Patent Application
20060263587
There is no government interest in this application.
The present invention relates in general to structures formed from bonding fiber-reinforced aerogel layers to each other and/or a non-aerogel surface with a binder layer. Said binder layer can also be utilized as a coating.
Aerogels, first prepared by Kistler in 1931 [S. S. Kistler, Nature, 1931, 127, 764], are a type of material structure rather than a specific material, and can be prepared by replacing the liquid solvent in a wet gel with air without substantially altering the network structure (e.g., pore characteristics) or the volume of the gel body. Supercritical and subcritical fluid extraction technologies are commonly used to extract the fluid from the gel without causing the collapse of the pores. “Aerogels” refers to “gels containing air as a dispersion medium” in a broad sense and include, xerogels and cryogels in a narrow sense. A variety of different aerogel compositions are known and may be inorganic, organic or organic-inorganic hybrids. Inorganic aerogels can be based upon metal alkoxides such as silica [S. S. Kistler, Nature, 1931, 127, 764], alumina [S. J. Teichner et al, Adv. Colloid Interface Sci. 1976, 5, 245], and various carbides [C. I. Merzbacher et al, J. Non-Cryst. Solid, 2000, 285, 210-215]. Organic aerogels include, but are not limited to, urethane aerogels [G. Biesmans et al, 1998, 225, 36], resorcinol formaldehyde aerogels [R. W. Pekala, U.S. Pat. No. 4,873,218], and polyimide aerogels [W Rhine et al, U.S.2004132845].
Organic-inorganic hybrid aerogels are mainly ormosil (organically modified silica) aerogels [D. A. Loy et al, J. Non-Cryst. Solid, 1995, 186, 44]. The organic components in these aerogels are either dispersed throughout or chemically bonded to the silica network. It is usually preferred to have covalently bound organic components in such structures to minimize the amount of washout during solvent extraction from the wet gel.
Low to moderate density aerogel materials (typically in the range of about 0.01 g/cm3 to about 0.3 g/cm3) are widely considered to be the best solid thermal insulators, and have thermal conductivities of about 12 mW/m-K and below at 37.8° C. and atmospheric pressure. Aerogels function as thermal insulators primarily by minimizing conduction (low density, tortuous path for heat transfer through the solid nanostructure), convection (very small pore sizes minimize convection), and radiation (IR absorbing or scattering dopants are readily dispersed throughout the aerogel matrix). Aerogel materials also display many other interesting acoustic, optical, and chemical properties that make them useful in both consumer and industrial markets. Since aerogels, particularly in low density form are fragile, they must be handled or processed with great care. This presents a significant limitation for the application of aerogels in certain sectors of the insulation market.
In the past two decades, many investigators have attempted to improve the mechanical properties of the aerogels materials. A few notable improvements are as follows: N. Leventis, et al, claim an increase in strength of silica aerogels by a factor over 100 through cross-linking the silanols of the silica hydrogels with poly (hexamethylenediisocyanate) [Nano Letters, 2002, 2(9), 957-960, U.S. 2004/0132846]. In this approach, highly reactived isocyanate substance was used as reinforcement agent, and was introduced into the silica network in the post gelation stage. Relatively large amounts of this toxic substance is needed for the preparation of high strength aerogel presenting many problems in large scale manufacturing. All of the examples taught in this approach are performed on the bench scale, with typical example for the aerogel dimension of less than 2 cm.
B. M. Fung et al developed a new class of high organic cellulose aerogel [M. M. Fung et al, Adv. Mater. 2001, 13, 644-646]. Cellulose aerogels demonstrate relatively high impact strength of up to 0.85 Newton meters, considerably greater than typical metal oxide aerogel materials. Fung teaches preparative examples on very small scales, and little data is provided on the material properties. Further studies on cellulose aerogels made according to Fund et al., shows that they have inferior thermal insulation properties compared to other aerogels such as those based on metal oxides like silica.
The development of fiber-reinforced aerogel composites such as those described by Stepanian et al. (U.S.2002/094426) has opened up many application areas for aerogel materials. The fiber reinforcement adds considerable toughness and resilience, particularly in high flexural strain applications. The flexibility of thin sheets (typically between about 0.1 mm and about 25 mm) allows for the manufacture of large sections of aerogel composites while retaining most of the useful qualities such as low density and low thermal conductivity. However, the flexural toughness of the fiber-reinforced aerogel composites tends to lower the effective stiffness of the composites such that large sections may not support their own weight when standing on edge.
Embodiments of the present invention describe strengthened fiber-reinforced aerogels without any of the aforementioned drawbacks. Such structures provide increased mechanical stability, ability to support more than their own weight when standing on edge, and a host of other benefits while maintaining the exceptional insulation properties of fiber-reinforced aerogels.
Embodiments of the present invention describe a structure comprising at least one fiber-reinforced aerogel layer and at least one binder layer, said binder layer comprising a silicon-containing organic material and where the binder layer is bonded to at least one surface of at least one fiber-reinforced aerogel layer.
Embodiments of the present invention describe structures comprising at least one fiber-reinforced aerogel layer and at least one binder layer, said binder layer comprising a silicon-containing organic material. Such structures are highly useful as thermal insulators, acoustic insulators or both. The binder layer can be used as a coating, an adhesive, or both for the fiber-reinforced aerogel composites. The unique combination of at least one binder layer and at least one fiber-reinforced aerogel layer, as described herein allows a variety of useful configurations for fiber reinforced aerogels such as but not limited to: adhesion to like and/or dissimilar surfaces, high strength coatings, molded fiber-reinforced aerogel forms, mechanically stable multi-ply structures, and a host of others.
In one embodiment, “aerogel materials” or “aerogel material” refers to aerogel particles and monoliths. Fiber-reinforced forms of aerogel materials may additionally comprise chopped fibers, mats, battings, felts or a combination thereof. Furthermore, aerogel materials can be prepared from inorganic, organic or hybrid organic-inorganic precursors, whereby the resultant chemical structure will comprise the same or substances derived there from. Density of aerogels is generally between about 0.03 g/cm3 and about 0.3 g/cm3 with surface areas typically between about 300-1000 m2/g with greater than 90% porosity. Surface treatment of aerogels with compounds such as silyalting agents yields a hydrophobic surface.
The development of fiber-reinforced aerogel materials has opened up many application areas for, and facilitated manufacture of aerogel materials while retaining their exceptional insulating properties thereof. As used herein, “blankets” or “aerogel blankets” refers to fiber-reinforced aerogel materials which comprise a batting. Aerogels reinforced with a batting is described in published U.S. patent application U.S.2002/094426 (Stepanian et al.) which hereby incorporated by reference. Of course fiber-reinforced forms of organic, inorganic and hybrid organic-inorganic aerogels can also be prepared. By way of a non-limiting example, a fiber-reinforced form of inorganic aerogels is described in Stepanian et al., the teachings of which can be used in an analogous manner for preparing organic or organic-inorganic hybrid forms of the same. A non-limiting example of fiber reinforced form of hybrid organic-inorganic aerogels can be obtained from U.S. patent applications 2005/0192367 and 2005/019366 (which is based on U.S. provisional application 60/594,359), all three which are hereby incorporated by reference. Another non-limiting example involves, silica-organic polymer hybrid aerogels with urea linkages, as described in U.S. provisional patent application 60/692,100 which is hereby incorporated by reference. Yet another non-limiting example involving silica-chitosan hybrid aerogels and variation thereof is described in U.S. provisional patent application 60/594,359 also incorporated by reference.
The binder layer in embodiments of the present invention may serve as a coating to coat at least one surface of at least one fiber-reinforced aerogel material. The binder may also serve as an adhesive to bind at least two layers of fiber-reinforced aerogels, or at least one layer of a fiber-reinforced aerogel to another surface, or any combination of the preceding. By “another surface” it is intended to include a surface that is chemically or structurally, dissimilar to that of said fiber-reinforced aerogels. Examples of such surfaces include but are not limited to non-aerogel forms of polymeric, ceramic or metallic surfaces.
In general, the binder layer is a silicon-containing organic material. Of particular interest are silicon-containing polymeric materials. This type of binder layer is formed from organic compounds (i.e. precursors) which contain at least one silicon atom, the polymerization and/or three-dimensional cross-linking of said compound yields the silicon containing organic material. Precursors can be in the form of monomers, oligomers or both and comprise a silicon-containing segment and an organic segment. A non-limiting example of the silicon-containing segment is an alkoxysilyl or silanol group. Examples of alkoxysilanes include mono-, di- or trialkoxysilanes where the alkoxy groups comprise 1 to 12 carbon atoms.
The organic segment can comprise acrylics such as but not limited to methacrylates, methyl methacrylates, ethyl methacrylates, propyl methacrylates, butyl methacrylates, and the higher alkyl or aryl chain relatives in the methacrylate series. Other polymerizable monomers may be reacted with the cross-linking silicon containing reagent such as, but not limited to: cyanoacrylate, styrene, and other activated olefinic monomers to form the organic segment.
The binder layer is applied to at least one surface of at least one fiber-reinforced aerogel and exposed to an energy flux suitable for initiating polymerization, cross-linking or both in said binder layer. Such energy forms include but are not limited to heat, electromagnetic energy, an infrared energy, an x-ray energy, a microwave energy, a gamma ray energy, acoustic energy, ultrasound energy, particle beam energy, electron beam energy, beta particle energy, an alpha particle energy, and combinations thereof.
Upon polymerization, cross-linking or both, the binder layer adheres to the surface of the fiber-reinforced aerogel and further strengthens itself. In the case of alkoxysilyl functionalized organic compounds (precursor of the binder layer) and silica fiber-reinforced aerogels layers, the application of a suitable energy flux results in hydrolysis/condensation of alkoxy groups thereby resulting in siloxane (Si—O—Si) linkages within the binder layer and between the binder layer and the silica aerogel material. Furthermore such bonding can also occur with another surface, as previously described. Therefore the binder layer can simultaneously bind, two or more surfaces, thereby acting in a sense as an all purpose adhesive for fiber-reinforced aerogels.
In the case of silicon-containing organic compounds comprising an alkoxysilyl segment and an acrylic moiety, both hydrolysis/condensation and free radical polymerization can take place. The free radical polymerization of acrylic moieties results in cross-linkages within the binder layer and may also form chemical bonds with at least one surface of an aerogel material (such as silica aerogels), or another surface or both.
The following is an example of a structure according to embodiments of the present invention wherein hybrid silica-PMMA blankets are utilized. This example is intended for further illustration of embodiments of the present invention without limiting the scope thereof in any way.
In this embodiment, trialkoxysilyl-containing polymethacrylate oligomers are used as a binder layer to attach at least two fiber-reinforced silica-PMMA aerogel blankets thereby forming a rigid insulation panel with increased insulation value (R-value), or to coat such hybrid aerogel blankets, or both. A two-ply structure comprising two hybrid aerogel blankets and a binder layer disposed there between can resist up to 4000 psi compression stress and 200 psi flexural stress, before rupture. The thermal conductivity values are typically below 16 mW/mK. Accordingly, structures with dimensions of up to 100 square feet and over 10 inch thick can be prepared. The binder layer, when used as a coating for these hybrid aerogel blankets provides an effective barrier to damage from contact with water or common organic solvents (such as THF, ethanol, etc.)
In one embodiment, the polymer content in the silica-polymer hybrid aerogel is less than about 90% or less that about 80% or less than about 70% or less than about 60% or less than about 50% or less than about 40% or less than about 30% or less than about 20% or less than about 10% or less than about 5% (wt).
Trialkoxysilyl-containing polymethacrylate oligomers can be prepared by thermal (usually in the presence of a radial initiator) or UV initiated polymerization between a methacrylate monomer and a cross linker such as trimethoxylsilyl propylmethymethacrylate (referred as TMSPM here after). Thermal initiated polymerization was used in this embodiment, unless stated otherwise. Suitable initiators include, but are not limited to: azobisisobutyronitrile, tert-butylperoxy-2-ethyl hexanoate, α,α-dimethoxy-α-phenyl acetophenone, 2-benzyl-2-(dimethylamino)-1-[4-4-morpholinyl)-butanone. The methacrylate monomer includes, but not limited to methylmethacrylate (referred as MMA there after), ethylmethacrylate (referred as EMA thereafter), butylmethacrylate (referred as BMA there after), hydroxyethylmethacrylate (referred as HEMA there after), hexafluorobutyl methacrylate (referred as HFBMA there after), etc.
The polymerization was carried out in lower alcohol solutions at elevated temperatures between 40° C. to 100° C. and preferably 70° C. to 80° C. To ensure a fast reaction the reactant concentration in alcohol solution needs to be in the range between 5 and 95 weight percent, and preferably from 40 to 70 weight percent. The mole ratio of TMSPM/methacylate monomer is in the range between 1 and 10 and preferably between 1 and 4. The resulting trimethoxysilyl containing polymethacrylate oligomer has a molecular weight between about 30,000 and about, 350,000 and is soluble in common organic solvents.
Generally the principal route for the formation of fiber reinforced silica-PMMA aerogel blankets can be followed according to published U.S. patent application 2005/019366. Here, trimethoxysilyl containing polymethacrylate oligomer was co-condensed with a silicon alkoxide such as tetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS) or partially hydrolyzed an oligomerized silicon alkoxide materials (e.g. polyethylsilicates commercially available as Silbond 50, Silbond 40, Silbond H-5 or Dynasil 40) in alcohol solution to form a hybrid alcogel. A suitable fibrous batting material with the x-y oriented tensile strengthening layers was added to the hybrid alcogel prior to its gelation. The fiber reinforced PMA/silica aerogel blanket is formed from this composite after surface modification and CO2 supercritical extraction.
In preparation of a panel, according to an embodiment, trimethoxysilyl containing polymethacrylate oligomer was coated onto the surface of a fiber reinforced silica-PMA aerogel blanket, before attaching to a second piece of the fiber reinforced silica-PMA aerogel blanket. A unique characteristic of the trimethoxysilyl-containing polymethacrylate oligomer (i.e. binder layer) is that it does not penetrate into the matrix of the aerogel thereby damaging the nanoporous structure, and/or significantly increasing the thermal conductivity.
The surface of a silica-PMA hybrid aerogel blanket has silanol (Si-OH) and methacrylate pendent groups. Trimethoxysilyl-containing polymethacrylate oligomer also contains methacrylate as well as alkoxysilyl pendant groups. The multiple layers of PMMA/silica aerogel blanket glued together by trimethoxysilyl-containing polymethacrylate oligomer (i.e. binder layer) were placed in an oven for the final fixing step. The curing temperature ranges between 40° C. to 100° C. and preferably 70° C. to 80° C. At elevated temperatures, and in the presence of a thermal initiator in the oligomer binder, the methacrylate functions in the oligomer binder will react with the similar functions on the surface of the hybrid aerogel blanket to form covalent bonds; the alkoxysilyl functions in the binder layer reacts with the silanol groups on the surface of the hybrid aerogel to form a strong Si—O—Si covalent bond thereto, as illustrated in
The following non-limiting examples are provided to further illustrated how to carry out the methods of the present invention. In the ensuing examples, weights are expressed as grams (g) unless stated otherwise. The MMA Monomer was purchased from Aldrich; cross-linker TMSPM was obtained from Ashland Chemicals as Dow Corning Z6030 silane, catalyst tert-butylperoxy-2-ethyl hexanoate was obtained from Degussa.
This example illustrates the formation of trimethoxysilyl containing polymethymethacrylate oligomer binder. 35.4 g of tert-butylperoxy-2-ethyl hexanoate were added to a mixture of 780 g of MMA, 975.0 g of TMSPM and 422 g of methanol, following by vigorous stirring at 68 to 78° C. for 40 minutes. The polymethymethacrylate oligomer binder was obtained as a viscous liquid in concentrated methanol solution. GPC shows 70% of the monomer was polymerized and the oligomer has Mw of 632570.
This example illustrates the formation of a rigid thermal insulation panel. The binder of example 1 was coated as a layer between three pieces of 1′×1′ foot fiber-reinforced PMA/silica hybrid aerogel blankets with a density of about 0.16 g/cm3. The three hybrid aerogel blankets were affixed to one another with a binder layer between every two blankets. The five-layer coupon was placed into an oven set at 75° C. for 2 hours. The resultant structure, which is in the form of a panel shows a density of about 0.17 g/cm3; thermal conductivity of about 13.9 mW/mK under ambient conditions and flexural strength at rupture of about 101 psi. The size of this rigid insulation panel is 1′×1′ foot and 2″ inches thick. This panel deforms lass than 10% under 17.5 psi compression. For much higher compression loading of 4000 psi, this panel recovery up to 90% of its original thickness within 2 hours after compression.
Ultra large size rigid aerogel insulation panels with over 90 square feet dimension can be prepared. For example, 30∴×3′ dimension and ⅛″ thick silica-PMA aerogel composite (two blankets and a binder layer as a glue) was prepared according to this approach. In theory, there is no limitation on the size of the composite prepared with the embodiments of the present invention. It is only currently limited by the space available for drying the composite sheet. Such high strength aerogel panels show good compression resistant properties (<10% under 17.5 psi, up to 98% recovery strain after 4000 psi loading). The resulting high strength aerogel panels also exhibit good flexural strength (resist 100 psi flexural pressure). The improvement of mechanical properties in this hybrid aerogels composite was achieved without sacrificing other inherent properties of aerogel such as low density and low thermal conductivity.
In one embodiment of the present invention, a shaped structure is formed from at least one binder layer and at least one fiber-reinforced aerogel layer. A fiber-reinforced aerogel layer can be shaped to a desired geometry and subsequently coated with a binder layer. Upon curing, the binder layer further rigidifies permanently maintaining the aerogel layer in the desired geometry. Of course the same can be practiced with two or more fiber-reinforced aerogel layers where these layers sandwich a binder layer, and/or are coated with a binder layer where again upon curing, the desired shape of the aerogel layers is achieved. Examples of such geometries include, but are not limited to: spherical, hemispherical, cylindrical, hemi cylindrical, half-pipe, annular, helical, navicular, corrugated, grooved, rippled, and various others. Without limiting the scope of this embodiment an example of a shaped structure using silica-PMMA hybrid aerogel layer or layers is described, and also illustrated in
In another embodiment, a structure comprises at least one fibrous layer in addition to at least one fiber-reinforced aerogel layer and at least one binder layer. The fibrous layer can comprise energy absorbing ballistic fibers such as polyaramids (e.g. Kevlar®) ultrahigh molecular weight polyethylene (e.g. Specra®), PBO as well as others, and can be in the form of a batting, a matt or a felt. The binder layer (such as a trimethysilyl-containing polymethacrylate) is capable of impregnating the fibrous layer thereby affixing the same to at least one fiber-reinforced aerogel layer after curing. The benefits of such structure include added reinforcement and insulation capability.
In another embodiment, a fiber-reinforced aerogel layer is adhered to another surface via the binder layer. For instance, if another surface contains silanols or other suitable reactive surface pendant groups, the binder layer can affix the aerogel layer there to via chemical bonds. Exemplary surfaces include but are not limited to polymeric, ceramic or metallic surfaces and non-aerogel forms thereof. In a sense, the binder layer can act as an all purpose glue for fiber-reinforce aerogel layers.
In another embodiment a protective layer is formed by coating a fiber-reinforced aerogel layer with at least one binder layer. A binder layer such as the trimethoxysilyl-containing polymethacrylates, when cured is an effective barrier to damage from contact with water or common organic solvents (such as THF, ethanol, etc.) Furthermore, the cured binder layer can be useful as an abrasion resistant, or corrosion resistant coating. The coatings can be 0.1 mm in thickness or greater depending on the desired application.
In one embodiment, additives are added to the fiber-reinforced aerogel layer for added performance. Examples of additives include, but are not limited to, biocide compounds, anti-fungal compounds, flame retardant compounds, opacification compounds or combinations thereof. Examples of opacification compounds are B4C, Diatomite, Manganese ferrite, MnO, NiO , SnO , Ag2O, Bi2O3, TiC, WC, carbon black, titanium oxide, iron titanium oxide, zirconium silicate, zirconium oxide, iron (I) oxide, iron (III) oxide, manganese dioxide, iron titanium oxide (ilmenite), chromium oxide, silicon carbide or mixtures thereof.
In another embodiment the binder layer comprises a polyacrylate, polymethacrylate, polybutylmethacrylate, polyethylmethacrylate, polypropylmethacrylate, poly (2-hydroxyethylmethacrylate), poly (2-hydroxypropylmethacrylate), poly (hexafluorobutylmethacrylate), poly (hexafluoroisopropylmethacrylate) and combinations thereof.
In another embodiment the fiber for the fiber-reinforced aerogel is selected from Polyester based fibers, polyolefin terephthalates, poly(ethylene) naphthalate, polycarbonates, Rayon, Nylon, cotton based lycra® (manufactured by DuPont), Carbon based fibers like graphite, precursors for carbon fibers like polyacrylonitrile(PAN), oxidized PAN, uncarbonized heat treated PAN such as the one manufactured by SGL carbon, fiberglass based material like S-glass, 901 glass, 902 glass, 475 glass, E-glass, silica based fibers like quartz, quartzel (manufactured by Saint-Gobain), Q-felt (manufactured by Johns Manville), Saffil® (manufactured by Saffil), Durablanket (manufactured by Unifrax) and other silica fibers, Polyaramid fibers like Kevlar®, Nomex®, Sontera® (all manufactured by DuPont) Conex® (manufactured by Taij in), polyolefins like Tyvek® (manufactured by DuPont), Dyneema® (manufactured by DSM), Spectra® (manufactured by Honeywell), other polypropylene fibers like Typar®, Xavan® (both manufactured by DuPont), fluoropolymers like PTFE with trade names as Teflon® (manufactured by DuPont), Goretex® (manufactured by GORE), Silicon carbide fibers like Nicalon® ( manufactured by COI Ceramics), ceramic fibers like Nextel® (manufactured by 3M), Acrylic polymers, fibers of wool, silk, hemp, leather, suede, PBO-Zylon fibers (manufactured by Tyobo), Liquid crystal material like Vectan® (manufactured by Hoechst), Cambrelle® fiber (manufactured by DuPont), Polyurethanes, polyamaides, Wood fibers, Boron, Aluminum, Iron, Stainless Steel fibers and other thermoplastics like PEEK, PES, PEI, PEK, PPS or any combination of the preceding.
In another embodiment, the silicon-containing organic compound is an organopolysiloxane or a non-organopolysiloxane.
In yet another embodiment, the binder layer is applied to the surface of the aerogel layer with a brush, or a double roll in a continuous or semi-continuous manner.
This application claims priority to U.S. provisional patent application identified by serial No. 60/631,217 (filed Nov. 24, 2004) which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60631217 | Nov 2004 | US |
