Classifications

• • 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

Definitions

• the present disclosure relates to hydrophobic aerogels and methods of making hydrophobic aerogels.
• the methods of the present disclosure include combining a hydrophobizing agent with an aerogel precursor prior to gelation rather than adding a hydrophobizing agent to an existing gel.
• Aerogels are a unique class of ultra- low-density, highly porous materials. The high porosity, intrinsic pore structure, and low density make aerogels extremely valuable materials for a variety of applications including insulation. Low density aerogels based upon silica are excellent insulators as the very small convoluted pores minimize conduction and convection. In addition, infrared radiation (IR) suppressing dopants may easily be dispersed throughout the aerogel matrix to reduce radiative heat transfer.
• IR infrared radiation
• Aerogels tend to be very hygroscopic due to the presence of hydroxyl groups on the surface. Unmodified aerogels absorb water and other organic solvents adversely affecting desired properties (e.g., surface area, porosity, and density) thereby degrading performance (e.g., thermal insulation). However, many applications of aerogels require exposure to water or atmospheric moisture. Therefore, methods are needed to prepare aerogels having hydrophobicity at ambient conditions as well as over a range of temperature and pressure conditions.
• FIG. 1 is an SEM image of the aerogel of Example 25.
• FIG. 2 is an SEM image of the hydrophobic aerogel of Example 27.
• the present disclosure provides methods of preparing a hydrophobic aerogel. Such methods comprise forming a surface-modified metal oxide gel from a sol comprising a solvent, a metal oxide precursor, and a hydrophobic surface modifying agent; and drying the gel to form the hydrophobic aerogel.
• the methods further comprise solvent-exchanging the hydrophobic, aerogel precursor with an alkyl alcohol to form a hydrophobic alcogel. In some embodiments, the methods further comprise supercritically drying the alcogel to form the hydrophobic aerogel.
• the solvent comprises water. In some embodiments, the solvent comprises an alkyl alcohol.
• the metal oxide precursor comprises an organosilane.
• the organosilane comprises a tetraalkoxysilane, optionally wherein the tetraalkoxysilane is selected from the group consisting of tetraethoxysilane, tetramethoxysilane, and combinations thereof.
• the organosilane comprises an alkyl-substituted alkoxysilane, optionally wherein the alkyl-substituted alkoxysilane comprises methyltrimethoxysilane.
• the organosilane comprises a pre -polymerized silicon alkoxide, optionally wherein the pre-polymerized silicon alkoxide comprises a polysilicate.
• the molar ratio of the hydrophobic surface modifying agent to the metal oxide precursor is no greater than 1. In some embodiments, the molar ratio of the hydrophobic surface modifying agent to the metal oxide precursor is at least 0.2.
• the sol comprises at least two moles of water per mole of metal oxide precursor. In some embodiments, the sol further comprises an acid, optionally wherein the acid is hydrochloric acid.
• the methods further comprise applying the mixture to a substrate prior to forming surface-modified metal oxide gel.
• the substrate is non-woven substrate.
• the substrate is a bonded web.
• the present disclosure provides aerogel articles made according to the methods of the present disclosure.
• the present disclosure provides hydrophobic aerogels made by the methods of the present disclosure.
• the first process involves the hydrolysis and condensation of a metal oxide precursor (e.g., alkoxysilane precursors) followed by supercritical drying. This process typically yields monolithic aerogels.
• the second process is a waterglass-based synthesis route that typically yields powders, beads, or granules.
• a typical method for making aerogels hydrophobic involves first making a gel.
• the methods of the present disclosure begin with a sol comprising a solvent, a metal oxide precursor, and a hydrophobic surface modifying agent.
• the solvent comprises water.
• one or more organic solvents such as an alkyl alcohol may be used.
• the sol may include both water and one or more organic solvents, e.g., a water/alkyl alcohol blend.
• the methods of the present invention are not particularly limited to specific metal oxide precursors.
• the metal oxide precursor comprises an organosilane, e.g., a tetraalkoxysilane.
• exemplary tetraalkoxysilanes include tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS).
• the organosilane comprises an alkyl-substituted alkoxysilane, e.g., methyltrimethoxysilane
• the organosilane comprises a pre-polymerized silicon alkoxide, e.g., a polysilicate such as ethyl polysilicate.
• the hydrophobic surface modifying agent combines with the skeletal structure formed by the metal oxide precursor to provide a hydrophobic surface.
• the hydrophobic surface modifying agent is covalently bonded to the metal oxide skeleton.
• the hydrophobic surface modifying agent may be ionically bonded to the metal oxide skeleton.
• the hydrophobic surface modifying agent may be physically adsorbed to the metal oxide skeleton.
• the hydrophobic surface modifying agent comprises two functional elements. The first element reacts with (e.g., covalently or ionically) or absorbs on to the metal oxide skeleton. The second element is hydrophobic.
• Exemplary hydrophobic surface modifying agents include organosilane, organotin, and organophosphor o us compounds.
• One exemplary organosilane is 1,1,1, 3,3, 3-hexamethyldisilazane (HMDZ).
• HMDZ 1,1,1, 3,3, 3-hexamethyldisilazane
• the gel may be supercritically dried using, e.g., supercritical carbon dioxide. After solvent removal, the resulting material is typically referred to as an aerogel.
• a solvent exchange step may precede the drying step.
• any known method of solvent exchange may be used with the methods of the present disclosure.
• the exchange solvent may be an alkyl alcohol, e.g., ethyl alcohol.
• the resulting gel is often referred to as an organogel as opposed to a hydrogel, which refers to gel wherein the solvent is primarily water.
• the exchange solvent is an alkyl alcohol
• the resulting gel is often referred to as an alcogel.
• the molar ratio of the hydrophobic surface modifying agent to the metal oxide precursor is no greater than 1, e.g., no greater than 0.8, or even no greater than 0.6. In some embodiments, the molar ratio of the hydrophobic surface modifying agent to the metal oxide precursor is at least 0.2, e.g., at least 0.3.
• the sol comprises at least two moles of water per mole of metal oxide precursor. In some embodiments, the sol comprises 2 to 5, e.g., 3 to 4, moles of water per mole of metal oxide precursor.
• the sol further comprises an acid.
• the acid is an inorganic acid, e.g., hydrochloric acid.
• the sol comprises between 0.0005 and 0.0010 moles of acid per mole or metal oxide precursor. In some embodiments, comprises between 0.0006 and 0.0008 moles of acid per mole or metal oxide precursor.
• the methods of the present disclosure may be used to form aerogel articles, e.g., flexible aerogel articles.
• the sol may be applied to a substrate prior to forming a gel. Gelation, solvent exchange (if used), and drying may then occur on the substrate.
• the substrate may be porous, e.g., a woven or nonwoven fabric. Exemplary substrates also include bonded web such as those described in U.S. Patent Application No. 11/781,635, filed July 23, 2007.
• BET Brunauer, Emmett, and Teller
• Aerogel cylinders were synthesized within plastic syringes with one end cut off. Once gelled, the aerogel cylinder was extracted from the syringe using the syringe plunger and dried. The diameter and length of the dried cylinders were measured and the volume calculated. The weights of the samples were measured on an analytical balance. The bulk density was then calculated from the ratio of weight to volume.
• the skeletal density was determined using a Micromeritics ACCUPYC 1330 helium gas pycnometer.
• the instrument uses Boyle's law of partial pressures in its operation.
• the instrument contains a calibrated volume cell internal to the instrument.
• the sample was placed in a sample cup, weighed and inserted into the instrument.
• the sample was pressurized in the instrument to a known initial pressure.
• the pressure was bypassed into the calibrated cell of the instrument and a second pressure recorded. Using the initial pressure, the second pressure, and the volume of the calibrated cell, the skeletal volume of the sample was determined.
• the skeletal density was then determined from the skeletal volume and the sample weight.
• Porosity The percent porosity was calculated from the measured bulk density (Pbulk) an d the and skeletal density (Pskeletal) using the following formula:
• Thermal Conductivity was measured at a mean temperature of 12.5 0 C using a LASERCOMP "Fox200" instrument.
• the sample was weighed and placed in a permeable cloth bag sealed with a draw string.
• the bag containing the sample was placed inside a stainless steel chamber.
• the bottom and top of this chamber were fitted with metal frits and O-rings.
• This chamber was inserted into a vessel rated to handle high pressure (40 MPa (6000 psig)). The outside of this vessel was heated by a jacket.
• Carbon dioxide was chilled to less than minus 10 degrees Celsius and pumped with a piston pump at a nominal flow rate of one liter per minute through the bottom of the unit. After ten minutes, the temperature of the unit was raised to 40 0 C at a pressure of
• Examples 1-6 TEOS-based aerogels with pre-hydrolyzation and surface treatment prior to gelation.
• a stock solution was prepared by mixing 209.39 grams of tetraethoxysilane (TEOS, 99+%) (Alfa Aesar) with 234.95 grams of ethanol (EtOH, 200 proof) (Aaper Alcohol), 54.09 grams of deionized water (H2O) and 0.701 grams of 1 Molar hydrochloric acid (IM HCl) (J.T. Baker) in a round bottom flask fitted with water cooled reflux condenser. The mixture was heated to 70 0 C for 1 hour under constant stirring. 1,1,1, 3,3, 3-hexamethyldisilazane (HMDZ) was used as a silylating/surface modifying agent to render the silica gel hydrophobic. In principle, other silylating agents can also be used for this purpose.
• the silylating agent here performs the dual role of modifying the surface and providing ammonia upon reaction with water, which acts as a catalyst for the hydrolysis and condensation of the silica precursor.
• Example 1 gelled but was not hydrophobic indicating insufficient surface treatment.
• Examples 2-4 gelled in less than one minute and were hydrophobic. In the case of Example 5, even though gelation occurred in less than one minute, the gel quality was poor and hence the sample could not be supercritically dried.
• Example 6 did not gel.
• Table 1 Formulations, gel times, and gel characteristics for Examples 1-6
• Examples 1-5 are shown in Table 2.
• the surface areas and densities of Examples 2-4 are typical of aerogels. These examples clearly demonstrate a process by which silica aerogels can be prepared in the presence of surface modifying agents in a time efficient manner when an appropriate amount of HMDZ is used for surface modification.
• Table 2 Characteristics of the aerogels of Examples 1-5.
• Examples 7-14 were prepared in a manner similar to Examples 1-6 except that the H2O/TEOS and EtOH/TEOS molar ratios were varied. Table 3 shows that the gel time trends for Examples 7-14 are similar to those for Examples 1-6. Examples 8 and 12 did not gel. Example 9 was not hydrophobic due to insufficient surface treatment, while Examples 7, 10, 11, 13, and 14 were hydrophobic. As summarized in Table 4, Examples 11 and 13 exhibited characteristic aerogel surface areas and densities. Table 3 : Formulations and gel times for Examples 7-14.
• Examples 15-20 TEOS-based aerogels without pre-hydrolyzation but with surface treatment prior to gelation.
• Tetraethoxysilane (TEOS, 99+%) (Alfa Aesar) was mixed with 1,1,1,3,3,3- hexamethyldisilazane (HMDZ, 99+%) (Alfa Aesar) in a glass beaker to prepare solution A.
• ethanol (EtOH, 200 proof) (Aaper Alcohol), deionized water (H2O) and 1 molar hydrochloric acid (IM HCl) (J.T. Baker) were mixed to form solution B.
• Solution B was added instantaneously to Solution A under vigorous stirring, such that the vortex formed by stirring approached the bottom of the container.
• Examples 15-17 did not gel within 15 minutes; however, gelation did occur after several days.
• Examples 18-20 did not gel even after several days (samples were observed for a period of two weeks).
• Higher HMDZ/TEOS ratios > 0.5 resulted in no gelation or very long gel times.
• Table 5 Formulations, gel times, and gel characteristics for Examples 15-20
• Example 16 was hydrophobic and had a surface area of 453 m ⁇ /g, a pore volume of 2.5 cc/g, a bulk density of 0.26 g/cc, a skeletal density of 1.48 g/cc, and a porosity of 83%.
• Examples 21-24 TEOS-based aerogels without pre-hydrolyzation, but with surface treatment prior to gelation.
• Solutions A and B were prepared and mixed as described for Examples 15-20.
• the molar ratios of the various reactants and the gelation times for these mixtures are listed in the Table 6.
• the molar ratios of H2O/TEOS and EtOH/TEOS were varied while the molar ratio of HMDZ/TEOS was held constant at 0.33.
• Those examples which resulted in gels were solvent exchanged three times with 75 ml of EtOH. After the final solvent exchange the samples were supercritically dried.
• TMOS-based aerogels without pre-hydrolyzation, but with Surface Treatment prior to Gelation.
• Tetramethoxysilane (TMOS, 98+%) (Alfa Aesar) was mixed with 1,1,1,3,3,3- hexamethyldisilazane (HMDZ, 99+%) (Alfa Aesar) in a glass beaker to prepare solution C.
• Methanol MeOH, 99.8%
• H2O deionized water
• IM HCl 1 Molar Hydrochloric acid
• Solution D was added instantaneously to Solution C under vigorous stirring, such that the vortex formed by stirring approached the bottom of the container.
• the molar ratios of the various reactants and the gelation times for these mixtures are listed in the Table 7. Those examples which resulted in gels were solvent exchanged three times with 75 ml of MeOH. After the final solvent exchange the samples were supercritically dried. Except for Example 29, all examples gelled in 1 minute or less. Example 29 shows longer gelation time and had a higher HMDZ/TMOS molar ratio. This is consistent with examples shown above for TEOS-based gels where higher HMDZ/TEOS ratios also resulted in longer gel times and, in some cases, no gelation was observed. Table 7: Formulations, gel times, and gel characteristics for Examples 25-29.
• Characteristics of the aerogels of Examples 25-29 are summarized in Table 8. The surface areas and densities are characteristic of TMOS-based aerogels. Example 25 was not hydrophobic and had a low HMDZ/TMOS molar ratio, which is consistent with results for TEOS-based aerogels. Examples 26-29 were all hydrophobic. Table 8: Characteristics of the aerogels of Examples 25-29.
• FIG. 1 is an image of the aerogel of Example 25.
• Figure 2 is an image of the hydrophobic aerogel of Example 27.
• Examples 30-41 TEOS-based aerogels derived from a commercially-available pre-hydrolyzed sol, surface treated prior to gelation.
• solution E 1,1,1, 3,3, 3-hexamethyldisilazane (HMDZ, 99+%) (Alfa Aesar) in a glass beaker to prepare solution E.
• ethanol EtOH, 200 proof
• H2O deionized water
• IM HCl 1 molar hydrochloric acid
• Solution F was added instantaneously to Solution E under vigorous stirring, such that the vortex formed by stirring approached the bottom of the container.
• Those examples which resulted in gels were solvent exchanged three times with 75 ml of EtOH. After the final solvent exchange the samples were supercritically dried.
• the aerogels were hydrophobic and had surface area and densities characteristic of aerogels.
• the aerogel of Example 31 was hydrophobic and had surface area of 690 m2/g, a pore volume of 1.9 cc/g, a bulk density of 0.14 g/cc, a skeletal density of 1.48 g/cc, and a porosity of 71%.
• Examples 42-49 TEOS-based aerogels prepared from a pre -hydro lyzed sol subject to further pre-hydrolysis and surface treatment prior to gelation.
• Ethyl polysilicate containing 45-47 wt% SiC>2 (SILBOND 50 from Silbond Corporation) was mixed with ethanol (EtOH, 200 proof) (Aaper Alcohol), deionized water
• Examples 50-60 TEOS/MTMOS-based aerogels with pre-hydrolyzation and surface treatment prior to gelation.
• Tetraethoxysilane (TEOS, 99+%) (Alfa Aesar) and methyltrimethoxysilane (MTMOS, 95%) (Aldrich) were mixed with ethanol (EtOH, 200 proof) (Aaper Alcohol), deionized water (H2O) and hydrochloric acid (HCl) (J.T. Baker) in a glass jar.
• EtOH 200 proof
• H2O deionized water
• HCl hydrochloric acid
• Examples 50-55, 0.005M HCl was used and for Examples 56-60 IM HCl was used.
• the glass jar containing the mixture was heated at 50 0 C for 45 minutes under constant stirring. While vigorously stirring, HMDZ was added to the mixture.
• the molar ratios of the various reactants in the final mixture are listed in Table 11.
• the resulting gels were solvent exchanged three times with 75 ml of EtOH. After the final solvent exchange the samples were supercritically dried.
• Table 11 Formulations, gel times, and gel characteristics for Examples 50-60.
• Examples 50-60 were all hydrophobic. Generally, the inclusion of MTMOS increased the gel time compared to the pure pre-hydrolyzed TEOS samples (e.g., Examples 59 and 60 showed an increase in gel time with increasing MTMOS content relative to Example 3, which did not contain MTMOS). Examples 50-60 also showed that surface modification prior to gelation can be used with other organosilanes (containing Si-
• silica precursors like tetraalkoxysilanes (e.g., TEOS and TMOS), pre-hydrolyzed TEOS, and pre-polymerized silicon alkoxides (e.g., SILBOND 50).
• Examples 61 and 62 Flexible aerogels with surface treatment prior to gelation on a nonwoven substrate.
• the gel precursor of Example 3 was prepared.
• the pre-hydrolyzed TEOS and HMDZ were cooled using dry ice prior to mixing in order to slow gelation.
• the mixture was coated onto a bonded fibrous flexible substrate.
• a 75-25 blend of 3d WELLMAN PET fibers and 6d KOSA PET fibers at 30 gsm was carded, corrugated, and bonded to 30 gsm of PP 7C05N strands wherein the corrugating pattern had 10 bonds per 2.54 cm (i.e.,
• Example 61 was prepared in the same manner as Example 61, except that the gel precursor of Example 54 was used.
• the thermal conductivity of the Example 62 measured at a mean temperature of 12.5 0 C, was 25.9 mW/m-K.

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