Cellular ceramic material and method of production thereof

A material having superior that insulating qualities which may be formed in flat sheets or any other desired configuration. The material in its basic form is produced by mixing an alkali metal halide or nitrate into an alkali metal silicate solution to form a polymerized silicate gel. The gel may be heated to cause intumescence and the formation of a dry cellular ceramic, or may be used in gel form, e.g., as a fireproof barrier which will expand upon exposure to heat. Physical properties of the end product are altered by the inclusion of one or more additives prior to the polymerization reaction. These additives may be any of a number of materials that do not significantly react with the alkali metal silicate solution or otherwise inhibit the formation of the gel.

BACKGROUND OF THE INVENTION 
The present invention relates to ceramic materials and methods of 
production thereof, and more specifically, to ceramic (i.e., inorganic, 
silicate-based) materials of potentially low density and good thermal 
insulating qualities. 
A number of alkali metal silicate-based insulation materials have been 
described in the prior art. Characteristic examples of such materials are 
evident from U.S. Pat. Nos. 4,297,252 (Caesar et al); 4,521,333 (Graham et 
al); 4,118,325 (Becker et al); and Great Britain Pat. No. 1,227,482. These 
patents describe well-known methods of hardening an aqueous alkali metal 
silicate composition to form a gel or solid product. The basic hardening 
techniques include chemical methods such as adding sodium or potassium 
fluorosilicate and/or organic gelling agents such as haloalcohols, amines, 
ketones, etc., or by heating the gel at low temperature (approx. 
100.degree. C.) to reduce water content. 
Though apparently well-suited to a number of applications, these techniques 
have a number of serious draw-backs. First of all, methods which involve 
heating to form the gel are expensive in terms of time and energy 
consumption. Furhermore, this gel can not be easily molded or extruded. 
Intumesced ceramics produced from these gels have very little structural 
strength because of their extremely low density, unless large amounts of 
filler materials have been incorporated into the starting silicate 
solution. Gels produced using organic hardening agents are easier to 
machine because of their plastic-like consistency. However, gels 
containing organic materials such as haloalcohols and amines present fire 
and toxicity hazards when heated to produce solidification or 
intumescence. 
Fluorosilicates are usually used in combination with organic hardening 
agents. When used alone, in sufficient quantity to cause solidification of 
the silicate solution, the resulting material does not exhibit 
intumescence (expansion and cell formation) when subjected to temperatures 
which normally cause this reaction to occur (around 1000.degree. F.) in 
gels produced by other means. Furthermore, when heated to high 
temperatures, the fluorosilicate may decompose to produce extremely toxic 
gasses such as silicon tetrafluoride and fluorine. 
It is a principle object of this invention to provide a simple, inexpensive 
technique for producing alkali metal silicate gels which offer a number of 
benefits over prior methods. 
A further object is to provide a method of producing alkali metal silicate 
gels which are non-toxic and can be easily molded and machined. 
Another object is to provide a method of producing silicate gels which can 
be heated (to cause intumescence) to produce a variety of low and high 
density insulation materials. 
Other objects will in part be obvious and will in part appear hereinafter. 
SUMMARY OF THE INVENTION 
In accordance with the foregoing objects, the invention contemplates a 
process for the formation of a silicate gel material which, in its basic 
form, is produced by the combination of one or more simple alkali metal 
halids or nitrates (of the form M.sub.1 X.sub.1 where M is the alkali 
metal, and X is the halide or nitrate; e.g. NaCl) with a solution of 
sodium or potassium silicate. Preferably, certain additives such as 
silicates, carbonates, oxides, and other materials which do not cause 
precipitation of the soluble silicate are mixed into the solution prior to 
mixing in the alkali metal halide or nitrate in order to vary the 
properties of the final product. Also, fibers or other filler substances 
may be added either prior to, simultaneously with, or after the mixing in 
of the alkali metal halide or nitrate. The solution may thereby assume the 
consistency of a slurry or paste, depending upon the quantity of the 
material added. 
After formation of the gel, the material may be molded to essentially any 
desired form and allowed to harden completely at room temperature. The 
material may also be applied to a substrate with a sprayer before the 
gelling (polymerization) reaction is completed. Since this reaction may 
occur very quickly, (within a few seconds) for some mixtures, it may be 
necessary to mix in the alkali metal halide or nitrate as the material is 
being sprayed, e.g., within the sprayer nozzle. 
The material may be left in the gel state to serve as a fireproof layer in 
packaging and construction materials. As such, the gel would react to heat 
and flame by expanding through intumescence to form a highly insulative 
and reflective barrier. The dehydration of the gel during this process 
also serves to carry away damaging heat. 
The gel may also be heated, preferably at around 1000.degree. F., to cause 
intumescence and the consequent formation of a cellular ceramic/glass 
foam. The surface of this foam may be quickly heated to its melting point 
to create a glazed surface.

DETAILED DESCRIPTION 
The present invention is directed to a process for the formation of easily 
manufacturable and machineable silicate gels which can be made to exhibit 
a wide range of properties to suit a number of different applications. 
The process primarily involves the combination of alkali metal silicates in 
aqueous solution with alkali metal halide and/or nitrate salts to produce 
a pliable, plastic-like gel. The use of these alkali metal halide and 
nitrate salts allows the production of materials which are not possible by 
any other known method, particularly with fluorosilicates commonly used in 
the prior art. For example, using equivalent quantities of potassium or 
sodium fluorosilicate (silicofluoride) in place of the alkali metal 
nitrate or halide in any of the compositions described by this invention 
results in a gel which will not intumesce to any noticeable degree. The 
heated gel merely hardens into a stone-like composition. By contrast, with 
the methods of the present invention, tough ceramic foams can be produced 
which are several times the volume of the starting gel. Furthermore, 
denser ceramics can be produced by the inclusion of additive substances as 
described in the following paragraphs. 
The alkali metal silicate used in this invention should be sodium or 
potassium silicate with a silicon to metal oxide ratio e.g.,(SiO.sub.2 
:Na.sub.2 O) betweeen 1:1 and 5:1 (preferably, between 2:1 and 4:1). The 
solution may also contain colloidal SiO.sub.2. However, the molar ratio of 
colloidal SiO.sub.2 to soluble silicate should preferably be no more than 
about 3:1. A commercially available sodium silicate solution (water glass) 
with a SiO.sub.2 :Na.sub.2 O ratio of around 3.4:1 and colloidal SiO.sub.2 
to soluble silicate ratio of around 2:1 appears to work well for most 
currently contemplated implementations of this invention. Gel formation 
optimally takes place when the alkali metal silicate solution 
concentration is between about 10 to 40% by weight. The rate of the 
polymerization reaction is most effected by the amount of gelling material 
used. FIG. 1 is an illustrative example of this relationship. It should be 
noted that the gel formation process using equal quantities of sodium or 
potassium fluorosilicate can take up to 20 times as long (10 to 20 minutes 
as opposed to 30 seconds to 1 minute) as with the alkali metal halide and 
nitrate salts. 
The materials used to polymerize the alkali metal silicate solution are 
simple monovalent alkali metal salts, specifically one or more of sodium, 
potassium, or lithium fluoride, chloride, bromide, iodide, or nitrate. The 
word "simple" implies that the molecular structure is of the form M.sub.1 
:X.sub.1 where M represents the alkali metal ion and X represents the 
halide or nitrate ion. Experimentation has shown that potassium and sodium 
salts are generally preferred over lithium in terms of reactivity and the 
quality of the gel formed. Experimental data further indicates that 
potassium and sodium chloride and nitrate can be used to satisfy most 
applications currently contemplated for this invention. Therefore, toxic 
substances such as fluorides and bromides can usually be avoided in 
producing materials for home insulation products and similar applications. 
The amount of alkali metal nitrate or halide required to polymerize the 
silicate solution can vary considerably depending upon solution 
concentration, additive materials, and the particular alkali metal halide 
or nitrate (gelling salt) used. In general, it appears that a minimum of 
about 1 part (by weight) of gelling salt to about 20 parts of low 
concentration (around 10%) silicate solution is required for gelling to 
occur within a reasonable time (on the order of several minutes or less). 
However, the optimum ratio appears to be about 1 to 4 parts gelling salt 
to about 10 parts silicate solution (prior to the incorporation of 
additives). Larger quantities of gelling salt may be used, but do not 
appear to be necessary. The gelling salt is usually added to the solution 
as a coarse or fine powder, but can be added as a strong solution or paste 
as long as its water content does not reduce the soluble silicate solution 
concentration to a level below which gelling cannot occur. 
Additive substances may be incorporated into the gel to create a variety of 
desired properties in the final product. These substances are preferably 
added to the silicate solution prior to the addtion of the gelling salt. 
Although theey could be added in a single step with gelling salt, this may 
prevent a good dispersion of these substances throughout the gel. These 
additive substances are basically silicates, oxides, carbonates, and other 
ceramic substances obvious to anyone skilled in the art, chosen to create 
certain properties in the final product. The only restriction on these 
substances is that they do not react with the silicate in solution to any 
extent which would seriously inhibit or prevent the polymerization 
reaction caused by the addition of the gelling salt. Hence, these maerials 
are for the most part non-aqueous-soluble and non-reactive in the alkali 
metal silicate solution at room temperature. Specific examples of such 
materials are silica gel (silicic acid), silicates such as aluminum, 
calcium, and zinc silicate, oxides such as aluminum and magnesium oxide, 
and carbonates such as magnesium, calcium, zinc, and lithium carbonate. 
Examples of some aqueous-soluble materials which do not adversely react 
with the silicate solution, and which appear to enhance the uniformity and 
strength of some intumesced compositions are alkali metal sulfates and 
phosphates. FIG. 2 illustrates the preferred method for mixing the raw 
materials to form the gel. 
The most noted general effect of using additive materials is the increase 
in hardness and density of the gel, and the increase in density and 
toughness of the intumesced gel (ceramic foam). Gels containing little or 
no additives may exhibit a great deal of expansion (up to several hundred 
percent) during heating to cause intumescence, while those with large 
quantities of additives may form dense materials exhibiting very little 
expansion. Ceramic foams with densities ranging from as little as a few 
hundredths of a gram per cubic centimeter to as high as a few grams per 
cubic centimeter can be produced by varying the types and amounts of these 
additive substances. Additive substances may also be included to create 
other effects such as coloring the final product. For example, copper and 
iron silicates may be added to produce blue and yellow ceramic foams or 
glazed surfaces (by heating the surface of the foam to induce 
self-glazing). 
For most applications envisioned at the present time, and for ease of 
manufacturing the gel, it appears that the ratio of additive substances to 
silicate solution should be between about 0:1 to 1:1. It should be noted 
that the insoluble (colloidal) SiO.sub.2 present in most commercial sodium 
silicate formulations is included in the above ratio. It is assumed that 
most implementations of this invention will utilize commercially available 
sodium or potassium silicate solutions which contain a molar ratio of 
colloidal silica to soluble silicate of around 2:1. The amounts of 
additive substances required to produce similar results using other sodium 
or potassium silicate solution formulations can therefore be easily 
calculated by considering the concentrations of insoluble SiO.sub.2 they 
contain. 
FIG. 3 illustrates some of the physical properties of a particular ceramic 
foam composition in terms of the ratio between additive substances and 
sodium silicate (including colloidal SiO.sub.2) in the solution. FIG. 4 
illustrates the results of an experiment to measure some of the insulative 
properties of these materials. In this experiment, a 1 centimeter thick 
sheet of ceramic foam containing approximately 2 parts additive substances 
to 10 parts commercial sodium silicate solution was used as one wall of an 
electric furnace. The furnace was quickly heated to 900.degree. F. and 
allowed to cool to room temperature over a period of 8 hours. Other 
experiments indicate that even very low density foams remain rigid at 
temperatures up to around 1100.degree. F. However, higher density ceramics 
made from gels containing additives such as magnesium oxide can remain 
rigid at temperatures exceeding 2000.degree. F. Such materials may have a 
variety of industrial applications. 
A further step in the process of this invention may include the addition of 
filler and reinforcing materials such as fibers, particulates (sand, 
crushed glass, etc.,), screen or mesh to the gel to enhance the strength 
of the final product. These could be added to the silicate solution, or 
mixed, pressed, layered, or folded into the gel during or after its 
formation. The total quantity of these substances that may be added is 
only limited by the saturation limits of the gel mixture. 
The silicate gel materials described above can be formed into sheets, cast 
around pipes, etc,. or molded into any desired configuration. The gel can 
be used as is to form a reactive barrier against heat and fire, or heated 
to cause intumescence and the formation of a ceramic foam. To form the 
ceramic foam, the gel is heated using a thermal source, preferably between 
about 900.degree. F. and 1300.degree. F., or microwave energy may be used. 
Either thermal or microwave heating (or some combination of the two) may 
be more appropriate for different gel compositions. The duration of the 
heating process should be sufficient to cause partial or complete 
intumescence of the gel (as desired), and will vary for different gel 
compositions and thicknesses. Complete intumescence of an average gel 
sample of 1 centimeter thickness takes approximately 5 to 8 minutes at 
1000.degree. F. 
The methods described above can be used to manufacture a number of useful 
materials. Among the possible applications for these materials are sheets 
of wall board, or similar construction materials which are light in 
weight, provide excellent thermal insulating qualities, are easily 
machined (sawed, drilled, ground, etc.), and which are non-toxic and 
fireproof. The material may also be used as bulk insulation or molded in 
various shapes to serve as an insulating layer or jacket on various items, 
including being cast in place directly around pipes, and the like. The 
properties of the material are controlled to suit the particular 
application by selection of the types and amounts of additive substances 
and fillers, process temperatures, and other variables. Finally, it should 
be noted that the material may be prepared in a plurality of layers, 
providing substrates having different properties, and diverse materials 
such as rods, wire mesh, etc. may be embedded or otherwise incorporated 
into the material for added strength or for other purposes. Likewise, the 
material of the invention may be deposited upon and/or bonded to layers of 
other materials, such as wood, paper, etc. 
A number of specific examples illustrating the general principles and scope 
of this invention are provided below. It will be understood that the 
examples are in no way intended to be limiting. 
EXAMPLE 1 
A strong, low-density ceramic foam with good heat insulating properties is 
formed by mixing or blending 1 to 3 parts sodium or potassium chloride 
into 10 parts of commercial sodium silicate solution. The resulting gel 
hardens quickly and can be used as is or heated at about 1000.degree. F. 
to intumescence. 
EXAMPLE 2 
Add 1 part potassium sulfate to 10 parts commercial sodium silicate 
solution. To this mixture, add 2 parts potassium nitrate (or sodium or 
potassium chloride) to cause polymerization. The gel may be heated at 
around 1000.degree. F. to form a light-weight ceramic foam, similar to 
that of Example 1. Preferably, the gel is first heated in a microwave oven 
to cause partial intumescence and is the further heated at about 
1000.degree. F. to essentially complete intumescence. This results in a 
more homogeneous foam structure. 
EXAMPLE 3 
A very tough, high density ceramic is formed by first mixing 4.5 parts 
silica gel, 1.8 parts alumina silicate, and 1.5 parts calcium carbonate 
with 20 parts of commercial sodium silicate solution. 4 parts of potassium 
nitrate are then added to polymerize the mixture. The gel hardens quickly, 
and can be molded and pressed into sheets. It is then heated between 
1000.degree. and 1300.degree. F. to form a tough, dense ceramic foam. 
EXAMPLE 4 
A high temperature ceramic is formed by first mixing 1 part silica gel and 
5 parts magnesium oxide with 10 parts commercial sodium silicate solution. 
1 part potassium nitrate is then mixed in to ploymerize the mixture. The 
gel is very dense, but can be molded and pressed into sheets. The gel can 
be heated at around 1000.degree. F. to produce a ceramic which can resist 
temperatures above 2000.degree. F. 
EXAMPLE 5 
A medium density tough ceramic foam which could be used for boards or 
ceiling tiles is formed by mixing 1 part silica gel and 0.2 parts calcium 
carbonate into 10 parts of commercial sodium silicate solution. 1 part 
sodium or potassium chloride is then mixed in to form a hard, polymerized 
gel. The gel is molded or pressed into sheets. It is then heated in a 
microwave oven to produce a ceramic foam. 
EXAMPLE 6 
Any of the above examples in which filler materials such as fibers and/or 
particulates (sand, crushed glass, etc.) have been incorporated into the 
gel.