said providing step (a) and/or said preparing step (b) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
(d) vaporizing said pore-replacement fluid under a pressure of about 150 psia or less to remove a substantial portion ofsaid pore-replacement fluid from said gel and produce an aerogel.
Although the present invention includes method of making aerogels in numerous and varied forms and sizes, in preferred embodiments the methods of forming an aerogel according to the present invention provide methods of forming monolithic aerogel, such methods comprising:
(a) providing an aged hydrogel, alcogel and/or hydro/alcogel having a volume of at least about 8 cubic centimeters ;
(b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing a substantial portion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95% by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a fluid selected from the group consisting of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) , cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) , cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) , 1-methoxyheptafluoropropane, and combinations of these, to produce an aged gel in which the liquid portion thereof comprises in substantial proportion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume of said liquid in said aged gel, is said pore-replacement fluid;
(c) before, during or after said preparing step (b) and/or said providing step (a) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
(d) vaporizing said HCFO-1233zd (E) under a pressure of less than about 30 psia to remove a substantial proportion of said pore-replacement fluid from said gel and produce an aerogel.
In preferred embodiments the methods of forming an aerogel according to the present invention provide methods of forming relatively large, monolithic aerogels, such methods comprising:
(a) providing an aged hydrogel, alcogel and/or hydro/alcogel in a mold having a volume of at least about at least about 80 cubic centimeters, more preferably at least about 100 cubic centimeters;
(b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing a substantial portion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95% by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel in said mold with trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) to produce an aged gel in which the liquid portion thereof comprises in substantial proportion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume of said liquid in said aged gel, is HCFO-1233zd (E) ;
(c) before, during or after said solvent preparing step (b) and/or said providing step (a) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
(d) vaporizing said HCFO-1233zd (E) under a pressure of less than about 30 psia to remove substantially all of said HCFO-1233zd (E) from said gel and produce a monolithic aerogel.
The present invention also provides methods of forming a silica aerogel comprising:
(a) providing an aged hydrogel or an aged alcogel and/or an aged hydro/alcogel;
(b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing a substantial portion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a pore-replacement fluid comprising, consisting essentially of, or consisting of a fluid selected from the group consisting of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) ; cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) ; cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) ; 1-methoxyheptafluoropropane; 1, 1, 1, 2-tetrafluoroethane; 1, 1, 1, 2, 3, 3, 3-heptafluoropropane; 1, 1, 1, 3, 3, 3-hexafluoropropane; pentafluoroethane;  2-chloro-1, 1, 1, 2-tetrafluoroethane; trifluoromethane, chlorodifluoromethane and combinations these, to produce an aged gel in which the liquid portion thereof comprises in substantial proportion, and preferably at least 50%by volume, more preferably at least 80%by volume, and even more preferably at least 95%by volume said pore-replacement fluid;
(c) before, during or after said providing step (a) and/or said preparing step (b) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
(d) vaporizing said pore-replacement fluid under a pressure of about 150 psia or less to remove a substantial portion of said pore-replacement fluid from said gel and produce an aerogel.
DETAILED DESCRIPTION
DEFINITIONS
As used herein, the following terms have the indicated meaning unless specifically indicated otherwise herein.
Aerogel: What remains when the liquid part of a wet-gel like material (including materials formed by sol-gel processes) is removed without substantially damaging the solid part, including any solid part formed from silica, cellulous, polyimide, TiO2 and the like. Aerogels as produced herein generally retain the original shape of the wet-gel like material and at least 50% (typically great than about 85%) of the wet-gel like material's volume.
Condensation: A condensation reaction occurs when two metal hydroxides (M-OH+HO-M) combine to give a metal oxide species (M-O-M) . The reaction forms one water molecule.
Cross-linking agent: A cross-linking agent can be an organic or an inorganic compound that forms a bond with a reactive side group accessible on a sol-gel like material to form a cross-linked sol-gel like material that can be dried to form a cross-linked aerogel.
Cross-linked aerogel: an aerogel having at least two side groups that are linked by a cross-linking agent that forms a bond with the side groups.
Gel Point: The point in time at which the network of linked oxide particles spans the container holding the liquid part of the wet-gel material. In a sol-gel process, the sol becomes a gel or gel-like material at the gel point.
Hydrolysis: The reaction of a metal alkoxide (M-OR) with water, forming a metal hydroxide (M-OH) .
Specific Surface Area: The specific surface area as measured using a Micromeritics ASAP2420 in which the degassing temperature is about 150℃ and in which the time is 4 about hours.
Substantial proportion: As used in connection with the volume of fluid means at least 50%by volume.
Supercritical fluid: A substance that is above its critical pressure and critical temperature. A supercritical fluid possesses some properties in common with liquids (density, thermal conductivity) and some in common with gases (fills its container) .
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The present invention relates to processes for the preparation of aerogels. According to the processes of the present invention, a “wet gel” is provided that has been aged. As used herein, the term “wet gel” refers to a gel consisting essentially of a solid matrix or network, preferably of metal oxide particles, preferably comprising, consisting essentially of or consisting of silica particles, and a liquid material that is contained in the matrix or network in a gel structure.
As will be appreciated by those skilled in the art, wet gels are frequently and most commonly formed by sol-gel type processes. In such processes generally, a liquid continuous phase is formed to contain a solid disperse phase, and such sols are frequently formed by either (i) growing the disperse solid phase in situ; or (ii) synthesizing solid nano-sized particles which are then dispersed into the liquid phase. Both such processes, and others that are known and available, can be used to form a wet gel according to the present methods.
Sols can be processed to form the wet, aged gels of the present invention. As is known, a sol is converted to a gel when the solid nanoparticles dispersed in the liquid potion of the sol join together to form a network of particles that spans the liquid. In some cases, this forming of the network occurs as a result of reactive surface groups on the nanoparticles that condense together to form bonds, and in other cases a binding agent may be added to help bond the particles to form the network. As this process of forming the network of solid proceeds, the sol transitions to a gel in which the solid is the continuous phase and the liquid is held within the solid network structure. During such gelation process, the viscosity of the sol increases and eventually approaches infinity (i.e.,  a “wet gel” is formed) . The point in time when the particle network extends across essentially the entire volume of the liquid causing it to immobilize is called the gel point.
The gel-point does not necessarily define the final desired properties of the solid network being formed, such as the strength and porosity of the solid network. The formation of a wet gel in which the solid network has the final desired properties, such as network strength and pore size, may include additional steps, such as adding additional catalyst and/or water and/or other ingredients intend to achieve the desired properties, together with adjustments to other sol conditions such as temperature and the time required to allow the required properties to develop. Such post-gel-point operations and processing is referred to herein broadly as an ageing process, and the end product thereof is referred to as a “wet, aged gel. ” As mentioned above, wet, aged gels in which the liquid is predominantly water are referred to as an aged hydrogel, and aged gels in which the liquid is predominantly alcohol (s) are referred to as an aged alcogel. Wet gels which have large proportions (i.e., greater than 20%) of each of water and alcohols are referred to herein for convenience as hydro/alcogels.
The present invention includes processes for the preparation of aerogels. Those skilled in the art will appreciate that aerogels have been formed as relatively small, discrete particles (including particles embedded in or coated onto fiberous materials, such as blankets and the like) , as thin films, and as relatively large, monolithic aerogels of various shapes and sizes, and all such forms can be made in accordance with the present methods. As used herein with respect to aerogels, the term “monolithic” means the aerogel is in substantially the shape in which it was converted from a wet-gel to aerogel, such as would occur for example by forming the aerogel by placing the wet-gel or sol-gel  in a mold and then forming the aerogel in the mold, and to cut or sliced pieces for blocks thereof. In contra-distinction, aerogels that are formed as plurality of small particles and then deposited and adhered to a blanket would not be within the meaning of a monolithic aerogel.
The present invention produces surprising and unexpected results, especially in connection the formation of monolithic aerogels, relating to the ability to form high porosity, monolithic articles and shapes that retain sufficient strength to avoid fracturing, breaking or substantial densification during the step of removing the pore-liquid of the present invention from the wet-gel of the present invention. This surprising and unexpected result of the present methods permits the formation of important monolithic articles having a hard-to-achieve combination of desirable properties, including very high thermal and/or electrical insulating value, high light transmittance and relatively high strength, using a process that does not require supercritical drying and has numerous and beneficial processing advantages compared to prior super-and sub-critical processes. One article that can advantageously formed according to the present methods, which heretofore has not been practically formed in large-scale commercial production, is insulating, monolithic aerogel with high levels of light transmittance for use as window panes in residential windows, office business windows and the like.
The present invention includes processes for the preparation of aerogels having a porosity of at least about 85%, more preferably at least about 90%, and even more preferably at least about 95%.
The present invention includes processes for the preparation aerogels, and preferably of monolithic aerogels having a volume of at least about 80 cm 3, having; (i) a  density of not greater than about 2.5 g/cm3; (ii) specific surface area of from about 450 m2/g to about 1000 m2/g as measured by BET; (iii) a porosity of at least about 50%; and (iv) a mean pore size of from about 1 to about 20 nm.
The present invention includes processes for the preparation monolithic aerogels having a volume of at least about 80 cm 3, a specific surface area of from about 450 m2/g to about 1000 m2/g as measured by BET, a porosity of at least about 50%, more preferably at least about 55%, and even more preferably at least about 60%.
As mentioned above, the processes of the present invention generally includes the steps of:
(a) providing an aged hydrogel or alcogel and/or hydro/alcogel;
(b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing a substantial portion by volume of the solvent of said hydrogel, alcogel and/or hydro/alcogel with HCFO-1233zd (E) ;
(c) optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
(d) vaporizing said HCFO-1233zd (E) under a pressure of less than about 30 psia to remove substantially all of said HCFO-1233zd (E) from said gel and produce an aerogel.
Each of these steps is described in detail below
a) Providing The Aged Hydrogel or Alcogel and/or Hydro/Alcogel Gel
Wet silica gels have been prepared, for example, by the aqueous condensation of sodium (or potassium) silicate in an acid medium (see for example S. S. Kistler, J. Phys Chem., 256 (1932) , pp. 52-64) . This is one efficient method of forming a wet gel, but the  salts formed as by-products inside the gel have to be removed by several washing cycles, which can be a time-consuming operation and therefore can be disadvantageous.
Certain processes use a sol-gel approach that helps to avoid the formation of undesirable by-products, and such processes are generally preferred herein, as disclosed in US Patent No. 3,672,833, which is incorporated herein by reference. In such sol-gel processes, a colloidal solution (i.e. a sol) is used that contains water, solvents, catalysts and the precursors of vitreous or ceramic materials. In this colloidal solution the inorganic polymerization reaction is carried out to obtain a gel, as described above.
Any molecule that can undergo hydrolysis and polycondensation and so form reactive “inorganic” monomers or oligomers can be used as a precursor in the sol-gel process (see for example R. K. Iller: “The Chemistry of Silica” , published by Wiley in New York in 1979 (given here as one of the references) ) . Typical precursors are metal alkoxides with the general formula M (OR) n, which suitably act as a source of “inorganic” monomers, with the advantage of being soluble in the usual organic solvents. In solution, alkoxides are both hydrolysed and condensed to form polymeric species with metal-oxygen-metal bonds [see for example H.J. Schmidt, Non-Cryst. Solids, No. 100 (1988) , p. 51, which is incorporated herein by reference) . The functional groups involved participate in the following three reactions, which are generally used to describe the sol-gel process and which can be written generally for metal oxides but are written in the following as a case of a silicon alkoxide,  Si (OR) 4:
Figure PCTCN2018095817-appb-000001
where R is an alkyl group, and in certain preferred embodiments R is C2H5.
As a result of hydrolysis and polycondensation, the sol forms a porous solid matrix that remains in the residual liquid reaction mixture. One advantage of this process is that it is readily adaptable to form the the wet gel is a monolithic body by placing the sol in a mold having the desired shape of aerosol to be produced, and then producing a wet gel having basically the same shape and size as the mold used.
As mentioned above, after reaching the gel-point, the gel should undergo an ageing process in which the wet gel is left for a period of time period, preferably at a temperature above room temperature, to help grow the solid portion of the gel to its final strength in which the backbone structure is reinforced relative to its condition at the gel-point. In preferred embodiments the aging temperature includes temperatures in the  range of from about 50℃ to about 100℃, and more preferably in 60℃ to 85℃, and the aging time is from 0.1 hours to 48 hours, and more specifically, 0.5 hours to 24 hours.
One advantage of the sol-gel process as described above is that the product formed has relatively low levels of chemically impurities and is and is highly homogeneous, the composition can be chosen from a wide range, relatively low temperatures are needed, and monolithic pieces with more or less the required shape can be obtained. However, one difficulty that it has been generally found in connection with such processes is the extremely difficulty of converting such wet gels, even after aging, to relatively large monolithic pieces that have a high porosity and which substantially retain the size and shape of the mold with no cracks and no substantial shrinkage. This because in processes previously used cracks are generally formed during the process of removing the liquid of the aged wet gel at sub-critical conditions of the liquid solvent in the gel. While not intending to bound by or to any particular theory of operation, prior techniques of drying the solvent from the gel resulted in the exertion of relatively high levels of pressure and stress on the silica backbone as the liquid that filed the pores exerted on the walls of the pores as it was converted from its liquid to vapor form, which in many cases occurred at relatively high pressures and temperatures, even though those temperatures and pressures were below the critical values for the solvent. As a result, the process of removing the solvent by heating heretofore caused the solid network to collapse and/or shrink, thus producing cracks and/or other defects in the resulting aerogel, which disadvantageously greatly reduced the volume of the gel, leading to an undesirable loss of intrinsic porosity.
Without being bound by any particular theory of operation, and as described before, applicants have come to appreciate that the great difficulty of preparing monolithic pieces by the sol-gel method under subcritical conditions using prior techniques was the combination of several negative effects occurring during drying, including: (1) the generation of very high capillary pressure as a result of the solvent which is being removed having a high surface tension; (2) the need to heat and/or pressurize the wet gel during removal of the solvent, thus weakening the silica backbone. As explained in connection with the preparing step (b) and the vaporizing step (d) of the present invention below, the sub-critical cracking and shrinking associated with prior processes is advantageously and unexpectedly avoided while using a process that is much less costly and time-consuming than super-critical processing and at the same time safer than prior solvent drying techniques.
b) Preparing the Wet, Aged Hydrogel or Alcogel and/or Hydro/Alcogel Gel for Sub-Critical Drying
Applicants have unexpectedly found that the advantages described herein can be achieved by carrying out aerogel preparation methods which include the step of preparing the aged wet gel for sub-critical drying by replacing a substantial portion by volume of the liquid component of wet gel, and in preferred embodiments substantially all of the liquid component of wet gel, with a pore-replacing fluid that avoids the difficulties of those used according to prior processes.
The present invention thus includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise- height method of less than 20 dynes per centimeter while at the same time having normal boiling point of less than 50C.
The present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 20 dynes per centimeter while at the same time having normal boiling point of less than about 40C.
The present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 20 dynes per centimeter while at the same time having normal boiling point of less than about 20C.
The present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 50C.
The present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 40C.
The present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 30C.
The present invention also includes processes as described herein in which the pore-replacing fluid has a surface tension as measured by the differential capillary-rise-height method of less than 15 dynes per centimeter while at the same time having normal boiling point of less than about 30C. The present invention also includes processes in which the pore-replacing fluid has a vapor thermal conductivity as measured at 20C by of less than about 15 W/mK, more preferably of about 10 W/mK or less.
Examples of preferred pore-replacing fluids of the present invention thus include fluids comprise at least 50%by weight, or at least 60%by weight, or at least 70%by weight, or at least 80%by weight, or at least 90%by weight, or consists essentially of, or consists of one or more of the following components, which have the surface tension, boings point and thermal conductivity properties as indicated:
Figure PCTCN2018095817-appb-000002
In preferred embodiments, the pore-replacing fluid of the present invention comprises at least 50%by weight, or at least 60%by weight, or at least 70%by weight, or at least 80%by weight, or at least 90%by weight, or consists essentially of, or consists of one HCFO-1233zd (E) .
In preferred embodiments, the pore-replacing fluid of the present invention comprises at least 50%by weight, or at least 60%by weight, or at least 70%by weight, or at least 80%by weight, or at least 90%by weight, or consists essentially of, or consists of one HCFO-1233zd (E) .
In preferred embodiments, the pore-replacing fluid of the present invention comprises, consists essentially of or consists of a combination of HCFO-1233zd (E) and isopropanol.
In preferred embodiments, the pore-replacing fluid of the present invention comprises, consists essentially of or consists of a combination of from about 50 percent to about 90 percent by volume of HCFO-1233zd (E) and from about 10 percent to about 50 percent by volume of isopropanol. In preferred embodiments, the replacement step is easily and relatively quickly achieved, without negatively effecting the strength or structure of the solid silica network or backbone, by immersing the wet gel in the replacement fluid under ambient temperatures (or in some cases moderately elevated temperatures) and ambient pressures, preferably and especially in embodiments in which the wet gel comprises a liquid component that comprises an alcohol (i.e., the wet gel is an alcogel) or an alchol/water mixture (i.e., the wet gel is an hydro/alcogel) .
A preferred but non-limiting example of specific method steps and sequences according to the present invention is described below in connection with Figure 1.
It will be appreciated that while Figure is depicted in connection with the use of 1233zd as the pore–filling fluid, the steps and sequences described in connection with that figure apply equally well to each of the pore–filling fluids described herein.
In Figure 1, a main reactor or mold 10 is provided with an aged, wet aerogel contained therein and as such the pores of the aged aerogel contain a liquid solvent, preferably an alcohol or an alcohol/water solvent. The temperature/pressure condition of the reactor/mold is brought to within the range preferred for the replacement step of the present invention, and this can be achieved, for example, by simply pumping the selected pore-filling fluid of the present invention into the reactor/mold. More specifically, the reactor/mold is at a pressure of less than about 150 psia, or preferably at a pressure of less than about 90 psi, or preferably less than about 60 psia and even more preferably less than about less than about 30 psia. Preferably, the reactor/mold is at a pressure of less than about 150 psia and a temperature of less than about 100℃, or preferably at a pressure of less than about 90 psi and a temperature less than about 80℃, or preferably less than about 60 psia and at a temperature less than about 60℃, or preferably less than about less than about 30 psia and a temperature less than about 50°. Preferably, the reactor/mold is at a temperature of less than about 100℃, or preferably at a temperature less than about 80℃, or preferably at a temperature less than about 60℃, or preferably at a temperature less than about 50° or preferably at a temperature less than about 50°.
In preferred embodiments of the present invention the reactor/mold is at about room temperature, and in such cases the pressure in the mold/reactor will correspond preferably to the saturation pressure of the pore-filling fluid chosen in accordance with the present invention. For example, for pore-filling fluid consisting of trans1233zd, at room temperature the reactor/mold will be only slightly above ambient pressure (less than about 0.3 MPa) . The preferred methods of the present invention utilize pore-fillin fluids  as described herein to achieve reactor/mold pressures that are only slightly above atmospheric pressure preferably, less than about 2 atmospheres.
It will be appreciated by those skilled in the art that the steps of the present methods can be carried out either continuously, batch –wise, or in a combination of continuous and batch wise processes. Thus, according to a preferred embodiment the pore-filling fluid is pumped continuously into the reactor/mold under conditions which allow a continuous withdrawal of a waste liquid comprising a mixture of the pore-filling fluid and the solvent material which had been in the pores of the aerogel, and in such a case of the continuous operation is maintained for a period sufficient to achieve and aerogel in which a substantial proportion of the fluid in the pores, and in preferred embodiments substantially all of the fluid in the pores, is the pore-filling fluid of the present invention. In an alternative embodiment, the step of replacing the solvent can occur in a one or more (preferably a series) of batch–wise operations, for example, by adding the reactor/mold with a predetermined batch of the pore–filling fluid and allowing it to soak for a period of time. During the soaking step in such a batch wise operation, a proportion of the solvent contained in the pores of the aerogel will be replaced by the pore-filing fluid according to the present invention, and as a consequence the fluid surrounding the aerogel will contain a mixture of pore-filling fluid and the solvent material, which is referred to hereinafter for the purposes of convenience as “waste fluid. ” . After the soaking, the waste fluid is withdrawn from the reactor/mold, and in those embodiments in which a series of batch operations are used to conduct the replacement step a second batch of pore-replacement fluid is added to the reactor/mold once the waste fluid from the previous batch has been removed.
According to preferred embodiments, the waste fluid is subjected to a separation step 11 in which the pore–filling fluid of the present invention is separated from the solvent that has been removed from the aerogel. According to preferred embodiments, the separation step produces a recycle stream of pore-filling fluid according to the present invention in which less than 10%by weight, even more preferably less than 5%by weight, and even more preferably less than 2%by weight of the recycle stream is the solvent that has been removed from the pores of the aerogel. One advantageous feature of the present invention is that the pore-filing fluids of the present invention are easily separated from solvents that are contained in the pores of the aerogel and thus permits a highly efficient and effective recycle stream which can be returned to the source of the pore-filling fluid, such as tank 12. For example, according to a preferred embodiment the separation step can be achieved by simply heating under relatively low pressure, for example atmospheric pressure or slightly above, the waste liquid to a temperature of about 30 –35℃ to form a vapor that will be relatively rich in the pore-filing fluid, and then condensing such enriched stream to produce a liquid stream having a relatively higher concentration of pore-filling fluid. As is known to those skilled in the art, such a step can be repeated over a series of temperatures and pressures, (for example as would occur in a distillation column) to achieve a relatively purified stream of pore-filling fluid for recycle for use again in the process. In addition, the waste-fluid that has been removed can be sent to a waste tank 13 and/or for further processing, such as possible further separation of solvent and pore-fluid to generate additional recycle streams and/or incineration, and the like.
As mentioned above, the replacement step is conducted for a time sufficient to ensure that a substantial proportion, and preferably at least about 75 volume percent, or at least about 85 volume percent, or at least about 95 volume percent of the pore volume of the aerogel contains the pore-filling fluid of the present invention. Those skilled in the art will appreciate in view of the disclosure contained herein that several methods and mechanisms are available to monitor and achieve the preferred level of replacement according to the present invention, and all such methods and mechanisms are within the broad scope of the present invention. In one preferred embodiment, the extent of replacement is monitored by monitoring the relative concentration of the pore-filling fluid in waste liquid leaving the reactor/mold. The concentration of the pore-filling fluid in the stream is believed to be an approximate representation of the volume concentration of the pore volume occupied by the pore-filling fluid in the aerogel, and accordingly in preferred embodiments this concentration is monitored and the removal step is continued until the concentration of pore-filling fluid in the waste stream is at or above a predetermined level, preferably at or above 50%by weight, were preferably at or above 75%by weight, or preferably at or above 85%by weight, or preferably at or above 95%by weight based on the total weight of the waste liquid stream. When such a liquid concentration is achieved, the replacing step of the present invention can be discontinued and the removing step can be initiated.
Thus, once the aerogel has had a substantial proportion of the pores filled with the pore-filling fluid of the present invention, and in particular such that at least about 75%, or at least about 85%, or at least about 95%by volume of the pore volume in the aerogel is occupied by pore-filling fluid of the p resent invention, the aerogel (preferably in a  reactor/mold) is subjected to the step of removing the pore-filling fluid under subcritical conditions. While it is contemplated that various steps for achieving such removal will be known and available to those skilled in the art based upon the teachings contained herein, in preferred embodiments the removal step comprises evaporating the pore-filling fluid, preferably by raising the temperature of the aerogel. One preferred embodiment of evaporating the pore-filling fluid comprises introducing a relatively hot, inert gas (such as nitrogen) into the reactor/mold containing the aerogel so as to vaporize a substantial proportion of the pore-filling fluid of the present invention under subcritical conditions. In particular, preferred embodiments are conducted such that during the drying step the temperature/pressure of the drying gas is higher than the saturation temperature pressure of the pore-filling fluid so as to cause evaporation of the pore-filling fluid while at the same time being below the critical conditions of the pore-containing fluid. Preferably, during the drying step the aerogel remains at a pressure of less than about 150 psia, or preferably at a pressure of less than about 90 psi, or preferably less than about 60 psia and even more preferably less than about less than about 30 psia. Preferably, the aero during the drying step is at a pressure of less than about 150 psia and a temperature of less than about 100℃, or preferably at a pressure of less than about 90 psi and a temperature less than about 80℃, or preferably less than about 60 psia and at a temperature less than about 60℃, or preferably less than about less than about 30 psia and a temperature less than about 50°. Preferably, the aerogel during the drying step is at a temperature of less than about 100℃, or preferably at a temperature less than about 80℃, or preferably at a temperature less than about 60℃, or preferably at a temperature less than about 50° or preferably at a temperature less than about 50°.
EXAMPLES
Comparative Example 1
A wet gel consisting of tetraethyl orthosilicate (TEOS -Si (OC2H5) ) as the source of silica and tetrabutylammonium hydroxide as a base catalyst in a solvent comprising isopropanol and water is formed. The wet gel is washed and then soaked in ethanol such that approximately all of the pore volume of the gel is occupied by ethanol and then aged according to standard ageing techniques. The washed, aged gel is then silyated with hexamethyldisilazane (HMDS) to enhance the strength of the solid portion of the aerogel according to standard ageing techniques.
The washed, aged gel having ethanol substantially filling the pores, which defined a starting wet volume, is then dried by placing the wet gel in an oven at 100C and ambient pressure for a period of approximately one (1) hour and the drying was done at ambient pressure in air. This subcritical drying operation resulted in substantial fracturing of the gel as it dried into many pieces much smaller than the starting volume of wet gel.
Example 1
Comparative Example 1 was repeated except that the washed, aged gel having ethanol substantially filling the pores was provided for use in a replacement step according to the present invention. Specifically, the aged, wet gel produced in Comparative Example 1 was soaked for about 24 hours in liquid transHFCO-1233zd at room temperature and pressure, and then the waste liquid containing a mixture of transHFCO-1233zd and ethanol was removed and a fresh batch of transHFCO-1233zd  was used to soak the wet gel. The wet gel was soaked in this second batch for 24 hours and the waste liquid was again removed and a third batch of transHFCO-1233zd was used to soak the wet gel for three days. The waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the third soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for four hours. The waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the fourth soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for final soaking period of 17 hours. The waste liquid that from this fifth batch was removed and found to have the following relative volume percentages of transHFCO-1233zd of 96.88%and ethanol of 2.34%and IPA of 0.78%. As a result of this replacement process, a substantial proportion of the pore volume of the gel contains transHFCO-1233zd.
The gel that was subject to the replacement step was then dried by placing the gel in a heated oven at a temperature of approximately 35C for about 2 hrs. After the replacing and drying step the size and shape of the dried gel substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step. The aerogel thus produced was tested and found to have the following properties:
BET (m 2/g) –799
Average Pore Size (nm) –3.59
Pore Volume (cm 3/g) –0.679
Density (g/cm 3) –0.9
Porosity (%) –59.9%
The pore size and volume is measured by Micromeritics ASAP2420 with a degassing temperature of 150C and degassing time of 4 hours. Density and porosity are determined by calculation as follows: pore volume = 1/density –1/ (SiO2 density) , where SiO2 density is 2.2 g/cm 3.
Example 2
Comparative Example 1 was repeated except in two respects. First, the gel was not silyated with hexamethyldisilazane (HMDS) , and second the washed, aged gel having ethanol substantially filling the pores was provided for use in a replacement step according to the present invention. Specifically, the aged, wet gel produced in as in Comparative Example 1, but without the strengthening step, was soaked for about 24 hours in liquid transHFCO-1233zd at room temperature and pressure, and then the waste liquid containing a mixture of transHFCO-1233zd and ethanol was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel. The wet gel was soaked in this second batch for 24 hours and the waste liquid was again removed and a third batch of transHFCO-1233zd was used to soak the wet gel for three days. The waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the third soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for four hours. The waste liquid containing a mixture of transHFCO-1233zd and ethanol resulting from the fourth soaking was removed and a fresh batch of transHFCO-1233zd was used to soak the wet gel for final soaking period of 17 hours. The waste liquid that from this fifth batch was removed and found to have approximately the same relative percentages of transHFCO-1233zd as reported in Example 1. As a result of this replacement process, a substantial proportion of the pore volume of the gel contains  transHFCO-1233zd. The gel that was subject to the replacement step was then dried by placing the gel in in a closed container a heated maintained at about 35C for 2hrs and then increased to 60C for 2hrs and finally increased to 180C for 1hr. After the replacing and drying step the size and shape of the dried gel substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step. The aerogel thus produced was tested and found to have the following properties:
BET (m2/g) –945
Average Pore Size (nm) –3.32
Pore Volume (cm3/g) –0.785
Density (g/cm3) –0.8
Porosity (%) –63.3%
Density and porosity are determined by calculation as follows: pore volume = 1/density –1/ (SiO2 density) , where SiO2 density is 2.2 g/cm3.
Example 3 –Batch Replacement with transHFCO-1233zd
A prepared, transparent, aged alcogel (with ethanol in the pores) was provided and then soaked for about 24 hours in isopropanol (IPA) at room temperature. After this soaking step, the alcogel was used in a replacement step according to the present invention. Specifically, the replacement step of the present invention involves a series of six (6) one hour soakings of the alcogel in a liquid bath of transHFCO-1233zd at room temperature and pressure. After each one hour soaking in the liquid bath of transHFCO-1233zd, the bath liquid (containing a mixture of transHFCO-1233zd and materials to be removed such as ethanol and IPA) is removed and a fresh batch of transHFCO-1233zd liquid was  used to soak the wet gel. As a result of this replacement process, a substantial proportion of the pore volume of the gel contains transHFCO-1233zd.
The gel after the above-described replacement step has a liquid with the following approximate concentrations by volume of components in the cells:
transHFCO-1233zd. –98.4 %
ethanol –0.8 %
IPA –0.8 %
The gel was then dried by first placing the gel in a closed container overnight at about room temperature. A pressure of about 1.2 atmospheres results in the container based on the saturation pressure of transHFCO-1233zd. The container is then placed in an oven maintained at a temperature of about 35℃ for 1hr. After the replacing and drying step the size and shape of the dried gel has substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step. The aerogel thus produced was tested and found to have the following properties:
BET (m2/g) –704
Average Pore Size (nm) –1.08
Pore Volume (cm3/g) –1.90
Density (g/cm3) –0.4
Porosity (%) –80.7%
Density and porosity are determined by calculation as follows: pore volume = 1/density –1/ (SiO2 density) , where SiO2 density is 2.2 g/cm3.
Example 4 –Continuous Replacement with transHFCO-1233zd/IPA (70/30)
A prepared, transparent, aged alcogel (with ethanol in the pores) was provided. The gel is placed in a vessel which has a continuous flow of fresh liquid comprising about 70%by volume of transHFCO-1233zd and 30%by volume of IPA is provided to the vessel to maintain a liquid level above the height of the gel in the vessel while continuously removing a liquid stream containing transHFCO-1233zd and dissolved/carried impurities (including ethanol) . The output liquid stream from the vessel was processed to remove the impurities and to recycle transHFCO-1233zd/IPA to use as fresh transHFCO-1233zd/IPA. This continuous replacement process is continued for about 3 hours. As a result of this replacement process, a substantial proportion of the pore volume of the gel contains the transHFCO-1233zd/IPA mixture. The gel after the above-described replacement step has a liquid that is estimated to be approximate 98 –99%by volume of 1233zd/IPA.
The gel was then dried by first placing the gel in a closed container overnight at about room temperature. A pressure of about 1.2 atmospheres results in the container based on the saturation pressure of transHFCO-1233zd. The container is then placed in an oven maintained at a temperature of about 35℃ for 1hr. After the replacing and drying step the size and shape of the dried gel has substantially the same as the size and shape of the starting wet gel, that is, essentially no fracturing or weakening of the gel structure occurred during the drying step. The aerogel thus produced was tested and found to have the following properties:
BET (m2/g) –779
Average Pore Size (nm) –1.02
Pore Volume (cm3/g) –1.99
Density (g/cm3) –0.4
Porosity (%) –81.4%
Density and porosity are determined by calculation as follows: pore volume = 1/density –1/ (SiO2 density) , where SiO2 density is 2.2 g/cm3.

Claims (11)

  1. A method of forming silica aerogel comprising:
    (a) providing an aged hydrogel or an aged alcogel and/or an aged hydro/alcogel;
    (b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing at least 50%by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a pore-replacement fluid comprising one or more of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) , cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) , cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) , 1-methoxyheptafluoropropane, and combinations of these, to produce an aged gel in which the liquid portion thereof comprises preferably at least 50%by volume of said pore-replacement fluid;
    (c) before, during or after said providing step (a) and/or said preparing step (b) , optionally strengthening the solid portion of said gel against collapse and shrinkage; and
    (d) vaporizing said pore-replacement fluid under a pressure of about 150 psia or less to remove a substantial portion of said pore-replacement fluid from said gel and produce an aerogel.
  2. The aerogel of claim 1 wherein at least 80%by volume of the solvent of said hydrogel, alcogel and/or hydro/alcogel is replaced with said pore-replacement fluid.
  3. The aerogel of claim 1 wherein at least 90%by volume of the solvent of said hydrogel, alcogel and/or hydro/alcogel is replaced with said pore-replacement fluid.
  4. The aerogel of claim 1 wherein at least 95%by volume of the solvent of said hydrogel, alcogel and/or hydro/alcogel is replaced with said pore-replacement fluid.
  5. The aerogel of claim of any of claims 1 –4 wherein said pore-replacement fluid comprises transHFCO-1233zd.
  6. The aerogel of claim of any of claims 1 –4 wherein said pore-replacement fluid consists essentially of transHFCO-1233zd.
  7. The aerogel of claim of any of claims 1 –4 wherein said pore-replacement fluid consists of transHFCO-1233zd.
  8. A method of forming monolithic aerogel comprising:
    (a) providing an aged hydrogel, alcogel and/or hydro/alcogel having a volume of at least about 80 cubic centimeters;
    (b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing at least 80%by volume of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a fluid comprising one or more of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) , cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) , cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) , 1-methoxyheptafluoropropane, and combinations of these, to produce an aged gel in which the liquid portion thereof comprises at least 80%by volume of said liquid in said aged gel is said pore-replacement fluid;
    (c) before, during or after said preparing step (b) and/or said providing step (a) , optionally, but preferably, strengthening the solid portion of said gel against collapse and shrinkage; and
    (d) vaporizing said pore replacement fluid under a pressure of less than about 30 psia to remove a substantial proportion of said pore-replacement fluid from said gel and produce an aerogel.
  9. The aerogel of claim 8 wherein at least 90%by volume of the solvent of said hydrogel, alcogel and/or hydro/alcogel is replaced with said pore-replacement fluid.
  10. The aerogel of any of claims 8 or 9 wherein said pore-replacement fluid consists essentially of transHFCO-1233zd.
  11. A method of forming silica aerogel comprising:
    (a) providing an aged hydrogel or an aged alcogel and/or an aged hydro/alcogel;
    (b) preparing said aged hydrogel or an aged alcogel and/or an aged hydro/alcogel for sub-critical drying by replacing at least 50%by volume, of the solvent of said hydrogel, alcogel and/or hydro/alcogel with a pore-replacement fluid comprising one or more of trans-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (E) ) , cis-1-chloro-3, 3, 3-trifluoro-propene (HCFO-1233zd (Z) ) , cis-1, 1, 1, 4, 4, 4-hexafluoro-2-butene (HFO-1336mzz (Z) ) , 1-methoxyheptafluoropropane, HCFC-123, HCFC-141b, HFC-365mfc, (C2F5C (O) CF (CF3) 2, HFC-245fa and combinations of these, to produce an aged gel in which the liquid portion thereof comprises preferably at least 50%by volume of said pore-replacement fluid;
    (c) before, during or after said providing step (a) and/or said preparing step (b) , optionally strengthening the solid portion of said gel against collapse and shrinkage; and
    (d) vaporizing said pore-replacement fluid under a pressure of about 150 psia or less to remove a substantial portion of said pore-replacement fluid from said gel and produce an aerogel.
PCT/CN2018/095817 2018-07-16 2018-07-16 Methods for forming aerogels WO2020014828A1 (en)

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CN105253892A (en) * 2014-06-11 2016-01-20 中国科学院苏州纳米技术与纳米仿生研究所 Color silica aerogel and preparation method thereof
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