CA1084710A - Low temperature manufacture of glass - Google Patents

Abstract

Abstract of the Disclosure The present invention relates to the production and chemical treatment of sodium and/or potassium hydrosilicate glasses containing about 5-50% by weight water within the glass structure, wherein the Na+ and/or K+ ions are capable of being partially completely replaced, on an electro-lytically equivalent basis, by protons and/or other monovalent and polyvalent cations species, to make glass articles whose alkali metal oxide content is substantially diminished or completely removed. Such glass articles can be either completely dense, or microporous, with pores ranging down to molecular size. Moreover, the chemical compositions of such microporous glasses can be further altered to include many other chemical species including polyvalent cations, anions and neutral molecular species, both organic and inorganic, and including complex ions such as metal amine complexes, where such species are capable of being sorbed on the porous glass by physical or chemical forces or both. The instant invention provides means for replacing part or all of the Na+ and/or K+ ions in an alkali silicate hydrosilicate amorphous body either with protons, or with protons plus other monovalent or polyvalent cations, in the nondestructive manner by reaction in aqueous solutions. In addition, it provides means for incorporating many other chemical species into the ion-exchange microporous body at low temperature, by sorption from solutions, either aqueous or nonaqueous. These latter species are not necessarily cations. The processes of the invention make possible the alteration of the chemical composition of alkali hydrosilicate body to any of a wide variety of other glass compositions having better chemical durability and other useful glass properties.

Description

On-.S~ Up-SLOOkey ~a08~71~

The present invention relates to the production and chemical treatment of sodium and/or potassium hydrosilicate glasses containing about 5-50% by weight water within the glass structure, wherein the Na~ and/or K~ ions are capable o~ being partially or completely replaced, on an electro-lytically equivalent basis, by protons andtor other mono-.. , ~
valent and polyvalent cations speciesj to make glass articles ; 10 whose alkali metal oxide content is substantially diminishedor completely removed. Such glass articles can be either completely dense, or microporous, with pores ranging down to .
molecular size. Moreover, the chemical compositions of such microporous glasses can be further altered to include many , . . .
` other chemical species including polyvalent cations, anions and neutral molecular species, both organic and inorganic, and including complex ions such as metal ammine complexes, ;-` where such species are capable of being sorbed on the porous , glass by physical or chemical ~orces or both i The ion exchange reactions, and also the subsequent - sorption o~ other chemical species onto the external and ;l internal surfaces of the microporous glass, can take place i;;~
I in aqueous solutions, and the exchange reactions are accom~
. . :
panied with a concurrent dehydration as the alkali metal `~
ions are exchanged with protons or other cations.
The process involves contacting the original hydro- ; ;
silicate ~lass first with the ion exchange solution to effect the ion replacement reaction. The same solution may contain the additional chemical species to be sorbed, or this optional step can be done serially in a second aqueous ;`~
.

,: .

-- ~ O ~ ~7 ~ ~

or nonaqueous solution. The rates of reaction are appre-ciable at room temperature and increase as the temperature is raised. Still further rate increases can be obtained by ~_~ increasing pressure and temperature, as in an autoclave.
The reacted product may thcrea~ter be further consolidated and dried by heating.
i It is well recognized that an amorphous body can be obtained by drying an aqueous solutlon of Na2U and/or K20 ` ~ silicate. However, such glassy bodies readily dissolve in H20 and in aqueous solutions of NaOH and KOH with the silica network being destroyed. It is believed that the following )~ ~
reactions are responsible for those phenomena. Thus, Reaction I describes the dissolution of the mass in H20 and Reaction II depicts the hydroxyl attack on the silica network:
.,~_ Reaction I -Si-ONa-mH20 + nH20 -~ water solution Reaction II _Si-O-Si_ ~ OH -~ decomposition products . : .
Polymeriæation of silica resulting from the contact of .' ~ simple alkali silicate glasses with acidic solutions is well ; ~ 20 known. Reaction III sets forth that phenomenon:
,_ r ~ Reaction III (a) Si-O-Na mH20 + H30 -~ -Si0+ H20 +
(b) 2 -SiOH -~ -Si-O-Si- + H20 However, that reaction commonly proceeds so rapidly and completely in acid solution that the silicate mass shrinks, cracks, and disintegrates. It can be observed that Reaction .: . I
III includes not only proton exchange for alkali metal ions but also a dehydration or splitting out of water, while the ~; ~ silica network undergoes polymerization and condensation , . :

-2-... . ; , .
~ .,,, I .
. , ultimately forming a silica glass. The exchange i5 SO rapid and irreversible that the presence of any signlficant con-~f,~ ~
1 ~ centration o protons tends to block exchange of other . cations or the alkali metal ions in the alkali silicate mass.
! The instant invention provides means for replacing part or all of the Na+ and/or K~ ions in an alkali silicate - h~drosilicate amorphous body either with protons as des-cribed in Reaction III, or with protons plus other monovalent or polyvalent cations, in the nondestructive manner by ` ~ reaction in aqueous solutions. In addition, it provides :~ , means for incorporating many other chemical species into the '.~i ion-exchange microporous body at low temperature, by sorption from solutions, either aqueous or nonaqueous. These latter species are not necessarily cations. The processes of the invention make possible the alteration of the chemical , ~ ~ composition of alkali hydrosilicate body to any of a wide i ;,, ' ~ariety of other glass compositions having better chemical , . . .
' , durability and other useful glass properties.
We have discovered that such nondestructive reactions are possible employing aqueous solutions comprising the - following composi~ional areas:
First, proton exchange for Na~ and/or K~ can result ~: I using aqueous solutions of ammonium hydroxide and/or ammon-ium salts;
Second, proton exchange can result using aqueous ,' !
solutions of sodium and/or potassium salts whose pH is ~,#: .. .' ~
:1 ,t I between 8.0 and 10.5 and whose concentrations are between about 30 gms/liter and satura~ion;
Third, proton exchange and cation exchange combined ~ - with sorption of species having an affinity for silica

-3-,~, .
~.. i . . . .

surfaces, can be carried out in a~ueous solutions of ammonium hydroxide ~nd/or ammonium salts containing ~lso monovalent and/or polyvalent metal cations including those complexed with ammonia and with carboxylic acids.
Fourth, proton and cation exchange combined with sorption of species having an affinity for silica surfaces, can be carried out in non-ammoniacal aqueous salt solutions having pH between 4 and 12, and containing about 30 to 500 grams/liter of salts of mono-, di-, tri- and/or tetravalent metal cations, including cations complexed with carboxylic acids.
Additionally, once a microporous glass has been produced by any of the above mentioned processes, nona~ueous ; solutions including organic solvents can be used as carriers to incorporate various chemical species into the body. rrhese species can include organic colorants, polymers and/or poly-; merizable organics.
Thus the present invention provides a method for producing a microporous silicate glass body which comprises `1 20 the steps of:
(A) altering the composition of a Na2O and/or K2O
hydrosilicate glass body containing between about 5-50% by weight H2O within its structure, wherein the anhydrous composi-tion of said glass body consists essentially, by weight on the oxide basis, of about 10-60% Na2O and/or K2O and 40-90% SiO2, the sum of Na2O and/or K2O ~ SiO2 constituting at least about 75~ of the total composition, and wherein said hydrosilicate glass contains up to about 25% by anhydrous weight total of .
such compatible metal oxides as BaO, A12O3, GeO2, SnO2, As2O3, ~-B2O3, SrO, MgO, ZnO, ZrO2, CaO, Sb2O3, and PbO and such anions as halides, carbonates, chlorates, bromates, iodates, cyanides, ~ _ 4 _ . .:

`" ~OB4710 sulfides, borates, phosphates, aluminates, plumbates, zin~ates, chromates, germanates, stannates, antimonates, and bismuthates;
by subjecting said hydrosilicate glass body to a reaction selected from the group~
(a) contacting said glass body with an aqueous solution of ammonium hydroxide and/~r ammonium salt to cause an exchange ~ . :
of protons from the solution for Na and/or K ions from the glass;
(b) contacting said glass body with an aqueous solution of sodium and/or potassium salt having a pH between about 8.0-10.5 and a concentration between about 30 grams salt/liter ; to saturation to cause an exchange of protons from ~he solution for Na+ and/or K+ ions from the glass : (c) contacting said glass body with an aqueous solution :
of ammonium hydroxide and/or ammonium salt also containing ~:-monovalent and/or polyvalent metal cations, including such :
cations complexed with ammonia and/or with carboxylic acids, ;:.
to cause an exchange of protons and/or cations from the 20 solution fox Na+ and/or K+ ions from the gla~s combined with `~
a sorption of species having an affinity for silica surfaces;
and (d) contacting said glass body with a non-ammoniacal aqueou~ salt solution having a pH between about 4-12 and ` containing about 30-500 grams salt/liter of monovalent and/or .:~ polyvalent metal cations, including such cations complexed with carboxylic acids, to cause an exchange of protons and/or cations from the solution for Na~ and/or K+ ions from the ' glass combined with a sorption of species having an affinity :
; 30 for silica surfaces;
: ~, ~ - 4(a) - : :
., ~ ,, . ' j, ~,........ . .. .

8~7~

(B) withdrawing said hydrosilicate glass body from contact wi th said aqueous solution selected from $ubparagraphs : (a)-(d); and then (C~ drying said body;
wherein said reaction is carried out at a temperature above ~he freezing point of the aqueous ~olution selected from subparagraphs (a)-(d), up to about 350C, but under such . pressure that volatilization of components from said solution - is inhibited.
In another embodiment.the present invention provides a method for producing a microporous silicate glass coating .
. on a substrate which is non-reactive with aqueous alkali metal silicate solutions comprising the steps of: -~:. applying an aqueous sodium and/or potassium silicate ~:l solution to a surface of said substrate; and . drying said solution to yield a hydrosilicate glass coating containing between about 5-50% by weight H2O withi.n its structure, wherein the anhydrous composition of said glass coating consists essentially, by weight on the oxide basis, of about 10-6~ Na2O and/or K2O and 40-90% SiO2, the sum of Na2O and/or K2O ~ SiO2 constituting at least 75~ of the total .
composition; and wherein said hydrosilicate glass coating i contains up to about 25~ by weight total of such compatible :: metal oxides as BaO, A12O3, GeO2, SnO2, As2O3, B2O3, SrO, MgO, ZnO, ZrO2, CaO, Sb2O3, and PbO and such anions as halides, .
carbonates, chlorates, bromates, iodates, cyanides, sulfides, ~ ~
,, I .
l borates, phosphates, aluminates, plumbates, æincates, chromates, ... ;1 germanates, stannates, antimonates, and bismuthates.
l The composition of said hydrosilicate glass coating is then altered by subjecting said coating to a reaction .
selected from the group:

4(b) -.,: I
, i ., , ., :.

7~L~

(a) contacting said glass coating with an aqueous siolution of ammonium hydroxide and/or ammonium salt to cause an exchange o~ protons from the solution for Na~ and/or K+ ions from the glass;
(b) contacting said glass coating with an aqueous solution of sodium and/or potassium salt having a pH between about 8.0-10.5 and a concentration between about 30 grams salt/liter to saturation to cause an exchange of protons from the solution for Na~ and/or K~ ions from the glass;
(c) contacting said glass coating with an aqueous solution of ammonium hydroxide and/or ammonium salt also containing monovalent and/or polyvalent metal cations, including such cations complexed with ammonia and/or with carboxylic acids, ~ , ; to cause an exchange of protons and/or cakions from the solution for Na~ and/or K~ ions from the glass combined with l a sorption of species having an a~Einity for silica surfaces; and -I (d) contacting said glass body with a non-ammoniacal ;~ aqueous salt solution having a pH between about 4-12 and I containing about 30-500 grams salt/liter of monovalent and/or 'i 20 polyvalent metal cations, including such cations complexed with carboxylic acids, to cause ar~ exchange of protons and/or cations from the solution for Na+ and/or K~ ions from the glass combined with a sorption of species having an affinity Eor silica surfaces.
~i The hydrosilicate glass coating is then withdrawn ; Erom contact with the above aqueous solution selected from subparagraphs (a)-(d); and then is dried. The above reaction ~,; i5 carried out at a kemperature above the freezing point of the l j aqueous solution, not exceeding about 350C, but under such pressure that volatilization o~ components from said solution is inhibited.

~ - 4(c) -. ,g~ ~ `.

.

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Ammonium ions, even in such strongly alkaline solutions as those of ammonium hydroxide ~N~40H), appear to be effective in dealkalizing, i.e., removing Na and K~ ions rom alkali hydrosilicate glasses, in a manner that does not disrupt the silica structure. Inasmuch as in most instances ; only a negligible quantity of ammonia (NH3), or nitrogen in any form, remains in the glass after tr~atment, we believe the reaction involves an exchange of ammonium ion (NH4 ) for Na or K ions~ ollowed by a splitting out of NH3, leaving a hydrogen ion (~1) attached to the silicon-oxyg~n (a silanol group). This mechanism is set out below.
'' '~ :

i' ~ ' ' ,~., ' ; ~ .
,..
. ,, ` ' . .

,,,.~ :, ' ; , .
, .
., .~ . .. .
,,; ,,, '~ '~
, .. ..
, .

~, :
.
, ~'"'`' ~'~
i~:
.~,"
, 30 _ 4(d) -,.
.';

. .
~, ' ~ . . ' ' -, ' ~ . . :
:.. . i , .- . ~ ~, .. .. . . :

NH40H -~ Na-O-Si'_ -~ MH~-O-Si- ~ NaOH
NaOH
NH4-0-Si_ -~ -SiOH -~ NH3~

In essence, the resultan~ reaction is an exchange of protons (H~) for Na+ and!or K~ .
This proton exchange, occurring in an alkaline environ- ,' ment, is no~ accompanied by the rapid condensation that occurs when protons replace alkali ions in an acidic medium.
Therefore, it does not lead to shrinking and cracking of the glass body while in solution, but appears to produce a sur- '` ' face layer of silica. This silica film increases in thick-ness with time in the solution and provides a protective layer that prevents dissolution of the hydrosilicate glass ,`
within. The rate of react'ion is believed to be con~rolled by the diffusion o NH4+ ions.
Behavior similar to that displayed with WH40H has al,so ,, .been found utilizing aqueous solutions of the salts thereof, e-g., NH4C1, NH4N03, (NH4)2C03, NH~HC03, (NH~)2B~07 4~2' ~H~
~58 4H2. and NH4G2H302.
In general, where NH40H i9 present alone, a concen~ra-tion o~ at least about 30% by weight is preferred to prevent dissolution of the glass and breakdown of the silica struc-ture. Where an ammonium salt is employed, a concentration of at least about 5 g/l is preferred.
We have also observed that the inclusion of other cations in the same ammoniacal solutions can lead to a partial or virtually complete replacement of Na~ and/or K~ ions from the glass with both NH4~ ions (or protons as described above) and such other catlons as Li+, Cu~~, Ag~, M ~2 C +2 pb~2 Ba+2 Zr+4, Zn~2, Cr+3, Mn -, Fe , Co Co+3, Ni~2, Cu+2, Cd+2, Ti+4, and Al~3. The latter exchange ' lOB4710 takes place more slowly than the proton exchange but chemical analyses of the final product have identified the presence of substantial quantlties of the cation. It is quite apparent that this phenomenon permits the production o~
glasses of wide ranging compositions.
A relatively few specimens, analyzed after treatment in borate or chloride solutions, have shown presence of sub-stantial boron or chlorine. We believe -that thése represent microporous glasses containing sorbed borate or chloride salt. Sorption also appears to be part of the mechanism by which low concentrations o silver ion and of copper ammine complex become concentrated in a dealkalized hydrosilicate glass.
As has been pointed out above, dealkalization of alkali hydrosilicate glasses in aqueous acidic solutions is well-known but the combined rates of proton exchange for alkali metal ions and dehydration are so rapid that the silica structure polymerizes and condenses, thereby leading to catastrophic shrinking and cracking of the glass.
Furthermore, the proton exchange is so much more rapid ~han an excha~ge with other cations that this latter exchange i9 essen~ially negligible.
Therefore, the following are some of the apparent advantages of treatment in nonacidic, aqueous ammoniacal solutions:
(1) slower, better controlled rate of ion exchange;
(2) slower, better controlled rate of dehydration since abo~t two molecules of H2O are apparently associated with each alkali metal ion such that removal of the latter also causes dehydration;

:

(3) better chan ~ ~ ~o ~er cations to compete with protons in the exchange;
(4) no breakup of the resulting siliceous article; and ~-(5~ frequently less polymerized silica network, al.low-ing easier molding of ion exchanged granules of hydrated ;~
alkali silicate glass by conven~ional molding techniques such as compression molding.
Cation exchanges are, however, possible in aqueous salt ; solutions wherein the salt is of a particular con~entration, commonly between about 30-500 grams/liter, such that the dissolution rate of the alkali hydrosilicate glass is sub-stantially negligible. Thus, Na~ and K~ ions can be exchanged with monovalent, divalent, trivalent, and tetravalent ion ~ .
cations.
With respect to monovalent cations, exchange of K+ ions for Na+ ions takes place when a sodium hydrosilicate glass ` is contacted with an aqueous solution of a potassium salt.
Ag+, Cu+, Tl~, and Li.+ ions can also replace either Na~ or K+.
These latter exchanges can be irreversible because of result-;¦ 2Q ant changes in the state of the solute or in the silica ;:
network. In general, these exchange reactions are carried out in neutral or only weakly acidic, neutral, or only weakly basic solutions to avoid dissolution or disintegra-tion of the glass. Hence, nitrates, chlorides, sulfates, ! and acetates have been found particularly successful.
, I
~
With respect to divalent cations, alkali hydro9ilicate glasses can undergo ion exchange reactions with such when contacted with non-ammoniacal aqueous salt solutîons of ~¦ intermediate concentrations (about 30-500 grams/liter). In ,~j 30 general, the alkali ions exchange most readily for - ~ protons but they also, albeit at a slower rate, exchange ~: -7-.. ~ . .

-with the divalent ions. The replacement reaction can be conducted in slightly acid, neutral, and slightly basic solutions, l.e., within a pX range of between abou~ 4-12, without damaging the silica network of the glass. Chlorides, nitrates, and acetates of calcium, barium, lead, magnesium, and zinc are examples o operable compositions. Chemical analyses of the exchange glasses have identified the pre-sence of substantial quantities of the divalent cations.
The glasses after the exchange reaction and/or sorption can be transparent or opaque. -The opaque glasses may con-tain crystalline oxides, hydroxides, and/or silicates pre-cipitated within the silica structure as the divalènt cations migrate into the glass, or, in some instances, microscopic voids or bubbles produced by exsolution of water from the hydrosilicate glass.
However, examination by means of scanning electron microscopes, porosimetry and other techniques has ~hown that dealkalization by proton exchange ultimately results in a microporous silica glass body. The pore size can range down to well below 40 Angstroms; and bodles wlth pore diameter~ below about 60 ~ are transparent.
This discovery, combined with some analytical deter-minations, has shown that sorption of borate and other chemical species in the microporous glass does occur, if the species has a hlgh a~finity or the silicate surface of the pores. Thus) ion exchange followed by adsorption affords a means for incorpora~ing some neutral molecules and anionic species into ~he body.
Subsequent consolidation of the microporous bady to a dense glass is accomplished by heating at temperatures below the deformation temperature of the glass.

~ 7 ~

Contact o-f alkali hydrosilicate glasses with aqueous ammoniacal solutions containing trivalen~ cations such as Al+3 and tetravalent cations such as Ti~4 and Zr~4 can result in an exchange o~ the cations with the Na~ and/or K+ ions as well as a proton exchange. However, contact of the glasses with aqueous, non-ammoniacal salt solutions of such examples as aluminum chloride, aluminum nitrate, TiC14, ZrC14, and boron chloride has resulted principally in proton exchange, i.e., the Na+ and/or K~ ions exchange for protons with very little exchange of the trivalent and tetravalent cations.
A special type of reaction occurs with complex cations such as the metal-ammine ions. Thus, in neutral or slightly basic solutions, the metal-ammine ion readlly exchanges with the Na~ and/or K+ ion in the alkali hydrosilicate glass.
For example, solutions of Cu(NH3)2+, Cu(NH3)4+2, Ni(NH3)4-~2, and Co(NH3)4~2 react with the alkali hydrosilicate glasses and impart an intense coloration thereto characteristic of ~he particular ion. Chemical analyses have shown that ammonia and the cation are retained in the glass while the alkali metal ion concentration decreased. The anionic component (Cl-, N0 3, S0=4) of the metal ammine complex salt did not enter the glass. It is not known whether this reaction is true ion exchange, or merely adsorption, but these ion complexes are relatively stable in the glass since the characteristic color persists to temperatures above 300C.
Independent evidence shows that copper ammine complex ion adsorbs strongly on silica gel, and the reaction in the present case may be similar.
It is postulated that -the exchange may involve an ion replacement reaction of the following types:
, _g_ .

1~8~7~

+2 H20 Cu(N113)~ -t 2Na-O-Si -~ -Si-o-Cu(NH3)~-o-Si_, or _si-O-Cu(NH3)2(H2o)2-o-si--Hence, there can be a partial and even essentially complete removal of the alkali metal ions with a corresponding pro-portion of the divalent cation and of NH3, as well as partial to virtually complete dehydration. Subsequent heat treatment of the exchanged glass, i.e., to temperatures above 300C, will decompose the complex ions and, ul~imately, the NH3 in the glass. This action results in the reduction of reducible metal oxides such as copper, unless the NH3 is caused to diffuse away.
However, not only are the many well-known metal-ammine complex ions soluble in aqueous NH~OH capable of being introduced into ~he alkali hydrosilicate glasses, but also aqueous amine complexing agents containing complexed metal ions as cations (but not as anions) can be substitutes for aqueous NH40H.
Aqueous solutions of NaHC03 having a pH around 8.4 act to dealkalize the glass bodies, presumably through a proton exchange, Such behavior is unexpected since the golution i5 neither ammoniacal or acidic. It is believed that the protons produced by dissociation of the bicarbonate ion (HC03 -~ H~ ~ C02 2) account or this reaction.
Finally, metal cations complexed with carboxylic acids, such as acetic, citric, and/or tartaric acid, can be exchanged for the Na~ and/or K~ ions in the glass. This embodiment of the instant invention can be especially useful in introduc-ing trivalent and tetravalent metal cations such as Alt3, Ti+4, and Zr~4.

~ 7~ ~

The alkali hydrosilicate glass articles operable in this invention can be produced either by steam hydrating a Na2O and/or K2O-SiO2 glass to the required water content or by partially dehydrating an appropria~e aqueous Na20 or K2O
silicate solution to the required water content. Especially useful compositions, on an anhydrous glass basis, consist essentially, by weight on the oxide basis, of about 10-60%
Na2O and/or K2O and 40-90% SiO2.
Although not necessary to the successful operation of the invention, various optional ingredients can be included in the base alkali metal silicate glass. Hence, for example, such compatible metal oxides as BaO, A12O3, GeO2, SnO2, As2O3, B2O3, SrO, MgO, ZnO, ZrO2, CaO, Sb2O3, and PbO can be added to the base composition. In general, the Na~ and/or K~ ions will exchange more rapidly than the ions o the additlve oxides but the presence of the latter can reduce the rate of exchange. Moreover, it is also possible to dissolve anion salts such as the halides, carbonates, chlorates, bromates, iodates, cyanides, sulfides, borates, phosphates, aluminates, plumbates, zincates, chromates, gérmanates, stannates, antimonates, ~ismuthates, etc., in the starting solutions thus enabling materials to be incorporated which have signi-ficant volatility at glass melting temperatures. These materials would subsequently proton exchange to remove the alkali metals. However, the ~otal of all such additions wiLl normally not exceed about 25% by weight.
The inclusion of certain anions in the starting alkali metal silicate material can lead to the production of trans-lucent, white or colored opaque glass bodies. Hence, the cation from the ion exchange solution can react with the soluble anion in the glass structure causing the in situ '~ ~ ~ 'f '; ' . ..
' ~' ~ . ', '' ; . ' ~ 4~ ~

precipitation of insoluble reaction products. For example, Ca~2, Ag~, and Pb-t2 cation in the solution can react with such soluble anions as F- t Cl-, and CrO3~2 to yield CaF2, AgCl, and PbCrO4, respectively.
Although the ion exchange reaction can be operable with water contents between about 5-50% by weight, a preferred water concentration in the hydrosilicate glass body before the ion exchange reaction is undertaken is about 10-30% by weight. Within that range the body is solid but'thermo-plastic. Where the water content exceeds about 50%, the '' body flows and behaves in the manner of a li~uid. Upon immersion into the ion exchange solutions t the body either undergoes dissolution or gelation to a useless paste. When ,the water content is less than about 5%, the rate of ion exchange becomes impractically slow in aqueous solutions.
Uniform dehydrationt without excessive gradients throughthe bodyt is re,quired to avaid wrinklingt crackingt or de~ormation. Dehydration can be accomplished by air-drying with or without heat t infrared light t high frequency elec-20 tric fields t or vacuum. Under proper conditions, thearticle shrinks progressively and uniformly in all dlmensions.
The ion exchange reactLon can be initiated at any stage of de'hydrationt provided that sufficient water remains in the glass to permit ion exchange and/or sorption at reason-able rates. Some ion exchange solutions have the capability of hardeni.ng the silica structure so that the overall dimensions of the articles remain constant from the time of contact with the exchange solution t even though further dehydration and dealkalization are taking place. In those instances where the silica network cannot shrink during the ion exchange reaction t removal of water and , -12-~ 7 ~ ~

alkali metal ions can lea~ ~o a microporous structure.
Hence, through proper coordinated control of dehydration conditions and ion exchange papameters, the dimension and degree of porosity o such articles can be determined.
These circumstances recommend a two-step process wherein the first step contempla~es ion exchanging protons, cations, and/or complex species in a solution which does not cause shrinkage and, hence, permits a rapid rate of exchange.
Thereafter, shrinkage can be effected, resulting in a dense body of a desired altered composition. Also, bodies exhi-biting controlled pore sizes up to about 50% total porosity can be achieved.
The ion exchange process, itself, contemplates con-tacting the hydrosilicate glass body with the aqueous solution (normally immersing the solid body into a bath of the solution) under controlled conditions of temperature and pressure ~or a sufficient period of time to cause the reaction to proceed to completion or as near completion as desired.
The reaction will take place, albeit very slowly, at -tempera-tures approaching the freezing point o~ the solution. Thereaction rates are appreciable at 25C , and continue to increase wlth higher temperatures. Temperatures below the boiling point of the solution avoid possible surace abuse of the glass and are less hazardous to the operator. Custo-marily, the exchange reaction will be carried out between about 25-100C. ~or periods of time ranging from a few minutes to a few days, depending upon the thickness of the glass body, the temperature employed, the solution composi-tion, t'ne water content of the glass, etc. The reacted product is removed from contact with the solution, rinsed in water to remove any adhering solution, and dried. This ~ 7 ~

drying step may be conducted at somewhat elevated temperatures, i.e., above 25C. In some ins~ances, the dried bocly will require firing at higher temperatures, but below its deforma-tion temperature, to consolidate the body still further.
The ion exchange reactions of ~his invention can also be carried out in an aclueous solution under pressure such that a temperature can be employed which would cause volati-lization of solution components at ambient pressure. For example, water and/or ammonia could be volatilized away.
The pressure utilized may be due solely to the vapor pres-sure of the volatile components of the solution or may be partly or mainly the result of inert gas additions to the reaction environment. Inasmuch as the ion exchange rate can be substantially increased through ~his mode of operation, it is particularly useful where East processing is desired and/or hydrosilicate glass bodies of low water contents are sought to be exchanged.
Gaseous pressures up to about 3000 psi and temperatures up to about 350C. are reasonably attained. Pressures o steam with ammonia andlor inert gas can vary widely with 3000 psi generally being deemed a practical limit. Thus, in either practice, higher temperatures and pressures have not appeared to alter the properties of the final product to any significant extent. As a matter of practical convenience, reaction environment of saturated steam at temperatures not exceeding about 300C. are preferred.
The following tables provide specific examples of the above-described types of ion exchange reactions. It will be appreciated that the reported examples are only illustrative of the present invention and ought not to be considered limiting. Furthermore, whereas the followin~ examples ~L~7~

involve only a single exchange reaction, it will be recognized that a sequence of ion exchanges is possible, or a reac~lon where two or more ions are present in a slngle contacting medium which can exchange with the alkali metal ions of the hydrosilicate glass.
Where the body to be treated is stated to be a glass, the composition thereof çonsisted essentially, by weight on the oxide basis, of about 74% SiO2 and 26% Na20 which had been hydrated in steam to contain a water content of about 25%. The hydration was carried out in accordance with the practice disclosed in United States Patents Nos 3,498,802 and 3,498,803. Thus, a ~lass-forming batch was melted and a ribbon of glass drawn from the melt having a thickness of about 0.010". This ribbon was fractured into numerous pieces, placed in an autoclavej and subjected to a H20-saturated atmosphere at 120C. and a pressure of 21 psig for about l l/2 hours to achieve complete hydratlon of the glass.
Where the body to be treated is stated to be a dried sodium silicate solution, a solution commercially-marketed by Philadelphla Quartz Company was employed wherein the initial composition thereoE conslsted essentially, by weight on the oxide basis, of about 8.3% Na20, 24.7% SiO2, and the balance H20. The solution, containing dissolved additives where notèd, was poured onto à flat surface or into a plastic mold and allowed to dry in the ambient atmosphere. The sheet or article so formed was then removed from the flat surface and broken into small pieces and granules. After this air drying, the body had a water content of about 15-30% by weight.
.

~0 8 ~

The pieces of glass and "dried" sodium silica~e were then i~nersed into a bath of the reported ion exchange solutions for the times and temperatures recorded in the tables. Thereafter, the pieces were removed from the bath, rinsed in tap water, and dried in air at about 75C. The compositions tabulated reflect chemical analyses of the final product wherein the SiO2 was obtained by difference.
In certain instances, very minor amounts of carbon, nitro-gen, and H20 were ound but those have been ignored here.
As has been observed above, the small pieces and granules of ion exchanged glass can be formed into bodies of various geometries and dimensions employing,conventional forming techniques such as compres,sion molding. Table I lists a number of examples using ammoniacal solutions as the ion exchange media. The term "Ac" refers to acetate and the term "S-35" refers to the aqueous sodium silicate solution described above,dried at room temperature. The NH40H reers to a concentrated aqueous solution containing 2~% NH3.

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The following two examples illustrate instances wh~re glasses can be made practicing the method of the present invention utllizing a naturally-occurring silica as ~he starting material.

Exam~le I

25 grams of a naturally-occurring amorphous silica having a particle size less than about 10 micro~s were blended into 30cc of a 50% NaOH aqueous solution and the mixture then placed into a TEFLON~-lined container. The container was transferred to an autoclave and heated to `
150C. for 16 hours in a H2O-saturated atmosphere. The resulting sodium silicate solution was poured onto a flat surface and allowed to dry. The resulting Eilm had a water content between about 15-30% by weight.
The film was thereafter broken into small pieces and immersed into a solution consisting of 20 grams (NH4)2CO3 in 80cc H2O for 16 hours at 50C. Analysis of the final product showed 97% SiO2 and 3% Na2O.

Example II

25 grams of AErican sand were pulverized such that the particles were less than`about 20 microns in diameter and these particles were blended into 30cc of a 50~/O NaOH
aqueous solution contained within a TEFLON~-lined vessel.
The vessel was transferred to an autoclave and fired and dried in like manner to the practice described in Example I.
Immersion of the dried material in a solution of 20 grams (NH4)2CO3 in 80cc H2O for 16 hours at 50C. produced a inal product analyzing 92.8% SiO2 and 7.2% Na2O.

~ 7 1 ~

Table II records a group of examples illustrating ion exchange reaction in non-ammoniacal salt solutions. As noted above with respect to Table I, the term "Ac" re~ers to acetate and "S-35" refers to the aqueous sodium silicate solution discussed above dried at room temperature.
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O o '~ -31-Table III reports a number of examples illustrating reactions with complex cations such as the metal-ammine ions. TETA referred to in Example 115 is triethylenetetra-mine.

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Table IV records a group of examples wherein the ion exchange was carried out in an autoclave under elevated pressures and temperatures. Except where otherwise noted, ~he starting material "S-3$" refers to the aqueous sodium silicate solution discussed above dried at room temperature.
The pressed discs of "S-35" consisted of air dried S-35 which had been placed into a steel mold and pressed under about 5000-7000 psi pressure while at a temperature o~ about 120C. to yleld a circular disc about 2" in diameter and about l/16" thick. In each instance, the glass bodies were ``
immersed into the reported solutions which were contained within glass vessels that were capable of being sealed. The glass vessels were then sealed and placed withln an autoclave and heated to about 120C.` to yield a pressure of about 15 psig within the glass vessel. This treatment was maintained for the periods of time set out in Table IV. -The nitric acid utilized was a concentrated (70.4%) aqueous solution.
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Table V lists a group of examples wherein pressed discs of air-dried "S-35" were prepared as described above. The discs were divided into two groups. The first group was placed into an autoclave and subjected for three minutes to a saturated steam atmosphere at 120C. and about 15 psig.
Thereafter, those discs and those of the second group were immersed into the reported solutions which were contained within glass vessels which were capable of being sealed.
The vessels were then sealed, placed into an autoclave operating at about 120C. so as to produce a pressure of !~
about lS psig within the vessels, and retained within the autoclave for 16 hours. The nitric acid used was the same as that reported above.

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Table VI records a group of examples wherein a dried potassium silicate solution comprised th~ starting material.
The potassium silicate solution employed was a commercially-marketed product of Philadelphia Quartz Company labelled "K-1" which consisted essentially, by weight on the oxide basis, of about 8.3% K20, 20.8% SiO2, and the balance H20.
Dried slabs, exhibiting water contents between about 15-30%
by weight, were made in the same manner as that described above with respect to dried "S-35". The slab was broken into small pieces and immersed into the reported ion exchange solutions for about 20 hours at 50C. Thereafter, ~he pieces were removed from the baths, rinsed in tap water, and dried in air at about 75C. The compositions tabulated report analyses of the final product wherein the SiO2 was obtained by difference. The presence of minute amounts of carbon, nitrogen, and H20 have been ignored, , ..

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Products of extremely pure SiO2 were made in the follow-ing manner. Slabs of "S-35" were made via the same procedure described above and air dried at 95C. to a water content of about 13%. Samples thereof were then immersed into three different aqueous solutions. The first solution consisted of five grams NH4Cl in 100 cc water ~ a few percent of a water soluble acrylic re$in. The second solution consisted of 10 grams (~H4)2C03 in 100 cc water ~ a few percent of a water soluble acrylic resin. The third solution consisted of ~0 grams NaHC03 + a few percent of a water soluble acrylic resin. (The resin acts to pick up and keep out o the solution any extra Na~.) The samples were retained within the solutions at 50C. or 16 hours.
Porosimetry measurements utilizing mercury intrusion indicated a pore size of less than about 50 R for the samples treated~in NH~Cl and (NH4)2C03 and about 60 ~ for those samples i~nersed in NaHC03 . An analysis of Na20 in the ~inal products reported about 0.03-0.1% in the bodies reacted with NH4Cl and (NH4)2C03 and less than about O.003% in those articles sub~ected to NaHC03.
Therefore, a method is provided ~or preparing extremely pure SiO2 bodies without the requirement of melting batch materials.
Another method for obtaining SiO2 bodies of high purity without the necessity of melting batch materials involves the leaching oE sodium silicate articles in concentrated sodium nitrate solutions. In carrying out this procedure, sheets of air-dried "S-35" were prepared as described above to a water content o about 25-30% by weight. On a dry basis, the sheets contained, by weight, about 21% Na20 and 79% SiO2. Two series of samples were leached in concentrated ' . ' g~7~0 aqueous NaN03 solution (-lQOg/lOOgH20) and numbers of the samples were subsequently leached in aqueous NH4N03 solu-tions (lOg/lOOgH20). Each series was run at a different temperature and the Na20 content remaining in each sample spectrographically analyzed. The effect of successive short-time immersions in two or three baths of NH4N03, rather than a single long-time immersion in one bath, was also studied. Table VII reports the various leaching pro-cedures conducted at 85C. and the retained Na20 in the final bodies. Table VIII reports the same data at 65C.

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It is apparent from 'these data that SiO2 bodies con-taining less than about 100 parts/million (PPM) Na20 can be produced without melting a glass-~orming batch. A further immersion in water containing a cation exchange resin can - lower the Na2O content to less than 10 PPM. An exposure of samples to several changes of NH~N03 for short ~imes, e.g., three baths for a total of four hours, is more effective in leaching out Na2O than is one exposure for an even longer time, e.g., one bath for eight hours.
These low Na2O-containing bodies can be consolidated into solid, non-porous fused silica articles by firing at temperatures above about l200C.
Customarily, the star,ting bodies operable in the instant Lnvention will consist of glass formed from a molten batch in the conventional manner or a dried'mass o an alkali silicate. Nevertheless, it is possible to utilize the ';
, products formed,by reacting solutions and,/or suspensions of soluble silicates with organic reagents to thereby polymerize the siLica into a self-supporting, porous or non-porous body. Illustrative of such practice are United ' States Patents Nos. 3,782,982 and 3,827,893.
In the former patent is disclosed the reactlon oE
formaldehyde, paraformaldehyde, glyoxal, and/or formamide with aqueous solutions, colloidal solutions, andjor sus-pensions oE sodium silicate and~or potassium silicate at temperatures between the reezing and boiling points of the respective solutions. The reaction reduces the pH of the aqueous silicate material and thereby causes the silica to polymerize. The latter patent describes a similar type reaction but the polymerizing agent is selected from the group consisting of methyl formate, ethyl formate, methyl ' ' ~
-~5-. ' acetate, ethyl acetate, and a relatively water-insoluble carbonate and/or hydroxide and/or borate and/or phosphate of magnesium, lead, zinc, chromium, lanthanum, and aluminum.
The resultant polymerized silica body can be dried in air at ambient temperature, similar to the S-35 and K-l, supra, to yield a porous, monolithic body containing in excess of 10% by weight water. The body can then be heated to moderately elevated temperatures to eliminate most of the water and leaving a much stronger, porous, monolithic body.
If desired, the body can be fired to higher temperatures to consolidate the body into a non-porous article.
Any of those forms of products will 'be operable in the ion exchange process of the present invention.
Although the above examples relied upon simple thermal difusion as the driving orce for the ion exchange reaction, it will be appreciated that an electrical potential can be applied to promote the reaction. In general, this practice is not required and, of course, involves providing the necessary circuitry and source of electrlc power. However, it can increase the rate o~ ion migration and, where speed is of vital importance, this embodiment can be useful.
The method of the instant invention also permits the ' '`' production of reinforcecl glass'boclies. This is very readily accomplished where an alkali silicate solution comprises a starting material. Thus, cloth, screening, fibers, granules, ribbon, etc. o the gla~s, metal, or organic polymers can be incorporated into the solution or impregnated t'hereby and will be retained in the dried body. In general, these reinforcing elements will be substantially inert to the subsequent ion exchange reaction.
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As illustrative of the manner of making composite bodies, pieces of cardboard, fiberglass cloth, brass screen, and a mat of alumina fibers were immersed into S-35 solution at room temperature. After an immersion of about five minute~, the articles were removed from the bath and allowed to dry in air at room temperature for two days. Thereafter, each body was immersed into a solution consisting of 10 grams (NH4)2C03S100 cc H20 for three days. Upon removal from the batch, a hard slab was observed wherein the sili-cate material was transparent. A hard slab was also achievedwhen a mat of alumina fibers was first immerséd into S-35 solution and then into a solution consisting of 30 grams Ca(Ac)2/100 cc H20-Silicate coatings can be ormed on various substrates which are non-reactive with alkali metal silicate solutions such as glass, ceramic, plastic, and metal by applying the alkali silicate solution thereon, drying, and then conducting the ion exchange reaction. Post firing for further consoli-dation of the coating can improve the dura~ility and abra-sion resistance thereof.

Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing a microporous silicate glass body which comprises the steps of:
(A) altering the composition of a Na2O and/or K2O
hydrosilicate glass body containing between about 5-50% by weight H2O within its structure, wherein the anhydrous composition of said glass body consists essentially, by weight on the oxide basis, of about 10-60% Na2O and/or K2O and 40-90%
SiO2, the sum of Na2O and/or K2O + SiO2 constituting at least about 75% of the total composition, and wherein said hydrosilicate glass contains up to about 25% by anhydrous weight total of such compatible metal oxides as BaO, Al2O3, GeO2, SnO2, As2O3, B2O3, SrO, MgO, ZnO, ZrO2, CaO, Sb2O3, and PbO and such anions as halides, carbonates, chlorates, bromates, iodates, cyanides, sulfides, borates, phosphates, aluminates, plumbates, zincates, chromates, germanates, stannates, antimonates, and bismuthates;
by subjecting said hydrosilicate glass body to a reaction selected from the group:
(a) contacting said glass body with an aqueous solution of ammonium hydroxide and/or ammonium salt to cause an exchange of protons from the solution for Na+ and/or K+ ions from the glass;
(b) contacting said glass body with an aqueous solution of sodium and/or potassium salt having a pH between about 8.0-10.5 and a concentration between about 30 grams salt/liter to saturation to cause an exchange of protons from the solution for Na+ and/or K+ ions from the glass;
(c) contacting said glass body with an aqueous solution of ammonium hydroxide and/or ammonium salt also containing monovalent and/or polyval-ent metal cations, including such cations complexed with ammonia and/or with carboxylic acids, to cause an exchange of protons and/or cations from the solution for Na+ and/or K+ ions from the glass combined with a sorption of species having an affinity for silica surfaces; and (d) contacting said glass body with a non-ammonia-cal aqueous salt solution having a pH between about 4-12 and containing about 30-500 grams salt/liter of monovalent and/or polyvalent metal cations, including such cations com-plexed with carboxylic acids, to cause an exchange of protons and/or cations from the solution for Na+ and/or K+ ions from the glass combined with a sorption of species having an affinity for silica surfaces;
(B) withdrawing said hydrosilicate glass body from contact with said aqueous solution selected from subparagraphs (a)-(d); and then (C) drying said body;
wherein said reaction is carried out at a temperature above the freezing point of the aqueous solution selected from subparagraphs (a)-(d), up to about 350°C, but under such pressure that volatilization of components from said solution is inhibited.

2. A method according to claim 1 wherein said hydrosili-cate glass body consists of a glass having the identified composition which has been hydrated in a H2O-containing environment to absorb at least an amount of H2O sufficient to render the hydrosilicate glass body operable in said ion exchange reaction selected from subparagraphs (a)-(d).

3. A method according to claim 1 wherein said hydrosili-cate glass body consists of an aqueous sodium and/or potassium silicate solution which is dehydrated to the required H2O
content, being an amount of H2O sufficient to render the hydrosilicate glass body operable in said ion exchange reaction selected from subparagraphs (a)-(d).

4. A method according to claim 1 wherein said hydrosilicate glass body is produced through the reaction of aqueous solutions, colloidal solutions, and/or suspensions of sodium silicate and/or potassium silicate at temperatures between the freezing and boiling points of the respective solutions with a polymerizing agent selected from the group consisting of formaldehyde, glyoxal, paraformaldehyde, formamide, methyl formate, ethyl formate, methyl acetate, ethyl acetate, and a relatively water-insoluble carbonate and/or hydroxide and/or borate and/or phosphate of magnesium, lead, zinc, chromium, lanthanum, and aluminum.

5. A method according to claim 1 wherein said pressure is provided by steam, ammonia, and/or inert gas.

6. A method according to claim 1 wherein said reaction is carried out in an environment of saturated steam at a temp-erature not exceeding about 300°C.

7. A method according to claim 1 wherein said reaction is carried out under the influence of an electrical potential.

8. A method according to claim 1 wherein said hydrosili-cate glass body is contacted successively with two or more of said solutions (a), (b), (c), and (d).

9. A method according to claim 1 wherein said microporous silicate glass body is contacted with a non-aqueous solution to incorporate various chemical species into the pores of the body, wherein said chemical species include organic colorants, polymers, and/or polymerizable organic materials.

10. A method according to claim 1 wherein, after drying, the microporous body is heated to an elevated temperature, but below the deformation temperature thereof, to further dehydrate and/or consolidate it.

11. A method for producing a microporous silicate glass coating on a substrate which is non-reactive with aqueous alkali metal silicate solutions comprising the steps of:

(A) applying an aqueous sodium and/or potassium silicate solution to a surface of said sub-strate;
(B) drying said solution to yield a hydrosilicate glass coating containing between about 5-50%
by weight H2O within its structure, wherein the anhydrous composition of said glass coating consists essentially, by weight on the oxide basis, of about 10-60% Na2O and/or K2O and 40-90% SiO2, the sum of Na2O and/or K2O + SiO2 constituting at least 75% of the total composi-tion; and wherein said hydrosilicate glass coating contains up to about 25% by weight total of such compatible metal oxides as BaO, Al2O3, GeO2, SnO2, AS2O3, B2O3, SrO, MgO, ZnO, ZrO2, CaO, Sb2O3, and PbO and such anions as halides, carbonates, chlorates, bromates, iodates, cyanides, sulfides, borates, phosphates, aluminates, plumbates, zincates, chromates, germanates, stannates, antimonates, and bismuthates;
(C) altering the composition of said hydrosilicate glass coating by subjecting said coating to a reaction selected from the group:
(a) contacting said glass coating with an aqueous solution of ammonium hydroxide and/or ammonium salt to cause an exchange of protons from the solution for Na+ and/or K+ ions from the glass;
(b) contacting said glass coating with an aqueous solution of sodium and/or potassium salt having a pH between about 8.0-10.5 and a concentration between about 30 grams salt/liter to saturation to cause an exchange of protons from the solution for Na+ and/or K+ ions from the glass;
(c) contacting said glass coating with an aqueous solution of ammonium hydroxide and/or ammon-ium salt also containing monovalent and/or polyvalent metal cations, including such cations complexed with ammonia and/or with carboxylic acids, to cause an exchange of protons and/or cations from the solution for Na+ and/or K+ ions from the glass combined with a sorption of species having an affinity for silica surfaces; and (d) contacting said glass body with a non-ammoniacal aqueous salt solution having a pH between about 4-12 and containing about 30-500 grams salt/liter of monovalent and/or polyvalent metal cations, including such cations com-plexed with carboxylic acids, to cause an exchange of protons and/or cations from the solution for Na+ and/or K+ ions from the glass combined with a sorption of species having an affinity for silica surfaces;
(D) withdrawing said hydrosilicate glass coating from contact with said aqueous solution selected from subparagraphs (a)-(d); and then (E) drying said coating;
wherein said reaction is carried out at a temperature above the freezing point of the aqueous solution, not exceeding about 350°C, but under such pressure that volatilization of components from said solution is inhibited.

12. A method according to claim 11 wherein said pressure is provided by steam, ammonia, and/or inert gas.

13. A method according to claim 11 wherein said reaction is carried out in an environment of saturated steam at a temp-erature not exceeding about 350°C.

14. A method according to claim 11 wherein said reaction is carried out under the influence of an electrical potential.

15. A method according to claim 11 wherein said hydro-silicate glass coating is contacted successively with two or more of said solutions (a), (b), (c), and (d).

16. A method according to claim 11 wherein said microporous silicate glass coating is contacted with a non aqueous solution to incorporate various chemical species into the pores of the coating, wherein said chemical species include organic colorants, polymers, and/or polymerizable organic materials.

17. A method according to claim 11 wherein, after drying, the microporous coating is heated to an elevated temperature, but below the deformation temperature thereof, to further dehydrate and/or consolidate it.