Method to improve fire-resistance of wood composites, cellular solids and porous materials

Abstract

Provided herein are methods of increasing the fire -resistance of porous materials by impregnating them with inorganic metal hydrates. Porous materials that have been treated with the inorganic metal hydrates are also provided. The treated materials exhibit increased fire resistance but the mechanical properties of the composites are not altered, e.g., the strength, flexibility, rigidity etc. of the composites is not decreased by the presence of the inorganic metal hydrates. Materials that can be treated in this manner include, for example, wood, cellular solids, honeycomb materials and porous concrete.

Description

[0001]METHOD TO IMPROVE FIRE-RESISTANCE OF WOOD COMPOSITES, CELLULAR SOLIDS AND POROUS MATERIALS CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No 63 / 607,163 filed December 7, 2023. BACKGROUND OF THE INVENTION Field of the Invention This invention generally relates to methods of improving the fire-resistance of wood composites, cellular solids and other porous materials. In particular, the methods comprise impregnating pores and / or crevices of the materials with inorganic metal hydrates.     State of Technology In most construction materials, fire resistance and fire retardancy are increased by adding brominated organic compounds and organophosphorus compounds. These compounds are not environmentally friendly and can be carcinogenic, and there is a need to replace them. Lowden and Hull (Fire Science Reviews 2013, 2:4) disclose that metal hydroxides such Al2(OH)3can be incorporated into wood and wood composites. These hydroxides decompose when heated and release water. For example: Al2(OH)3= Al2O3+ 3 H2O. However, the decomposition temperature of these hydroxides is comparatively high, on the order of 200 ºC, which is sufficient to ignite or significantly damage a wood composite. In addition, Nicole M. Stark, et al., (Polymer Degradation and Stability 95 (2010) 1903-1910) describe the use of metal hydroxides as fillers when incorporated into composites during fabrication. However, incorporation during fabrication poses issues of compatibility of the fillers with adhesives and other additives and limits the concentration of metal hydroxides that can be used. Metal oxides such as Fe3O4 or SiO2, or their precursors, have been incorporated into wood composites. They are fire retardant due to the formation of ceramic-like layers when heated. For example, a procedure for the incorporation of Fe3O4is described in CN 106625930 B. In this work, the authors impregnated wood with ferrous iron phosphate, which reacted to form Fe3O4nanoparticles inside the pores of the wood composite. However, in general ceramic precursors are difficult to incorporate into wood products, and they are often applied only to the surfaces. In addition, ceramic materials do not provide cooling, and they become effective only at high temperatures, when the material has already ignited. Khelfa et al. (2013 J Anal Appl Pyrolysis 101:111–121) describe impregnating wood composites with chlorides such as MgCl2. Flame retardancy was attributed to the metal salt catalyzing dehydration of cellulose    The potential of these metal salts is, however is limited. They are corrosive and can form    noxious hydrogen chloride on decomposition. Regarding the addition of metal hydrates into honeycombs, this has been reported for example in CN105985132B. However, this application describes that the procedure is for energy storage and temperature control of exhaust pipes, not for increased fire resistance. In general, the prior art methods are not environmentally friendly, and / or are costly and / or do not provide sufficient fire resistance to prevent ignition at suitable, realistically low temperatures. SUMMARY OF THE INVENTION 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. Provided herein are methods of increasing the fire-resistance of porous and / or cavitated and / or cellular materials by impregnating (infiltrating) them with inorganic metal hydrates, generally after the materials are made (post-manufacturing). Materials that have been so treated are also provided. The methods are relatively facile, cost-effective, and environmentally friendly, and provide protection against ignition of treated materials at low temperatures, compared to the prior art. Composites that have been impregnated with inorganic metal hydrates exhibit increased fire resistance compared to prior art composites but the mechanical properties of the composites are not altered, e.g., the strength, flexibility, rigidity etc. of the composites is not decreased by the presence of the inorganic metal hydrates within the pores and / or cavities. Composites that can be treated in this manner include, for example, wood, various cellular solids, honeycombs and porous concrete. DESCRIPTION OF THE DRAWINGS Figure 1A-B. Digital camera images of post-burn hemp board samples, with and without metal hydrate infiltration. The samples were mounted vertically and placed under either: A, a candle for 60 seconds; or B, a propane flame for 10 seconds. The flames propagated through most of the samples without metal hydrates. In the samples with metal hydrates the flames did not propagate and extinguished after removal of the flame source. DETAILED DESCRIPTION Disclosed herein are methods to improve the fire resistance of wood composites, cellular materials and porous materials such as open cell foams. Fire resistance is increased by filling the voids of those materials with at least one type of inorganic hydrated salt. The hydrated metal salt acts as a solid water reservoir. Upon heating, for example by flames, the water bound to the inorganic salt detaches from the salt, absorbing energy and cooling nearby material. In other words, by applying heat, the water molecules are removed from the crystal structure, producing the anhydrous form of the metal. In addition, once freed from the salt, the water absorbs additional energy when it evaporates. Since evaporation is a cooling process, this also helps to subdue flames. The hydrate salts are advantageously    environmentally friendly and non-toxic. DEFINITIONS Wood composites include a range of different derivative wood products, all of which are created by binding the strands, fibers or boards of wood together. It is also known as manmade wood, engineered wood and wood-plastic composite (WPC).Wood composite is usually made from the same hardwoods and softwoods used for lumber, except sawmill scraps and wood waste are generally used. In some aspects, wood composites are created by mixing ground wood particles with at least one heated thermoplastic resin. Some methods combine and process the materials into pellets which are re-melted and formed into their final shapes while others create the final product by a one-step mixing and extrusion process. Both virgin and recycled thermoplastics are used with polyethylene-based products being the most common ones. UV stabilizers, colorants, coupling agents and lubricants can also be added to create a product specifically targeted to its application, forming both solid and hollow shapes. Plywood is considered the original composite wood product, manufactured from sheets of cross-laminated veneer which are bonded with moisture-resistant adhesives under heat. Fiberboard is another example, made by combining wood fibers with wax and a resin binder under high temperatures and pressure, while particleboard is manufactured from wood chips or sawmill shavings pressed with a synthetic resin. Oriented strand board is made from strands of wood arranged in layers and bonded together using moisture-resistant adhesives. These are then cross-oriented to give the panels strength and stiffness. Laminated timber is created using dimensional timber glued together into structural columns or beams, while laminated veneers bond thin wooden veneers into a large billet which can be used for rafters, beams, columns and joints. Other known wood composites include wood-plastic composites. Fire resistance is a material's ability to prevent or slow the spread of heat, flames, and hot gases in certain conditions. It's often measured by how long a material can withstand a standard fire test. Fire resistance testing is a process that evaluates a material's ability to withstand fire and retain its structural integrity. The test method involves exposing a material to a controlled temperature and time curve in a furnace, while simultaneously applying a load to simulate real-world conditions. The duration of the test can range from 30 minutes to a full day. The furnace's temperature is controlled according to an internationally accepted time-temperature curve. The specimen may fail in three ways: the side not exposed to the flame reaches a certain temperature, the specimen collapses, or voids widen. Fire rating is determined based on how long a specimen can contain a fire and maintain its structural integrity. Fire resistance ratings (FRR) are defined as the time in minutes or hours that a material or assembly can withstand fire exposure under specific conditions. The three key factors are: time until failure, temperature required to cause failure and strength of load capacity at a certain temperature. For example, a building code might require a FRR of 90, i.e. the assembly must resist fire, in each of the three ways, for 90 minutes. Fire resistance ratings are commonly assigned to walls, floors, and ceiling assemblies. Assemblies can be tested to ASTM E119 or UL 263 standards, in which there can be no passage of flame or gases hot enough to ignite cotton waste on the unexposed side, and the temperature on the unexposed surface of the assembly cannot increase more than 250° F (average) or 325° F (individual) above the thermal couple temperature. Three criteria may be measured: stability, the assembly’s ability to withstand the applied load for the duration of the test; integrity, the assembly’s ability to stay intact without cracks or fissures when it is exposed to fire; and insulation, the ability of the assembly to slow the transmission of heat from one side to the other. An "inorganic metal hydrate" refers to a compound in which water molecules are incorporated into the crystal structure of an inorganic metal salt. The metal ions are bound to a specific number of water molecules (a definite ratio) within the crystal lattice, often called "water of crystallization" or "water of hydration". The water molecules are an integral part of the crystal and are either bound to a metal center or have crystallized with the metal complex. Water molecules are not simply dissolved in the compound, but are part of the crystal lattice, forming bonds with the metal ions. “Cellular” applies to honeycombs, but also to foams with holes, which are called “cells”. METHODS In some aspects, the disclosure provides methods of rendering a porous or cavitated material fire resistant by impregnating pores and / or cavities of the porous material with at least one type of metal hydrate salt. Generally, impregnation is carried out after fabrication of the material and the only limits to the loading of the metal salts are the solubility of the salts in the solvent (e.g. water, methanol, dimethylsulfoxide, etc.) and the volume and accessibility of the pores and / or cavities in the host material. The added salts advantageously do not affect the mechanical properties of the materials, since they do not interact with the adhesives. The treated (impregnated) materials are “fire-resistant” in that, when exposed to a direct flame and / or to temperatures higher than the binding energy of the water to the metal salt, water is released. These temperatures will depend on the metal salt. For example, for MgSO1308129174, water starts being released around 100 ºC.    Release of water from the treated materials cools the treated materials itself and / or adjacent materials and / or extinguishes the flame. Accordingly, in some aspects, a flame or heat that would otherwise spread to adjacent materials (e.g. doors, a roof, a battery, etc.) and set them on fire (or heat them to the point of explosion) is extinguished by contact with the treated materials disclosed herein. Thus, the heat and / or flames are extinguished and / or are prevented from spreading for a period of time sufficient to permit detection of the danger and / or for other safety measures to be implemented, e.g. the use of fire extinguishers, built-in sprinkler systems, intervention by professional firefighters, etc. The treated materials work in conjunction with these other measures to prevent or lessen damage that would otherwise occur. Further, the treated materials advantageously do not release toxic gases or substances upon release of the water molecules. In some aspects, water is released from the impregnated material during a fire and ignition of structures behind the treated material (e.g. a honeycomb, a cellular solid, a solid with cavities, a human, a tree, etc.) is prevented. Hence, the materials are used as fire barriers, with cooling functionality (but not as energy reservoirs).            Types of metal hydrate salts that are used in the practice of the methods include but are not limited to: Epsom salt: magnesium sulfate heptahydrate (MgSO4·7H2O) Borax: sodium tetraborate decahydrate (Na2B4O7·10H2O) Cobalt chloride: Cobalt(II) chloride hexahydrate (CoCl2·6H2O) Gypsum: Calcium sulfate dihydrate (CaSO4·2H2O) Glauber’s salt (sodium sulfate decahydrate, Na2SO4∙10H2O); washing soda (sodium carbonate decahydrate, Na2CO3∙10H2O); the double salts known collectively as alums (M++ 2SO4∙M 32(SO4)3∙24H2O, where M+is a monopositive cation, such as K+or NH4+, and M3+is a tripositive cation, such as Al3+or Cr3+; Copper(II) sulfate pentahydrate (CuSO4⋅5H2O) Copper nitrate hexahydrate (Cu(NO3)2⋅6H2O) Iron acetate tetrahydrate (Fe(CH3COO)2⋅4H2O) Lithium sulfate mono-, di- and pentahydrates (Li2SO4·H2O; Li2SO4·2H2O; and Li2SO4·5H2O) Manganese chloride tetrahydrate (MnCl2⋅4H2O) Iron nitrate nonahydrate (Fe(NO3)3⋅9H2O) Zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O) Tin chloride pentahydrate (SnCl4⋅5H2O) Sodium thiosulfate pentahydrate (Na2S2O3⋅5H2O) Nickel(II) chloride hexahydrate (NiCl2⋅6H2O) Calcium sulfate dihydrate (CaSO4⋅2H2O) Iron(II) sulfate heptahydrate (FeSO4⋅7H2O) Zinc sulfate heptahydrate (ZnSO4⋅7H2O) Aluminum sulfate octadecahydrate (Al2(SO4)3⋅18H2O) Sodium carbonate decahydrate (Na2CO3⋅10H2O) Calcium chloride hexahydrate (CaCl2⋅6H2O) Sodium acetate trihydrate (NaCH3COOH⋅3H2O) Barium hydroxide octahydrate (Ba(OH)2⋅8H2O) Sodium phosphate dodecahydrate (Na3PO4⋅12H2O) Sodium pyrophosphate decahydrate (P2O7⋅10H2O⋅4Na) Calcium carbonate decahydrate (CaCO3⋅10H2O) Calcium nitrate tetrahydrate (Ca(NO3)2⋅4H2O) Manganese sulfate heptahydrate (MnSO4⋅7H2O) Calcium oxide hydrate (CaH2O2) Calcium hydroxide hydrate (CaH4O3) In order to impregnate porous and other materials, the inorganic metal hydrates are dissolved or suspended in at least one solvent to form a solution comprising at least one inorganic metal hydrate. Suitable solvents include but are not limited to: water, methanol, ethanol, acetone, butanol, dimethylsulfoxide ,and other polar solvents where the metal hydrate salts may be soluble, and mixtures of these. The amount of the one or more inorganic metal hydrates that are dissolved in the solution generally ranges from about 1 gram per liter to about 350 grams per liter, such as from about 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80m 85, 90, 95, 100, 110, 120, 130, 140, 150, 1690, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 289, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, or 400 grams per liter, including all whole or decimal integers in between these ranges of values. The precise amount depends on the solubility of the hydrated salt in a specific solvent, the temperature of the solvent, and on the amount of metal salt loading that is desired for a particular application. For example, MgSO4⋅7H2O (Epsom salt) has a room temperature water solubility of about 350 grams per liter or, very roughly, about 30% by volume. Hence, one can fill pores of a material for about 30% of their volume using magnesium sulfate, but not more. A higher concentration would require use of a salt with higher solubility, or use of a higher temperature. For example, the solubility of MgSO4is about 3 mol / Liter around 20 ºC, but it increases to about 5 mol / Liter at 80 ºC. So, one could increase the loading inside the pores by using warm water instead of room temperature water. The example is not limited to MgSO4 or water. Many hydrated metal salts show the same behavior as MgSO4, and higher solubility is often noticed for increasing temperatures. One should also notice that a range of water loss temperatures could be achieved by dissolving in water (or another solvent) salts with different water release temperatures. So, for example, one could dissolve Na2SO4 (which has a low water release temperature, around 30 ºC) and MgSO4 (which releases water at around 100 ºC). The resulting composite would release water first around room temperature and then around 100 ºC. Similarly, one could use three or more salts and achieve a nearly continuous water release as a function of temperature and cover temperature ranges from about room temperature to about 500 ºC, such as about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 400, 425, 450, 475 , 500, 525, 550, 575 or 600, including all whole or decimal integers in between these ranges of values. In addition to a solvent and the inorganic metal salts, the solutions may contain other components, examples of which include but are not limited to: various wetting agents such as coordinating compounds that increase the solubility of the metal salts, such as amines for mono-and divalent metals, and phenols or glycols for trivalent salts, such as salts containing Al3+and Fe3+; and the like.    Any set of parameters may be used for the solutions, as long as the solutions are sufficiently non-viscous and free-flowing to permit entry into the pores and / or crevices of the material to be impregnated, and as long as the H2O remains attached to the metal and is available to detach / be released when exposed to heat and / or flame. In order to infiltrate the pores and / or crevices of a material, the material is generally submerged in the solution, although in some aspects, the solution may be sprayed or coated onto the material, or the material can be placed inside an autoclave, or a vacuum can be applied to the material. Those of skill in the art will recognize that the time and conditions of exposure of the material to the solution varies depending, for example, on the size of the pores and crevices, the length of pores or channels to be infiltrated, the volume of the pores, cavities, etc. in the material, and the ambient conditions. For example, for thicker materials that are very porous (e.g., that have a large pore volume) it may be necessary to submerge the material in the solution for several hours (e.g. about 1-24 hours) or even days, optionally with constant shaking, tipping, rotation, etc. or other movement. Pressure (e.g. pumping, or placement inside an autoclave) may also be applied. In contrast, for very thin materials or very porous materials (e.g. sheets, narrow diameter rods, fabric, etc.) especially those having holes that penetrate the entire width or thickness of the material, the exposure time may be much shorter, such as a few minutes (e.g. about 1-30 such as 1, 5, 10, 15, 20, 25 or 30 minutes) or a few hours (such as about 1, 2, 3, 4, 56, 7, 8, 9 or 10 hours). Generally, the goal is to fill at least about 5-100% of the pore or crevice volume, such as at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100%, e.g. by displacing air or other gases (or even other liquids) that are within the pores, channels, cracks, crevices, holes, etc. of the material. In general, impregnation is conducted so as to prevent trapping air, etc. within pores and channels of the material. In general, the surface openings of pores, cavities, channels, etc. range from nanosized to much larger. For example, in sol-gel materials, pores can be as small as a few nanometers, typically from about 1 nm to about 100 nm. In commercial foams, pores are on the order of 1-20 microns; in wood, one encounters a wide range of pore sizes. For example, in Scots Pine the pore volume is 4.5% for pores below 80 nm, about 16% for pores 80-500 nm, about 17% for pores 0.5 to 2 microns, and about 63% for pores between 2 and 58 microns (Eur. J. Wood Prod. (2011) 69:649–657, DOI 10.1007 / s00107-010-0504-0). In cellular materials, for example paper honeycombs, the pores (called cells in the honeycomb industry) may range from about 1 mm to about 25 mm. Thus, pore sizes ranging from about 1 nm to about 75 microns are encompassed. In some aspects, especially in the case of materials with e.g., internal hollow cavities or chambers, those internal cavities or chambers may be very large. For example, a rectangular solid (e.g. a board) may have an internal chamber that extends for most of the length and width of the rectangle so that when the chamber is filled (via one or more surface openings), the solution containing the inorganic metal hydrates will mostly adhere to the surface of the walls and    constitutes an internal “layer” of fire protection.    After exposure to the solution of inorganic metal hydrates, the fire-resistant material or product is typically dried to remove the solvent(s), e.g. under ambient conditions or under increased temperature (but not high enough to release the H2O from the metal) and / or pressure, and / or by applying a vacuum. The type of drying and the length of time required to dry the material varies according to the size of the material, the size of the pores / channels / cavities, etc. as will be readily understood by those of skill in the art. After removal of the solvent, the fire-resistant product is typically stored according to standard industry practice until use.        Examples of materials to which the method is applied include but are not limited to: Wood and any type of wood composites which have pores or other openings. The pores can be as small as a few nanometers and as big as millimeters; Open cell foams (polymeric, metallic, ceramic, etc.); Honeycombs and honeycomb sandwiches. For example, paper honeycombs and honeycomb sandwiches commonly employed in the shipping industry, or Nomex® (aramid) or Kevlar® (para-aramid) honeycombs, used for example in protective clothing for emergency workers, the military, fire fighters, etc.; Porous concrete; Materials with apertures, for example drilled wood boards, or materials assembled in such a way that one or more apertures or cavities are included in the assembly. For example, a metallic bar with cavities drilled into it, and sealed by a metallic plate; Any material that comprises pores and / or cavities and / or apertures or other openings may be rendered fire resistant by the present methods. In some aspects, the material is a porous material having pores extending internally throughout the material with the terminus of at least one pore, and typically of a plurality of pores, being surface exposed (open at the surface) to allow the liquid solution of inorganic metal salt(s) to enter the pores. Pores are generally interconnected throughout all or part of the interior of the material. The solution penetrates the material through the surface exposed termini of pores and displaces e.g. air or another liquid that is present within the porous material. In other aspects, the material contains internal chambers or voids which may be of any shape, and which have at least one surface exposed opening to permit ingress of the inorganic metal hydrate solution. In other aspects, the material comprises cavities, cracks, holes or channels that are open to the surface and the solution penetrates the material by filling the cavities, holes or channels. The cavities, holes or channels may or may not extend through the material. In other words, the cavities, holes and channels may have two external openings at the surface of the material or may have only one opening at the surface of the material.    In other aspects, the material that is treated comprises both pores and apertures.   USES The fire-resistant products that are prepared using the methods disclosed herein are used in a variety of different ways, including both home and industrial construction. Treated wood composites are used, for example, to replace steel for joists and beams in building projects, as well as for outdoor deck flooring, railings, fencing, benches, window and door frames, cladding and landscaping work, furniture (including flat-pack furniture), boards, shingles, siding, interior and exterior trim (e.g. crown molding, quarter round, shoe molding, chair rails, etc.), shelving, roofing, various types of panels, insulation (e.g. for buildings, for electrical wiring, in battery or small appliances, etc.), in automobile components, laminates, various sheets and membranes, joint seal systems, and the like. In some aspects, the treated material is used “as is” after drying. In other aspects, the treated material is further processed e.g. by being trimmed, cut or otherwise formed into a shape that fits its intended use, if the treatment took place before the cutting / shaping. For example, treated material destined for use in clothing may be shaped and sown or glued into a garment; treated material intended for use in a battery may be trimmed and fitted onto a variety of battery sizes and shapes; and so on.    When exposed to flames and / or a temperature of about 100 ºC, water is released from the inorganic metal hydrate within the material. In some aspects, these low release temperatures prevent ignition. In other aspects, the low release temperatures lead to self-extinction of flames, as shown in Figure 1. The release temperature can be higher or lower than 100 ºC depending on the metal hydrate that is being used. For example, sodium sulfate decahydrate releases water starting at about 30 ºC, and is therefore a good candidate for use at low temperatures. If one used calcium hydroxide, the release temperature would be around 500 ºC. In our experience, magnesium sulfate heptahydrate represents a valid choice, since it releases its water at temperatures that are close to those where organic compounds may start oxidizing, burning and releasing fumes. However, the specific metal hydrate will have to be chosen depending on the application. And one should bear in mind that multiple salts can be added to a solution to create a range of release temperatures. Where a range of values is provided, it is understood that each intervening value, to at least 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. In the description of the invention herein, it is understood that a word appearing in the singular encompasses its plural counterpart, and a word appearing in the plural encompasses its singular counterpart, unless implicitly or explicitly understood or stated otherwise. Furthermore, it is understood that for any given component or embodiment described herein, any of the possible candidates or alternatives listed for that component may generally be used individually or in combination with one another, unless implicitly or explicitly understood or stated otherwise. Moreover, it is to be appreciated that the figures, as shown herein, are not necessarily drawn to scale, wherein some of the elements may be drawn merely for clarity of the invention. Also, reference numerals may be repeated among the various figures to show corresponding or analogous elements. Additionally, it will be understood that any list of such candidates or alternatives is merely illustrative, not limiting, unless implicitly or explicitly understood or stated otherwise. In addition, unless otherwise indicated, numbers expressing quantities of ingredients, constituents, reaction conditions and so forth used in the specification and claims are to be understood as being modified by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the subject matter presented herein. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the subject matter presented herein are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. All patents and publications mentioned in the specification are indicative of the level of those skilled in the art to which the invention pertains. All patents and publications are herein incorporated by reference in their entirety to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. EXAMPLES Example 1. An exemplary hemp wood board was submerged in a saturated aqueous solution of MgSO4. The volume of the solution was about 10 times the volume of the board, and the board was flipped about every 2 hours, for 12 hours. The board, typically 1 inch (25.4 mm) thick, was then dried in air at room temperature and / or in an oven at temperatures around 50 ºC. Drying time was 2-24 hours. After drying, the board, or a slice of it, was suspended and a flame (a candle or a propane flame, although an alcohol burner could also be used) was then placed underneath, as shown in Figure 1. The board smoked and / or was ignited by the flame, but the flame and / or the smoke self-extinguished once the candle or propane flame was removed from underneath the board. Example 2. One places an open-cell plastic foam, for example a phenolic acoustic foam, in a saturated solution of metal hydrate. One follows the same procedure as in Example 1. Once exposed to a flame, the plastic foam does not burn or burns sluggishly and smokes. Smoke and flame cease once the flame source is removed. Example 3. One takes a board of plywood and follows the procedure reported in Example 1. The results are identical to those of example 1, with flame self-extinguishing and smoke ceasing, after the flame source is removed. While the invention has been described in terms of its preferred 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

CLAIMS We claim:

1. A method of increasing fire resistance of a material comprising pores and / or cavities, the method comprising exposing the material to a liquid solvent comprising at least one inorganic metal hydrate for a period of time and under conditions sufficient to permit the liquid solvent to at least partially fill the pores and / or cavities with the at least one inorganic metal hydrate, and removing the liquid solvent from the pores and / or cavities.

2. The method of claim 1, wherein the material is wood, a composite, a cellular solid, a honeycomb material, or porous concrete.

3. The method of claim 1, wherein the at least one inorganic metal hydrate is magnesium sulfate heptahydrate (MgSO4·7H2O).

4. The method of claim 1, wherein the liquid solvent dissolves at least 1 gram per liter of the at least one inorganic metal hydrate.

5. The method of claim 1, wherein the liquid solvent is water, dimethylsulfoxide or methanol.

6. The method of claim 1, wherein the liquid solvent further comprises a compound that coordinates with the metal of the at least one inorganic metal hydrate to increase solubility of the at least one inorganic metal hydrate.

7. The method of claim 6, wherein the at least one inorganic metal hydrate is a divalent or trivalent inorganic metal hydrate and the compound is an amine.

8. The method of claim 6, wherein the at least one inorganic metal hydrate is a trivalent or tetravalent metal ion and the compound is a phenol.

9. The method of claim 1, wherein the pores range in size from 1 nm to 75 microns.

10. A material comprising pores and / or crevices, wherein the pores and / or crevices are impregnated with at least one inorganic metal hydrate.

11. The material of claim 10, wherein the material is wood, a composite, a cellular solid, a honeycomb material or porous concrete.

12. The material of claim 10, wherein the at least one inorganic metal hydrate is magnesium sulfate heptahydrate (MgSO4·7H2O).

13. The material of claim 10, wherein the pores range in size from 1 nm to 75 microns.

14. The method of claim 1, wherein the at least one organic metal hydrate is selected from the group consisting of: magnesium sulfate heptahydrate (MgSO4·7H2O); sodium tetraborate decahydrate (Na2B4O7·10H2O); cobalt(II) chloride hexahydrate (CoCl2·6H2O); calcium sulfate dihydrate (CaSO4·2H2O); sodium sulfate decahydrate, Na2SO4∙10H2O); sodium carbonate decahydrate, Na + + 2CO3∙10H2O); M 2SO4∙M 32(SO4)3∙24H2O, where M+is a monopositive cation and M3+is a tripositive cation; copper(II) sulfate pentahydrate (CuSO4⋅5H2O); copper nitrate hexahydrate (Cu(NO3)2⋅6H2O); iron acetate tetrahydrate (Fe(CH3COO)2⋅4H2O); lithium sulfate mono-, di- and pentahydrates (Li2SO4·H2O; Li2SO4·2H2O; Li2SO4·5H2O); manganese chloride tetrahydrate (MnCl2⋅4H2O); iron nitrate nonahydrate (Fe(NO3)3⋅9H2O; zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O); tin chloride pentahydrate (SnCl4⋅5H2O); sodium thiosulfate pentahydrate (Na2S2O3⋅5H2O); nickel(II) chloride hexahydrate (NiCl2⋅6H2O); calcium sulfate dihydrate (CaSO4⋅2H2O); iron(II) sulfate heptahydrate (FeSO4⋅7H2O); zinc sulfate heptahydrate (ZnSO4⋅7H2O); aluminum sulfate octadecahydrate (Al2(SO4)3⋅18H2O); sodium carbonate decahydrate (Na2CO3⋅10H2O); calcium chloride hexahydrate (CaCl2⋅6H2O); sodium acetate trihydrate (NaCH3COOH⋅3H2O); barium hydroxide octahydrate (Ba(OH)2⋅8H2O); sodium phosphate dodecahydrate (Na3PO4⋅12H2O); sodium pyrophosphate decahydrate (P2O7⋅10H2O⋅4Na); calcium carbonate decahydrate (CaCO3⋅10H2O); calcium nitrate tetrahydrate(Ca(NO3)2⋅4H2O); manganese sulfate heptahydrate (MnSO4⋅7H2O); calcium oxide hydrate (CaH2O2); and calcium hydroxide hydrate (CaH4O3).

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