Ambient processing of phenolic aerogels as cores of vacuum insulated panels (VIPS)

Abstract

Methods of safely preparing mechanically strong phenolic aerogels, e.g. for use in Vacuum Insulated Panels (VIPs), are provided. The aerogel compositions are made using a substance that is a solid and is safe to handle when initially mixed with polymerizable hydroxylated aromatic monomers. The addition of a catalyst causes the substance to break down into an electrophilic linking agent and the monomers then polymerize, forming a wet gel. Drying of the wet gel produces an aerogel that is mechanically strong enough to withstand evacuation and be used as a core of a VIP.

Description

[0001] AMBIENT PROCESSING OF PHENOLIC AEROGELS AS CORES OF VACUUM INSULATED PANELS (VIPs)

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims benefit of United States provisional patent application 63 / 546,817 filed November 1, 2023.

[0004] STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0005] This invention was made with government support under contract DE-8676 awarded by the United Stated Department of Energy. The United States government has certain rights in the invention.

[0006] BACKGROUND OF THE INVENTION

[0007] Technical Field.

[0008] The invention generally relates to improved methods of preparing mechanically strong phenolic aerogels, e.g. for use in Vacuum Insulated Panels (VIPs). In particular, the invention provides mechanically strong aerogel compositions made using solid substances that generate electrophilic linking agents upon catalysis.

[0009] Description of Related Art

[0010] Phenolic aerogels comprise an important class of organic aerogels with many diverse applications. They display a series of excellent characteristics such as low density-high porosity, large sound absorption-low sound velocity, and low heat conductivity, good dimensional stability, thermal stability, chemical and fire resistance, and anti-ablation performance. Such attributes render phenolic aerogels highly amenable to a variety of uses, including thermal protection and insulation, electronic components, foundry technology, architecture, lightweight composites, filtration, energy generation (e.g., electrodes in fuel cells), energy storage (e.g., electrodes for batteries and supercapacitors), and other green energy technologies, nonlinear optics, cosmetics, pharmacy (drug delivery), apparel, sportswear, and so on. Several methods of making phenolic aerogels have been published and / or patented (see e.g., US 48743218, US 4997804, US 5420168, US 945084, US 7005181, US 7723262, US 8436061 and US 2013236714, the complete contents of each of which is hereby incorporated by reference in entirety). Such methods generally involve dissolving precursors such as resorcinol (or other hydroxylated aromatics) and an electrophilic linking agent such as formaldehyde, furfural or furfuryl alcohol, and a catalyst such Na2COa or an acid such as HC1 in a solvent such as water or ethanol. After a time varying from a few hours to a few days, a gel is formed.

[0011] Unfortunately, in all these cases, the electrophilic linking agent is very hazardous, and, in the case of formaldehyde, is a dangerous volatile gas. Thus, realizing cost-effective, scalable, and safe manufacturing methods of phenolic aerogels with excellent comprehensive performance is still difficult.

[0012] SUMMARY OF THE INVENTION

[0013] Other features and advantages of the present invention will be set forth in the description of invention that follows, and in part will be apparent from the description or may be learned by practice of the invention. The invention will be realized and attained by the compositions and methods particularly pointed out in the written description and claims hereof.

[0014] Provided herein are methods of making mechanically strong phenolic aerogels in which the hazardous electrophilic linking agents are replaced by substances that generate electrophilic linking agents only under acidic conditions. The methods are cost-effective, scalable, and safe, and the aerogels made by the methods exhibit excellent mechanical properties. They are strong enough to withstand evacuation and be used as a core of a VIP.

[0015] In some aspects, exemplary substances that break down to form electrophilic linking agents are mixed, in solid form, with polymerizable hydroxylated monomers in a suitable solvent, forming a precursor mixture. The substances and the hydroxylated monomers dissolve in the solvent but the substances remain intact and the hydroxylated monomers remain in a monomeric state. This precursor mixture is stable and can be safely and conveniently stored indefinitely. When a practitioner of the method desires to form a phenolic aerogel, polymerization of the monomers is initiated by adding a catalyst which causes the substances to dissociate into an electrophilic linking agent, e.g. formaldehyde.

[0016] In some aspects, the phenolic aerogels made by the methods disclosed herein are open cell foam aerogels and, when formed using the methods disclosed herein, are advantageously mechanically stronger than aerogels obtained by using an equivalent molar concentration of e.g. an aqueous formaldehyde solution.

[0017] It is an object of this invention to provide a method of making a wet phenolic gel, comprising forming, in a solvent, a precursor solution by mixing i) monomers of at least one hydroxylated aromatic compound and ii) at least one compound that breaks down into an electrophilic linking agent upon addition of a catalyst; and adding the catalyst to the precursor solution to form a gelation solution in which the at least one compound dissociates to form the electrophilic linking agent, wherein the electrophilic linking agent reacts with the monomers to form polymers, thereby forming a wet phenolic gel. In some aspects, the catalyst is an acid, a base, a salt that modifies the pH of the precursor solution, or heat, or a combination of two or more of these. In some aspects, the at least one compound that breaks down into an electrophilic linking agent is trioxane or paraformaldehyde. In further aspects, the at least one hydroxylated aromatic compound includes resorcinol, phenol and / or bisphenol A. In additional aspects, the solvent is acetonitrile. In other aspects, the catalyst is a concentrated acid and in additional aspects, the concentrated acid is concentrated HC1 or concentrated H2SO4. In some aspects, the method further comprises the steps of removing the gelation solvent from the wet phenolic gel and drying the wet phenolic gel to achieve a phenolic aerogel. In some aspects, the step of removing the gelation solvent is performed by one or more solvent exchanges. In other aspects, solvents utilized in the one or more solvent exchanges include one or more of isopropanol and acetone.

[0018] The disclosure also provides a phenolic aerogel formed by the methods disclosed and claimed herein. In some aspects, the phenolic aerogel has a compressive strength of from 1 to 500 MPa. In further aspects, the phenolic aerogel has a density lower than 0.25 g / cm2, a surface area of 70 m2 / g and a thermal conductivity of 0.025 W / mK.

[0019] The disclosure also provides a vacuum insulated panel (VIP) comprising the aerogel disclosed and claimed herein. DETAILED DESCRIPTION

[0020] Provided herein are methods of making phenolic aerogels in which hazardous electrophilic linking agents are initially replaced by substances that, when acidified, break down to form electrophilic linking agents. The substances are mixed with suitable hydroxylated monomers in a suitable solvent to form a stable precursor solution that can be stored safely and indefinitely. Polymerization of the monomers in the precursor solution is initiated by the addition of a catalyst . Addition of the catalyst (often an acid, a base, an inorganic salt that modifies the pH of the precursor solution when dissociated or heat)to the precursor solution causes the substances to break down into electrophilic linking agents and polymerization of the monomers ensues.

[0021] Exemplary substances that can be used in this manner include but are not limited to trioxane and paraformaldehyde. The acid catalyzed breakdown reaction is illustrated Scheme 1 for the exemplary substance trioxane.

[0022] Scheme 1. Trioxane is synthesized by reacting formaldehyde with water. The equilibrium of the reaction is shifted to the left (leading to formaldehyde generation) in acidic solution. Paraformaldehyde can also be formed and dissociated in a similar manner.

[0023] Accordingly, when a catalyst is added to the precursor mixture, the substance that generates electrophilic linking agents breaks down, forming (releasing), e.g. formaldehyde, which functions as a linking agent and the hydroxylated monomers in the mixture polymerize. Since substances such as trioxane and / or paraformaldehyde dissociate into an electrophilic linking agent only in the presence of a suitable catalyst (e.g., under acidic (trioxane, paraformaldehyde) or basic (paraformaldehyde) conditions, or in the presence of sufficient heat, etc.) precursor solutions which are kept at around neutral pH (between about 6.5 and 7.5) and at ambient temperature remain stable, dissociation and polymerization do not occur and formaldehyde does not escape from the initial precursor mixing vessel, a safety advantage.

[0024] Trioxane and / or paraformaldehyde, which are solids at room temperature, are easy to handle and are advantageously less hazardous than alternative linking agents. In contrast, gaseous formaldehyde can readily escape from mixing vessels or solutions. Solutions of formaldehyde, furfuryl alcohol, or furfural, are extremely hazardous. The exemplary trioxane and / or paraformaldehyde pose fewer hazards and are easier to work with than commonly used electrophilic linking agents during polymerization reactions. Therefore, using the methods described herein, one can safely prepare large (or small) batches of precursor solutions and keep them safely stored for extended periods of time.

[0025] It is significant to note that aerogels produced with the procedure reported herein, for example, in Examples 1 and 2, were strong enough to withstand the stresses induced by atmospheric pressure, i.e. they exhibited high mechanical strength. By mechanical strength we refer here to the compression modulus. This mechanical strength is paramount for applications in Vacuum Insulated Panels (VIP). In VIPs, a porous core is evacuated and sealed inside a polymeric envelope. If the porous core is not sufficiently strong, the forces induced by the pressure differential cause the aerogel to collapse.

[0026] As a rule of thumb, one should employ cores with compression modulus >~ 10 MPa. This compression modulus ensures that the core will not compress or shrink more than 1% when used in a VIP application. A modulus < 5 MPa will result in a compression on the order of 10% of the core, with a decrease in porosity, increase in density and an increase in thermal conductivity. We have also observed that aerogels produced by dissociation of trioxane are mechanically stronger than aerogels obtained by using an equivalent molar concentration of formaldehyde aqueous solution. For example, Table 1 of Example 1 shows results obtained when 4 g of trioxane are added to a precursor solution. Trioxane has a molecular mass of 90.078 g / mol, hence 4 g corresponds to 0.044 moles of trioxane. As shown in Scheme 1 above, dissociation of trioxane yields 3 molecules of formaldehyde. If one were to replace trioxane with a 37% by weight formaldehyde solution (molecular weight = 30.026 g / mol) , one would need 0.044 * 3 *30.026 / 0.37 = 10.71 grams of formaldehyde solution. Without entering into the details of stoichiometry and coordination chemistry it is likely that coordination of the water introduced by the formaldehyde solution coordinates strongly to the hydrophilic phenolic precursors and slows the polymerization reaction. Without being bound by theory, the slower reaction leads to weaker links between the particles making up the aerogel backbone. Definitions

[0027] As used herein “aerogel” refers to a solid material of extremely low density, generally produced by removing the liquid component from a conventional gel. Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. In particular aspects, the aerogels that are made as described herein are phenolic aerogels.

[0028] Trioxane refers to 1,3,5-Trioxane, sometimes also called trioxin, a chemical compound with molecular formula C3H6O3. Trioxane is a solid at room temperature.

[0029] Paraformaldehyde is a linear polymer represented by the chemical formula H0(CH20)nH, where "n" represents a variable number of repeating formaldehyde units (- CH2O-) linked together. Essentially, it is a chain of formaldehyde molecules connected by oxygen atoms, appearing as a white solid at room temperature.

[0030] A vacuum insulated panel (VIP) is a form of thermal insulation comprised of a gastight enclosure (membrane wall) surrounding a rigid core, from which the air has been evacuated. The membrane wall prevents air from entering the panel after evacuation. The rigid core is typically a panel of a rigid, highly porous material, such as an aerogel, which supports the membrane walls against atmospheric pressure once the air is evacuated. Chemicals (known as getters) are generally included to collect gases leaked through the membrane or off-gassed from the membrane materials. When gas molecules strike the getter material, they combine with them chemically or by absorption. Thus, the getter removes small amounts of gas from the evacuated space. The getter is usually a coating applied to a surface within the evacuated chamber.

[0031] As used herein, “precursor solution” refers to a solution of a plurality of at least one type of polymerizable monomeric hydroxylated aromatic monomers trioxane or paraformaldehyde in a suitable solvent or mixture of solvents. In the “precursor solution”, the monomers are monomeric (not polymerized) and the trioxane or paraformaldehyde is intact, i.e., is not dissociated or broken down to form formaldehyde.

[0032] As used herein, “gelation solution” refers to a precursor solution to which a catalyst, for example, an acid, has been added. In a gelation solution, the exemplary trioxane or paraformaldehyde dissociates into formaldehyde and polymerization of the monomers occurs with formaldehyde serving as the electrophilic linking / cross -linking agent.

[0033] As used herein, “gelation solvent” refers to the solvent or mixtures thereof in which the monomers and trioxane or paraformaldehyde are dissolved to make the precursor solution. The gelation solvent is retained during the polymerization (gelation) step, i.e. it is the same solvent (or mixture of solvents) that is present in the gelation solution, although it contains a catalyst, for example, an acid.

[0034] As used herein, an exchange solvent (which may be a first exchange solvent) is a solution comprising a solvent or a mixture of solvents that is used to replace a previous solvent (such as a gelation solvent) that contains a wet gel. An exchange solvent is miscible with the solvent it replaces (such as a gelation solvent) and with a second exchange solvent by which it will be replaced (which may be a drying solvent). Generally, a drying solvent (see below) is not miscible with the gelation solvent and hence the need for solvent exchanges to arrive at a highly volatile drying solvent. Multiple exchanges using first, second third, etc. exchange solvents may be made. During an exchange of solvents, the wet gel remains intact.

[0035] As used herein, a “drying solvent” is the last solvent that contains a wet gel before the wet gel is dried, i.e. the drying solvent is not replaced but the wet gel is dried from the drying solvent, e.g. by evaporation or another suitable means, e.g. sublimation.

[0036] Linking agents, also known as cross-linking agents, create chemical bonds between molecules (for example monomers), effectively connecting them together to form a network structure (e.g. polymers). Mixtures of monomers and oligomers may also be crosslinked.

[0037] In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair.

[0038] An oligomer is a molecule made up of a few repeating units (e.g. monomers), usually between three and ten. Oligomers can have a variety of structures, such as linear chains, closed rings, or more complex structures. They generally have a molecular weight of several thousand or less.

[0039] A polymer is a macromolecule made up of a large number of repeating units (e.g. monomers). Polymers have long chains of repeating molecules called monomers that intertwine to form complex structures. Polymers generally have a high molecular weight, e.g., more than ten thousand.

[0040] Making the gel

[0041] The methods disclosed herein generally involve steps of combining (mixing) at least one polymerizable hydroxylated aromatic monomeric compound and at least one source of an electrophilic linking agent, generally trioxane or paraformaldehyde, to form an initial precursor solution. This first precursor solution is generally anhydrous, with suitable solvents being, for example: anhydrous acetonitrile; acetone; alcohols such as methanol, ethanol, ethylene glycol, n-propanol, n-butanol and isopropanol; water and polar protic solvents, such as dimethylsulfoxide, and in general any solvent that can dissolve the monomers, typically polar solvents, including chlorinated solvents, such as dichloromethane; and mixtures of these.

[0042] Hydroxylated aromatic monomeric compounds useful in the methods include but are not limited to: phenol; resorcinol; catechol; various isomeric benzenetriols such as phloroglucinol (benzene- 1, 3, 5-triol, hydroxyquinol (1,2,4-benzenetriol), and pyrogallol (1,2,3-benzenetriol); o-, m- and p- cresols (methyl phenols) such as hydroxytoluene, toluenol, benzol (cresylic acid); bisphenol A, phenol, hydroquinone, hydroxybenzoic acids, dyhydroxybenzoic acids, such as resocylic acid, triihydroxybenzoic acids, hydroxycynnamic acids, such as caffeic acid and ferulic acid, hydroxycoumarins such as scopoletin; flavonoids such as isoflavone, flavonol and flavone, tannins and gallotannins, such as tannic acid, gallic acid, ellagitannin, or catechin; stilbene derivatives such as piceatannol or resveratol; hydroxylated ligans such as pinoresinol and matairesinol; lignins, including water-soluble lignin derivatives, such as sulfolignin, etc., and combinations of these. In some aspects, the hydroxylated aromatic monomer is phenol or resorcinol.

[0043] The concentration of monomers in the precursor solution is generally in the range of from about 1 millimole / liter to about 0.75 mol / liter. For example, the concentration is generally about 1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750 millimole / liter, including all values in between these values to 0.01 millimole / liter. In other words, the concentration ranges from about 1.10, 1.20, 1.30, 1.40, 1.50 millimole / liter ... up to about 0.70, 0.71, 0.72, 0.73, 0.74 or 0.75 mol / liter.

[0044] Other monomers and mixtures thereof that can be or can be combined with the hydroxylated aromatic monomers include: silica, carbon, metal oxide, agar, cellulose, polyimide, chalcogens (e.g. S, Se, etc.), calcium selenide, pectin, polyurethane, polyicosyanurate, phenolic, polyisocyanurate, polyurethane, polystyrene, polyimide, pectin, cellulose, polyurethane, lignin, chitosan, and to combinations of an oxide and an organic component, such as silica-phenolic aerogels, etc.

[0045] Because the initial solution is anhydrous, it does not have a pH value and the trioxane or paraformaldehyde remains intact. Alternatively, if water is present, the pH is kept at about neutral (pH - 7.0, such as about 6.7, 6.8, 6.9, 7.0, 7.1, 7.2 or 7.3, including all values in between to 0.01 pH values) so that the trioxane or paraformaldehyde does not dissociate.

[0046] Mixing of the precursor(s), trioxane or paraformaldehyde and solvent(s) takes place in any suitable container, such as a mold in which it is desired to form a gel, or in a storage container (e.g. a bulk storage container) suitable for storing the precursor solution. Since the initial precursor solution is stable (i.e. trioxane or paraformaldehyde therein does not break down and the monomers do not polymerize) it can be safely stored for extended periods of time. For example, the precursor solution is stable for at least several days (e.g. for about 1-7 days), or for several weeks (e.g. for about 1-4 weeks), or for several months (e.g. for about 1- 52 weeks), or even years (e.g. for about 1-10 years).

[0047] The amount of trioxane or paraformaldehyde that is added to a precursor solution generally ranges from about 1 millimole / liter to about 1 mole / liter. For example, the concentration is generally about 1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000 millimole / liter (i.e. 1 mol / liter), including all single digit quantities in between these values to 0.01 millimole / liter. In other words, the concentration ranges from about 1.1, 1.2, 1.3, 1.4, 1.5 millimole / liter ... up to about 0.95, 0.96, 0.97, 0.98, 0.99 or 1.00 mol / liter. Typically, the concentration of trioxane is kept comparable to that of the hydroxyl groups of the monomers. So, for example, if the total concentration of hydroxyl groups (from the monomer, not the solvent) in the solution is 0.1 mol, one will add 0.1 mol of trioxane or paraformaldehyde .

[0048] In order to maintain the stability of the precursor solution, the following storage conditions are used: the precursor solution is generally stored at a temperature of from about -50 °C to about 40 °C (e.g. about -50, -45, -40, -35, -30, -25, -20, -15, -10, -5, 0, 5, 10, 15, 20, 25, 30, 35 or 40 °C in an anhydrous state, e.g. not exposed to any source of H2O, and not exposed to ambient gases such as air. However, the precursor solution can also stay exposed to air and in water (at the pH indicated above) during storage without damaging its properties. The solution may be kept under vacuum but evaporation of the solvent must be prevented. The precursor solution may be in a container that is hermetically sealed (airtight) thus preventing the passage of air, oxygen, or other gases. The parent solution can also be frozen if necessary.

[0049] Alternatively, in some aspects, the precursor solution is not stored but is used to form a gel shortly after mixing.

[0050] When it is desired to form a gel, a suitable amount of a catalyst, such as an acid or other catalyst as described herein, is added to the precursor solution, thereby forming a gelation solution.

[0051] Generally, the catalyst is a concentrated acid, examples of which include but are not limited to: hydrochloric acid, nitric acid, sulfuric acid, hydrobromic acid, citric acid, acetic acid, ascorbic acid, etc. The amount of acid that is added can vary but is generally in the range of from about 1 millimol / liter to about 1 mol / liter, such as about 1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000 millimole / liter (i.e. 1 mol / liter), including all values in between to 0.01 millimole / liter. In other words, the concentration ranges from about 1.1, 1.2, 1.3, 1.4, 1.5 millimole / liter ... up to about 0.95, 0.96, 0.97, 0.98, 0.99 or 1.00 mol / liter. The final pH of the gelation solution generally ranges from about 1 to about 3, such as about 10, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9 or 3.0.

[0052] Water must also be added to facilitate trioxane hydrolysis. Typically, one adds water in a concentration ranging from 1 / 100 to 10 times the concentration of trioxane or paraformaldehyde.

[0053] In other aspects, breakdown is achieved by the addition of organic or inorganic salts that modify the pH of the precursor solution when dissociated. Generally, the pH is modified to be sufficiently acidic or basic to cause the dissociation of the exemplary trioxane or paraformaldehyde. Examples of such organic or inorganic salts include but are not limited to: aluminum chloride, zinc chloride Mg(OH)2, CaCOa various amines, sodium acetate, ammonium chloride, aluminum nitrate, and other organic and inorganic compounds that are known to those of skill in the art.

[0054] In yet other aspects, release of an electrophilic agent is achieved by a temperature increase. For example, the temperature may be raised to from about 20 to about 130 °C, such as from about 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 110, 125 or 130 °C, including all integers between these values to 0.1 degrees centigrade. In some aspects, the temperature for catalysis is from about 20 to about 120 °C. The temperature is raised to any suitable value, up until the boiling point of the solvent, i.e. the temperature for catalysis is not above the boiling point of the solvent and is usually not equal to but is below the boiling point of the solvent. As disclosed herein, trioxane or paraformaldehyde advantageously serves as the source of the electrophilic linking agent. Upon addition of a catalyst to the precursor solution (thereby forming a gelation solution), the trioxane or paraformaldehyde dissociates, forming formaldehyde which acts as an electrophilic linking agent to cause polymerization of the monomers.

[0055] The step of combining is performed under conditions suitable for polymerizing the hydroxylated aromatic compound, thereby forming a wet gel. Such conditions include but are not limited to:

[0056] Temperature, gelation is often performed at ambient (e.g. room) temperature, such as at from about 21 °C, but it can be performed at a temperature in the range of from about 15 to about 200 °C, such as at about 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 °C, including all numeric values in between. The highest polymerization temperature will depend on the boiling point of the solvent. For example, if acetonitrile is used, the highest temperature of polymerization will be the boiling point of acetonitrile, which is between 81 and 82 °C.

[0057] Pressure', gelation is generally performed at ambient pressure but it can be carried out in a pressurized vessel, such as an autoclave, or a high pressure vessels, such as those manufactured by Parr, Inc.

[0058] Reaction time', reaction times for polymerization vary depending, e.g. on the monomers that are used, the concentration of monomers and linking agents, the temperature, etc. The time required can range from a few minutes to hours to days.

[0059] Those of skill in the art will recognize that polymerization of monomers is generally not an “all or nothing” reaction. In other words, “polymerization” of monomers proceeds, for example, by linking monomers to each other, e.g. to form oligomers; then by attaching monomers to oligomers that have already been formed and / or to polymers that already been formed; or by attaching oligomers to oligomers; or by attaching oligomers to polymers; or by attaching polymers to other polymers; etc., thereby forming extended chains which may be straight or branched or both. Generally, the longer polymerization is permitted to proceed, the denser the gel that is formed due to extensive crosslinking. Further, some monomers, oligomers or polymers may remain unreacted (unattached) to other gel components and are removed when excess solvent is removed (e.g. by washing in one or more solvents), as discussed below. Once a gel is formed, it is dried. Drying is carried out by any of several known methods. For example, in some aspects, the aerogels are dried by direct evaporation of the gelation solvent, with or without a vacuum. In such aspects, the gelation solvent is generally highly volatile, e.g., methanol, ethanol, acetone, acetonitrile, isophorone, 2-butanone, pentane, hexane, heptane, octane, cyclohexane, benzene, isopropyl alcohol, butyl alcohol, diethyl ether, petroleum ether, ethyl acetate, tetrahydrofuran, chloroform or mixtures thereof. The solvent can be also less volatile than the common organic solvents described above, for example, water, methylisobutyl ketone, 2-propanone, dioxane, toluene, acetic acid, chlorobenzene, nitromethane, or pyridine. Mixtures of these solvents, including highly volatile and less volatile solvents, are also possible.

[0060] In other aspects, which may be preferred, the aerogels are dried via solvent exchange. Typically, solvent exchange involves removing as much gelation solvent as possible from the container in which the gel was formed and then submerging the gel in about lOx the gel volume of a different solvent (usually a more volatile drying solvent) that is miscible with the gelation solvent but which does not dissolve or react with the gel matrix and / or the polymerized components of the pore walls. The drying solvent displaces the gelation solvent and excess reactants (including unreacted monomers) around and within the pores of the gel, and the exchange is typically performed multiple times (e.g. 1-20 times, such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 12, 13, 14, 15, 16, 17, 18, 19 or 20 times), with or without agitation. Exchanges are performed until unwanted, excess reactants and gelation solvent are removed, e.g. to a level at which at least about 90% or more of the gelation solvent is removed / replaced, and usually less than about 1% of the previous solvent is present. This time required varies depending on the solvents, the conditions (e.g. temperature), the thickness of the gel, gel pore size, etc., but is generally at least several hours and can be one or more days. Solvent exchanges generally are performed in hermetically sealed containers to prevent evaporation.

[0061] In some aspects, one or more exchanges with intermediate exchange solvents may be used in between the gelation solvent and the final drying solvent. For example, if the desired gelation solvent and the desired final drying solvent are not miscible or are only partially miscible, an intermediate solvent may be used that is miscible with both the gelation solvent and also with the drying solvent. Thus, the gelation solvent is replaced by an intermediate solvent during a first stage of the solvent exchange, and then the intermediate solvent is replaced by the final drying solvent in a second stage of the solvent exchange. In fact, multiple intermediate solvents and multiple stages of solvent exchange may be performed until a desired level of displacement / replacement / removal of all previous solvents and excess reactants is attained. For example, a precursor / gelation solvent may be acetonitrile, which is first exchanged with isopropanol, which is then exchanged with acetone. In another example, the gelation solvent can be ethanol, which is exchanged with acetone and then with hexane.

[0062] The equilibration time for a solvent exchange caries and can be as little as about 1-2 hours. However, the equilibration time may be as long as about two or more days for thicker gels. The solvent exchange may be enhanced by application of pressure (positive or vacuum).

[0063] Finally, the drying solvent is removed, e.g. by decanting excess solvent and then by evaporation. Drying is generally performed at ambient temperature and pressure and does not require hermetic sealing. However, drying may be via sublimation at a suitable temperature, if a solidified drying solvent is used, e.g., at a temperature at which the solvent is solid. Those of skill in the art will recognize that the “solid” drying solvent is introduced into the gel in a liquid state, the temperature is adjusted (usually lowered) to solidify the solvent, and then the gel is dried via sublimation of the solid solvent.

[0064] Examples of suitable drying solvents include but are not limited to: acetone, isopropanol, pentane, hexane, methanol, ethanol, acetone, acetonitrile, isophorone, 2- butanone, pentane, hexane, heptane, octane, cyclohexane, benzene, isopropyl alcohol, butyl alcohol, diethyl ether, petroleum ether, ethyl acetate, tetrahydrofuran, chloroform or mixtures thereof. In alternative aspects, the gelation solvent is exchanged with a solvent that freezes or solidifies) at ambient temperature and is then removed by sublimation. Examples of suitable solvents that freeze at ambient temperature that are use include but are not limited to: tertbutanol, camphene, naphthalene, camphor, menthol, dioxane, cyclohexane, dichlorobenzene, and the like.

[0065] Other (optional) components

[0066] In some aspects, other materials are added in order to modify the properties of the composite materials, e.g. to increase strength, heat / fire resistance, to make the materials more hydrophobic, or more hydrophilic, or to increase their optical density, including optical density in the infrared, or to increase or decrease electric conductivity. For example, loose fiberglass or other filler materials may be added to the aerogel precursor solution or mixed to the already-poured aerogel precursor solution before gelation occurs. Other examples of fillers include but are not limited to wood flour, cellulose fibers, metal nanoparticles or metal fibers, carbon fibers, graphene, glass microspheres, polyimide fibers, carbon black, and graphite. Alternatively, these filler or other materials may be added to the drying solution composition and the fillers could be deposited on the walls of the open cell foam in during drying.

[0067] In some aspects, a mat of fiberglass is placed at the bottom of the mold. The precursor solution is then poured on top of the fiberglass mat, which becomes incorporated into the composite gel. Alternatively, loose fiberglass is added to the precursor solution or mixed to the already-poured precursor solution before gelation occurs.

[0068] In other aspects, a sheet of paper is first placed at the bottom of the mold. The precursor solution is then poured on top of the fiberglass mat, which becomes incorporated into the composite gel. Alternatively, loose cellulose fibers or granules or even shredded paper are added to the precursor solution or mixed to the already -poured precursor solution before gelation occurs.

[0069] PROPERTIES OF THE PHENOLIC AEROGELS

[0070] The final dried phenolic aerogel made by the methods disclosed herein generally has a density (bulk density) ranging from about 0.05 to about 0.50 g / cm2, such as about 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 or 0.50 g / cm2, including all decimal fractions between these values to 0.00 places.

[0071] The surface area of a dried phenolic aerogel made by the methods disclosed herein can vary and is generally from about 1 to 300 m2 / g, such as about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290 or 175, 200, 225, 250, 275, 300 m2 / g, , including all digits and decimal fractions thereof between these values to 0.0 places.

[0072] The thermal conductivity of a dried phenolic aerogel made by the methods disclosed herein is very low and generally ranges from about 0.001 to about 0.1 W / mK , such as about 0.001, 0.005, 0.010, 0.015, 0.020, 0.025, 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060, 0.065, 0.070, 0.075, 0.080, 0.085, 0.090, 0.095 or 0.100 W / mK g / cm2at ambient temperature, including all decimal fractions between these values to 0.000 places. In some aspects, the thermal conductivity of the aerogels ranges between 0.01 and 0.04 W / m-K at ambient temperature, e.g. about 0.01, 0.02, 0.03 or 0.04 W / m-K at ambient temperature (e.g. room temperature, e.g., 19-32°C, and often about 20°C) and ambient pressure (e.g., 0.76 atm- 1.2 atm, and often about 1 atm (at sea level)). They are thus efficient thermal insulators and perform very well in VIPs.

[0073] As discussed above, aerogels produced by the present methods are mechanically stronger than aerogels obtained by using an equivalent molar concentration of formaldehyde aqueous solution. For example, the compression modulus (which is related to bulk density) of the aerogels is generally in the range of from about 1 MPa to about 750 MPa, such as about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 3909, 400, 410, 20, 430, 440, 450, 460, 470, 480, 490, 500, 550, 600, 650, 700 or more, e.g. up to about 750 MPa. In some aspects, the MPa ranges from about 5 to about 500 MPa.

[0074] Those of skill in the art are well versed in how to measure such parameters.

[0075] USES OF THE DRIED GELS

[0076] The dried phenolic gels made by the methods described herein are used for any of a variety of purposes, examples of which include but are not limited to: insulated cabinet structures for appliances such as refrigerators, freezers, stoves, etc.; for insulation of buildings; in temperature stabilizing cargo compartment; in thermally insulated packaging used, e.g. in the transportation of temperature sensitive goods, for example pharmaceuticals, biological samples, vaccines and the like; and for other purposes. In some aspects, before use, the dried gel is surrounded by a plastic foil sleeve to prevent puncture of the gel.

[0077] Accordingly, the dimensions of a phenolic aerogel made as described herein (after drying) can range from a few centimeters in length, width and thickness to a few centimeters in thickness but much larger is length and width, to fit the intended purpose or to allow cutting and trimming of the dried aerogel. For example, in industrial applications, the size of a dried phenolic aerogel as described herein can be hundreds of centimeters or even meters in length and / or width.

[0078] The present aerogel materials have a wide variety of applications / uses. For example, phenolic aerogels are frequently applied in the thermal insulation of hypersonic vehicles on account of their unique advantages such as high carbon residue rate, high-temperature resistance, and good thermal insulation performance. They are used in small and large-scale industrial manufacturing. It is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[0079] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

[0080] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Representative illustrative methods and materials are herein described; methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention.

[0081] All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference, and are incorporated herein by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual dates of public availability and may need to be independently confirmed.

[0082] It is noted that, as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as support for the recitation in the claims of such exclusive terminology as "solely," "only" and the like in connection with the recitation of claim elements, or use of a "negative" limitations, such as "wherein [a particular feature or element] is absent", or "except for [a particular feature or element]", or "wherein [a particular feature or element] is not present (included, etc.)...".

[0083] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present invention. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.

[0084] The invention is further described by the following non-limiting examples which further illustrate the invention, and are not intended, nor should they be interpreted to, limit the scope of the invention.

[0085] EXAMPLES

[0086] Example 1.

[0087] Phenolic aerogels were synthesized by reacting the precursors and concentrations thereof as shown in Table 1.

[0088] 'Fable I: Precursors and concentrations used in a typical synthesis

[0089] The precursors from Table 1 were mixed and poured into a hermetically sealed mold, typically pre-heated, and placed in an oven at a temperature > 60°C for a time between 1 and 3 hours, during which the precursors react to form a gel.

[0090] The gel was then removed from the mold and placed in a bath containing an excess of isopropanol. Typically, the volume of isopropanol is lOx the volume of the sample. In this part of the process, the isopropanol diffuses inside the porous gel, while gelation solvent and reaction by-products diffuse out of it. The isopropanol was then decanted from the container and replaced with acetone, typically lOx times the volume of the sample. In this part of the process, acetone replaces isopropanol and residues from the gelation solvent and reaction by-products. The acetone solvent exchange was repeated a second time to ensure that the concentration of non-acetone solvent inside the pores of the gel is < 1%. During all these solvent exchange stages, the gel was kept inside a hermetically sealed container to prevent solvent evaporation. Once acetone exchange was complete, the gel was removed from solvent and covered with a plastic sheet or placed inside a plastic bag. The sheet (or the bag) was not hermetically sealed and allows for slow evaporation of the acetone. The gel was then allowed to air dry at ambient temperature and pressure for 24-48 hours until all acetone was evaporated out of the gel’s pores. The physical properties of the resulting materials are reported in Table 2.

[0091] Table 2: Physical properties of aerogels obtained following the procedure described in

[0092] Example 1.

[0093] Example 2.

[0094] Phenolic aerogels were synthesized by reacting the precursors and concentrations thereof as shown in Table 3.

[0095] Table 3: Precursors and concentrations used in a typical synthesis

[0096] The precursors from Table 3 were mixed and poured into a hermetically sealed mold, typically pre-heated, and placed in an oven at a temperature of 70°C for a time between 1 and 12 hours. Gelation occurred after about 1 hour. The gels were left to cure for a few hours inside the oven. The gel was then removed from the mold and allowed to air dry at ambient temperature and pressure for 24-48 hours until all the gelation solvent (a mixture of water and acetonitrile) had evaporated out of the gel’s pores. The physical properties of the resulting materials are reported in Table 4.

[0097] Table 4: Physical properties of aerogels obtained following the procedure described in

[0098] Example 2,

[0099] It is most important to note that aerogels produced with the procedure reported in Examples 1 and 2 had a compression modulus of at least 10 MPa and were therefore able to withstand the stresses induced by atmospheric pressure without collapsing. This mechanical strength is paramount for applications in Vacuum Insulated Panels (VIP). In VIPs, a porous core is evacuated and sealed inside a polymeric envelope. Materials with a low compressive modulus will collapse under the pressure differential induced by evacuation.

[0100] While the invention has been described in terms of its several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. Accordingly, the present invention should not be limited to the embodiments as described above but should further include all modifications and equivalents thereof within the spirit and scope of the description provided herein.

Claims

CLAIMSWe claim:

1. A method of making a wet phenolic gel, comprising forming, in a solvent or mixture of solvents, a precursor solution by mixing i) monomers of at least one hydroxylated aromatic compound and ii) at least one compound that breaks down into an electrophilic linking agent upon addition of a catalyst; and adding at least one catalyst to the precursor solution to form a gelation solution in which the at least one compound dissociates to form the electrophilic linking agent, wherein the electrophilic linking agent reacts with the monomers to form polymers, thereby forming a wet phenolic gel.

2. The method of claim 1 , wherein the at least one catalyst is an acid, a base, a salt that modifies the pH of the precursor solution or heat, or a combination of two or more of these.

3. The method of claim 1, wherein the at least one compound that breaks down into an electrophilic linking agent is trioxane or paraformaldehyde.

4. The method of claim 1, wherein the at least one compound breaks down into an electrophilic linking after a temperature increase.

5. The method of claim 1, wherein the at least one hydroxylated aromatic compound includes resorcinol, phenol and / or bisphenol A.

6. The method of claim 1, wherein the solvent or mixture of solvents is or comprises acetonitrile.

7. The method of claim 1, wherein the catalyst is a concentrated acid and, optionally, heat.

8. The method of claim 7, wherein the concentrated acid is concentrated HC1 or concentrated H2SO4.

9. The method of claim 1, further comprising a step of cross-linking unreacted monomers and oliogomers and / or polymers formed during the adding step.

10. The method of claim 1 wherein the step of adding is performed more than a month after the forming step.

11. The method of claim 1 wherein the step of adding is performed less than a day after the forming step.

12. A method of making a phenolic gel, comprising forming, in a solvent or mixture of solvents, a precursor solution by mixing i) monomers of at least one hydroxylated aromatic compound and ii) at least one compound that breaks down into an electrophilic linking agent upon addition of a catalyst; adding at least one catalyst to the precursor solution to form a gelation solution in which the at least one compound dissociates to form the electrophilic linking agent, wherein the electrophilic linking agent reacts with the monomers to form polymers, thereby forming a wet phenolic gel; removing the solvent or mixture of solvents from the wet phenolic gel: and drying the wet phenolic gel to achieve a phenolic aerogel.

13. The method of claim 12, wherein the step of removing the solvent or mixture of solvents is performed by one or more solvent exchanges.

14. The method of claim 13, wherein solvents utilized in the one or more solvent exchanges include one or more of isopropanol and acetone.

15. The method of claim 12, wherein the step of drying is performed by supercritical drying.

16. The method of claim 12, wherein the step of drying is performed by freeze drying.

17. The method of claim 12, wherein the step of drying is performed by evaporation of porefilling solvent at room temperature and pressure.

19. The method of claim 12, wherein the step of drying is performed by evaporation at below ambient pressures and temperatures.

20. The method of claim 12, wherein the step of drying is performed at ambient temperature by sublimation.

21. A phenolic aerogel formed by the method of claim 12.

22. The phenolic aerogel of claim 21, wherein the phenolic aerogel has a compressive modulus of from 1 to 500 MPa.

23. The phenolic aerogel of claim 22, wherein the compressive modulus is equal to or greater than 10 MPa.

24. The phenolic aerogel of claim 21, wherein the phenolic aerogel has a density lower than 0.25 g / cm2and a thermal conductivity of 0.040 W / mK at ambient temperature and pressure.

25. The phenolic aerogel of claim 21 configured as part of a vacuum insulated panel.

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