17 D u r a b l e G l a s s by Reconstitution of H y d r a t a b l e Sodium Silicate Glasses R. BARTHOLOMEW, W. HAYNES, and R. SHOUP Corning Glass Works, Sullivan Science Center, Corning, NY 14831 Sodium silicate glasses containing from 15 to 21 wt. % Na0 were hydrated in an autoclave at about 140°C to contain up to 40 wt. % water. These hydrosilicates were then dealkalized in salt solutions (< 100°C) to as little as 100 ppm residual alkali. Kinetics of dealkalization depended on alkali content and extent of hydration of the glass. The pore structure of the dealkalized body was dependent on salt concentration, temperature and pH of the leach bath. It, also, depended on alkali and water content of the hydrosilicate regardless of its origin, hydrated glass or dehydrated sodium silicate solution. Consolidation of 1 to 4mm thick porous bodies to coherent transparent glass was accomplished above about 1200°C. Uniformity of pore structure affected transparency. 2
The role of water in glass has been studied extensively in recent years. It has been concluded from both infrared'i>2.»2»£) and NMR^-^ data that in addition to hydroxyl groups, molecular water exists in glasses of high water content (> 1 wt. % H 0). Conventionally melted commercial glasses contain usually less than 0.1 wt. % water, present as hydroxyl groups. Certain borate and phosphate compositions have reported water contents approaching one weight percent. To synthesize glasses with water contents greater than a few tenths of a percent high pressures and temperatures, attainable in an autoclave, are required '-^. Such glasses, containing mostly molecular water, are called hydrosilicates. Work on hydrosilicates has led to the finding that glasses based on silica either, (1) take up a fixed water content, giving stable transparent glasses, (2) pick up water continuously and finally form gels, or (3) take up water to a certain concentration where they crystallize to form hydroceramics.^ ' 2
0097-6156/82/0194-0277$06.00/0 © 1982 American Chemical Society
SOLUBLE
278
SILICATES
Anhydrous sodium s i l i c a t e g l a s s e s which are composed of be tween 12 and 21 wt. % Na 0 a r e d i f f i c u l t to form i n t o u s e f u l , durableglass o b j e c t s because of t h e i r r a p i d phase s e p a r a t i o n or c r y s t a l l i z a t i o n . However, they hydrate e a s i l y i n an autoclave to c o n t a i n up to 40 weight percent water. These sodium hydros i l i c a t e s are thermoplastic i n behavior, but more importantly the molecular water provides a means f o r d e a l k a l i z i n g the s i l i cate s t r u c t u r e . The o b j e c t i v e o f t h i s paper i s to d e s c r i b e a process which was found to be capable of r e c o n s t i t u t i n g a p o o r l y durable an hydrous a l k a l i g l a s s i n t o a durable high s i l i c a g l a s s w i t h the a i d of molecular water. Except f o r dimensional shrinkage, the s t r u c t u r e r e t a i n e d i t s i n t e g r i t y and shape during the v a r i o u s stages. These i n c l u d e h y d r a t i o n (or dehydration of s i l i c a t e s o l u t i o n ) , d e a l k a l i z a t i o n i n s a l t s o l u t i o n s , d r y i n g and f i r i n g to c o n s o l i d a t e a t temperatures greater than about 1200 C ( F i g u r e 1). 2
Experimental Sample P r e p a r a t i o n . Two methods were used to produce sodium s i l i c a t e g l a s s samples f o r t h i s study. The primary method used c o n v e n t i o n a l g l a s s m e l t i n g techniques to produce compositions ranging from 12 to 21 wt. % Na 0. Batch i n g r e d i e n t s , A f r i c a n sand, sodium carbonate, and sodium n i t r a t e , were melted a t 1600 C f o r s i x hours i n platinum c r u c i b l e s , poured i n t o p a t t i e s and f i n e ground i n t o 1 1/2" diameter d i s c s w i t h t h i c k n e s s of one to four m i l l i m e t e r s . These anhydrous d i s c s were f u l l y hydrated i n a one c u b i c f o o t a u t o c l a v e under saturated steam c o n d i t i o n s and stored i n c o n t r o l l e d r e l a t i v e humidity d e s i c c a t o r s a t room tem perature. The second method f o r sample production c o n s i s t e d of pouring S-35, a P h i l a d e l p h i a Quartz sodium s i l i c a t e s o l u t i o n , i n t o p o l y ethylene molds (1 1/2" diameter) to â depth approximating one m i l l i m e t e r and s t o r i n g i n c o n t r o l l e d r e l a t i v e humidity d e s i c c a t o r s at room temperature. Dehydration of the S-35 s o l u t i o n produced s o l i d g l a s s samples. Thicker S-35 h y d r o s i l i c a t e was produced by dehydration i n an autoclave to avoid wrinkled s u r f a c e s . Various sample water contents were achieved by s p e c i f i c humidity environments produced by using saturated s a l t s o l u t i o n s . R e l a t i v e h u m i d i t i e s from 30 to 75% produced water content ranges from 22 to 35 wt. %. A time p e r i o d approximating a week was needed to reach e q u i l i b r i u m sample weights. Water contents were measured by l o s s - o n - i g n i t i o n techniques. Lack of an anhydrous core confirmed complete sample h y d r a t i o n . 2
D e a l k a l i z a t i o n . The r a t e of d e a l k a l i z a t i o n and c a l c u l a t e d d i f f u s i o n c o e f f i c i e n t s were determined by monitoring the r e l e a s e of Na ions from the hydrated samples i n t o the ammonium n i t r a t e l e a c h s o l u t i o n . A l l d e a l k a l i z a t i o n was accomplished i n a c l o s e d system measured by c a l i b r a t e d pH, r e f e r e n c e , and Na i o n e l e c t r o d e s .
17.
BARTHOLOMEW ET AL.
Reconstituting
Sodium
Silicate
Glass
SODIUM SILICATE SOLUTIONS
SODIUM SILICATEl GLASSES
DEHYDRATE
HYDRATE HYDROSILICATE BODY DEALKALIZE POROUS SILICA BODY
DRY CONSOLIDATE DENSE SILICA GLASS Figure 1.
Reconstitution
process.
279
S O L U B L E SILICATES
280 +
The m i l l i v o l t output from the N a i o n e l e c t r o d e was monitored by a pH meter and r e c o r d e r . The temperature of the l e a c h s o l u t i o n was kept constant through the use of a c i r c u l a t i n g water bath. T h i s i n v e s t i g a t i o n was mainly concerned w i t h the i n f l u e n c e of water content on the r a t e of d e a l k a l i z a t i o n a t 5Q°C and a pH of 8. Further experimentation centered around d e a l k a l i z a t i o n v a r i a b l e s such as temperature, pH, l e a c h s o l u t i o n c o n c e n t r a t i o n and time. T h i s was accomplished by u s i n g a Metrohm End Point/pH Stat T i t r a t o r . The predetermined pH was c o n t r o l l e d by automatic a d d i t i o n of t i t r a n t to the l e a c h s o l u t i o n . C o n s o l i d a t i o n . Once d e a l k a l i z a t i o n was complete, the r e s u l t i n g porous samples were removed from the l e a c h s o l u t i o n , r i n s e d w i t h warm d i s t i l l e d water, and allowed to a i r dry a t room temperature. Other d r y i n g techniques that were used included exchange of methanol f o r water f o r the purpose of d r y i n g w i t h s o l u t i o n s of lower s u r f a c e t e n s i o n . T h i s helped to overcome some c r a c k i n g problems i n the small pore porous bodies by c a p i l l a r y forces. C o n s o l i d a t i o n was done i n tube furnace i n flowing helium at temperatures from 1200°C to 1450°C. Hold times at 800 to 1000°C were used to help d i s p e l water vapor which evolved as s i l a n o l groups on the h i g h s u r f a c e area s i l i c a were combined. Very r a p i d h e a t i n g schedules could produce foamed bodies as water was trapped i n the porous body. R e s u l t s and D i s c u s s i o n H y d r o s i l i c a t e Compositions; Hydration/Dehydration. For the purpose of demonstrating the f e a s i b i l i t y of the r e c o n s t i t u t i o n process the work r e p o r t e d here used o n l y cut d i s c s or p l a t e s of sodium s i l i c a t e g l a s s . I n t h i s way the dimensions of the sample could be e a s i l y c o n t r o l l e d and d e a l k a l i z a t i o n data was more meaningful. In those cases where t h e r m a l l y molded items, such as l e n s shapes, were processed the r e s u l t s were i d e n t i c a l . Table I shows s e v e r a l sodium s i l i c a t e g l a s s d i s c s (2mm χ 38mm d i a . ) that were hydrated at 140°C i n 100% r e l a t i v e humidity i n an a u t o c l a v e . The water contents range from about 21% to 41% f o r g l a s s c o n t a i n i n g 12.4% and 21.4% Na 0, r e s p e c t i v e l y . Hydra t i o n was d i r e c t l y p r o p o r t i o n a l to a l k a l i content and exposure time at 140°C. I t i s a l s o d i r e c t l y p r o p o r t i o n a l to autoclave temperature. 2
17.
Reconstituting
BARTHOLOMEW E T AL.
Hydration
o f Sodium S i l i c a t e TABLE I .
Temp.
Wt/o Na 0** 2
21.2 21.4 18.0 15.5 12.4
Sodium
Silicate
Glass*
Time (hrs)
140°C 140°C 140°C 140°C 140°C
281
Glass
Wt.% H 0 2
31.9 41.2 32.7 32.5 20.9
4 10 5 6 10
* Sample s i z e - 2mm χ 38mm d i a . **Remaining composition was S i 0 « 2
D e a l k a l i z a t i o n . Various s a l t s o l u t i o n s were tested f o r de a l k a l i z a t i o n o f h y d r o s i l i c a t e bodies. The s a l t s somewhat b u f f e r e d the s o l u t i o n s and a l s o retarded d i s s o l u t i o n o f the hydrosilicate. Sodium n i t r a t e s o l u t i o n s d e s p i t e the common sodium i o n were capable o f reducing the a l k a l i l e v e l i n the sample by about 50% i n s e v e r a l hours. The most e f f e c t i v e and l e a s t contaminating s a l t was Ν Η ^ Ν 0 . Exposure of 2mm χ 38mm d i s c c o n t a i n i n g 21 wt/o Na 0 to two baths o f 0.6M NHi*N0 s o l u t i o n over a 24 hour p e r i o d at pH 7 reduced the r e s i d u a l a l k a l i to about 100 ppm. Several bath changes over a short p e r i o d were p r e f e r a b l e to exposure o f the sample to a s i n g l e bath f o r long times. The e f f e c t s o f water content and pH o f the d e a l k a l i z a t i o n solution(0,6M NH^N0 ) on sodium i o n r e l e a s e a r e shown i n F i g u r e 2. A t constant water content, lower pH (7 vs 8) always r e s u l t s i n f a s t e r removal of sodium i o n from the h y d r o s i l i c a t e . Likewise, at constant pH, h y d r o s i l i c a t e s with highest water contents r e l e a s e sodium i o n a t a f a s t e r r a t e . In a d i s c u s s i o n o f d i f f u s i o n k i n e t i c s f o r d e a l k a l i z a t i o n of N a 0 - S i 0 - H 0 glasses i t appears best to d e s c r i b e i t as an i o n exchange r e a c t i o n . 3
2
3
3
2
2
2
S i -0 Na n(H 0) glass +
2
The
+
f- H 0 £=Si-0H (n + l - y ) H 0 + Na (H 0)y (1) glass solution solution 3
e q u i l i b r i u m constant
2
Κ i s given by:
glass a Ξ Si-OH(hydrous) glass solution
Κ =
A
ΛΤ
Na
+
a
2
H 0
solution Na
+
(2)
+
3
For the s o l u t i o n the reference s t a t e i s chosen such that l i m § + •+ 1 and s i m i l a r l y f o r H 0 , where $ i s the a c t i v i t y c o efficient. The reference s t a t e f o r the s o l i d exchange i s that i n +
N a
3
SOLUBLE
282
SILICATES
Figure 2. Effect of water content and solution pH on dealkalization rate. Key for % H O and pH: O, 32.4%, 7; A, 34.2%, 8; •, 24.4%, 7; and Φ, 24.4%, 8. g
17.
Reconstituting
BARTHOLOMEW ET AL.
Sodium
Silicate
Glass
283
which a l l o f the exchangeable c a t i o n s a r e of the i o n i n q u e s t i o n . The f a c t that the g l a s s p r e f e r s to e x i s t i n the d e a l k a l i z e d state when placed i n ΝΗ^Νθ3 s o l u t i o n (pH = 8), i . e . the e q u i l i b r i u m described i n (1) l i e s f a r to the r i g h t , i n d i c a t e s a very l a r g e v a l u e f o r K. Thermodynamic a n a l y s i s was not pursued because o f t h i s h i g h s e l e c t i v i t y o f the matrix f o r protons compared to sodium. D i f f u s i o n l i m i t s f o r the exchange process described i n equa t i o n (1) can be a r r i v e d a t u s i n g the f o l l o w i n g assumptions. There i s no d i s s o l u t i o n o f the S i 0 network (experimentally v e r i f i e d ) . The f l u x e s of the two i n t e r d i f f u s i n g species a r e equal i n magni tude because o f the e l e c t r o n e u t r a l i t y requirement. The more mobil i o n i s slowed down by e l e c t r i c f i e l d p o t e n t i a l s , while the slow i o n i s a c c e l e r a t e d . D i f f u s i o n i s u n i d i r e c t i o n a l and the sample i s s e m i - i n f i n i t e i n the d i r e c t i o n p a r a l l e l to the d i r e c t i o n o f d i f f u sion. The f o l l o w i n g boundary c o n d i t i o n s hold f o r the d i f f u s i o n o f sodium ions out o f the g l a s s : 2
C - C , χ > 0, t = 0 ο
(3)
C - 0, χ = 0, t > 0.
(4)
S o l u t i o n of F i c k ' s Law dc _ d^c dt dx
(5)
z
+
y i e l d s the r e s u l t that M (g. equiv. N a ) , the t o t a l amount of d i f f u s i n g substance which has l e f t the g l a s s a t time t i s given by, M = 2C · A(^~) (6) t
1
1
t
/
/
2
2
Q
2 -1 where D i s the mean i n t e r d i f f u s i o n c o e f f i c i e n t (cm sec ) , t i s time (sec), C c o n c e n t r a t i o n o f N a ions per u n i t volume i n i t i a l l y i n the g l a s s a t t = 0 (expressed i n e q u i v a l e n t N a cm 3) and A i s the area. The mean i n t e r d i f f u s i o n c o e f f i c i e n t i s i n dependent o f c o n c e n t r a t i o n o f the exchanging i o n s . I f concentra t i o n dependence o f the i n t e r d i f f u s i o n c o e f f i c i e n t e x i s t s , then the v a l u e f o r D obtained from equation (6) i s i n r e a l i t y the mean i n t e g r a l i n t e r d i f f u s i o n c o e f f i c i e n t (10). A plot of versus l / 2 ( F i g s 2 and 3) should be l i n e a r with a slope of 2C . A (£)l/2 from which D can r e a d i l y be c a l c u l a t e d . The q u a n t i t y C i s obtained from, +
Q
+
t
u r e
C
- 2 (lOO-x)yp
(7)
M
^ N a 0 where χ i s wt % H 0 i n the g l a s s , y i s the wt % Na 0 i n the g l a s s (dry b a s i s ) , ρ i s the d e n s i t y of the hydrated g l a s s and M is the molecular weight o f Na 0. The mean i n t e g r a l i n t e r d i f f u s i o n c o e f f i c i e n t obtained f o r three d i f f e r e n t Na 0 l e v e l s i n the s t a r t i n g compositions a r e p l o t t e d as a f u n c t i o n o f i n i t i a l water content i n the g l a s s 2
2
2
fJa
2
2
Q
284
SOLUBLE
15
20
25
%H 0 2
Figure
3.
(IN
30
35
40
HYDROSILICATE)
Effect of water and soda content on diffusion coefficient. Να 0: X , 15.5%; Δ , 18.0%;and 0,21.2%. 2
SILICATES
Key for
%
17.
BARTHOLOMEW ET AL.
Reconstituting
Sodium
Silicate
Glass
285
(Figure 3). I t i s obvious from these data that the mean i n t e g r a l i n t e r d i f f u s i o n c o e f f i c i e n t , D, increases with i n c r e a s i n g water content and w i t h decreasing Na 0 content of the g l a s s . The ex p l a n a t i o n f o r the water content case can be explained on the b a s i s of a more expanded s t r u c t u r e allowing increased d i f f u s i o n r a t e . However, i t i s not c l e a r why the d i f f u s i o n should be slower as a l k a l i content increases unless i t i s r e l a t e d to some minimum water requirement f o r N a t r a n s p o r t , or phase s e p a r a t i o n . i i Porous Structures and C o n s o l i d a t i o n . The s i l i c a body ob t a i n e d a f t e r d e a l k a l i z a t i o n was found to vary d r a m a t i c a l l y i n pore s t r u c t u r e . The soda content of the o r i g i n a l h y d r o s i l i c a t e body as w e l l as d e a l k a l i z a t i o n c o n d i t i o n s a f f e c t e d the f i n a l porous s t r u c ture* F i g u r e 4 shows SEM's of the c e n t r a l cores of s t r u c t u r e s ob tained by d e a l k a l i z i n g h y d r o s i l i c a t e s c o n t a i n i n g from 16 to 21% Na 0. A f t e r d e a l k a l i z i n g them at 80 C f o r 24 hours, the pore s i z e was d i r e c t l y p r o p o r t i o n a l to the o r i g i n a l Na 0 content. More im p o r t a n t l y , the pore d i s t r i b u t i o n was very broad i n the higher a l k a l i c o n t a i n i n g bodies. High d e a l k a l i z a t i o n temperatures were found to c o n t r i b u t e to poor pore s i z e d i s t r i b u t i o n s and l a r g e i n t e r n a l pore s i z e s . F i g u r e 5 shows SEM scans of the p o r o s i t y of d e a l k a l i z e d bodies from t h e i r surface to t h e i r c e n t r a l core. The o r i g i n a l h y d r o s i l i c a t e had 18% Na 0 and was d e a l k a l i z e d i n 0.6M, ΝΗι>Ν0 . These p i c t u r e s (5a) show that 24 hours a t 80 C creates a broad gradient i n pore s i z e across the s t r u c t u r e ' s c r o s s s e c t i o n . On the other hand, one hour at 80 C followed by 23 hours at 50 C produces a more uniform pore s t r u c t u r e (5b) i n the same h y d r o s i l i c a t e composition. When bodies with pore s t r u c t u r e s s i m i l a r to Figures 5a were c o n s o l i d a t e d a t 1200 C or higher a dense g l a s s with a porous cen t r a l core was obtained (Figure 6a,b). However, when a sample s i m i l a r to Figures 5b was c o n s o l i d a t e d a transparent dense high s i l i c a g l a s s was obtained. (Figure 6c). 2
+
2
2
2
3
Conclusions A process was developed that i s capable of transforming p o o r l y durable sodium s i l i c a t e glasses (12 to 21% Na 0) i n t o dur able high s i l i c a g l a s s e s . Sodium h y d r o s i l i c a t e s w i t h up to 40% H 0 were prepared by e i t h e r dehydrating commercial sodium s i l i c a t e s o l u t i o n s or by a u t o c l a v i n g anhydrous glasses of comparable com p o s i t i o n s . Regardless o f t h e i r o r i g i n , d e a l k a l i z a t i o n k i n e t i c s of these h y d r o s i l i c a t e s favored high water content and lower a l k a l i content. Uniform pore d i s t r i b u t i o n i n d e a l k a l i z e d s t r u c t u r e s was r e quired f o r a t t a i n i n g dense transparent g l a s s on c o n s o l i d a t i o n at > 1200°C. H y d r o s i l i c a t e s w i t h Na 0 between 15 and 18 wt/o and de a l k a l i z e d a t 50° to 60°C i n 0.6 ΝΗι*Ν0 s o l u t i o n s were most l i k e l y to have the d e s i r e d pore s t r u c t u r e s . High s i l i c a , low expansion g l a s s up to 4mm t h i c k was obtained by t h i s approach. Shape and s i z e l i m i t a t i o n s may e x i s t . 2
2
2
3
286
Figure
S O L U B L E SILICATES
4.
Porosity
as a junction of alkali content in hydrosilicate. Na O: a, 16%; b, 18%;and c,21%. z
Key for
%
BARTHOLOMEW ET AL.
Reconstituting
Sodium
Silicate
Glass
287
Ε
f
l a
§4?
So
Il si
1
SOLUBLE
288
Figure
6.
SILICATES
Consolidated silica glass. Key: a, cross section with opaque core; magnified porous layer; and c, transparent glass body.
b,
17. BARTHOLOMEW ET AL.
Reconstituting Sodium Silicate Glass
Literature Cited 1.
Schloze, H.; Glass Ind. 1966, 47, 546, 622, 670.
2.
Boulos, Ε. N.; Kreidl, N. J., J. Can. Ceram. Soc. 1972, 41, 83.
3.
Ernsberger, F. M.; J. Am. Ceram. Soc. 1977, 60, 91.
4.
Bartholomew, R. F.; Butler, B. L.; Hoover, H. L.; Wu, C. Κ., J. Am. Ceram. Soc. 1980, 63, 481.
5.
Bartholomew, R. F.; Schreurs, J.W.H., J. Non-Crystall. Solids, 1980, 38/39, 679.
6.
Bartholomew, R. F.; Tick, P. Α.; Stookey, S. D., J. NonCrystall. Solids, 1980, 38/39, 637.
7.
Moriya, Y.; Nogami, M., J. Non-Crystall. Solids, 1980, 38/39, 667
8.
Takata, M.; Tomozawa, M., J. Am. Ceram. Soc. 1980, 63, 710.
9.
Wu, C. K., J. Non-Crystall. Solids, 1980, 41, 381.
10. 11.
Garfinkel, H. M. "Membranes, A Series of Advances:, Eisenman, G., Ed., Marcel Dekker, Ν. Y., 1972, Vol. 1, p. 199.
Doremus, R. Η., "Glass Science," John Wiley and Sons, N.Y., Ν. Y. 1973. RECEIVED March 2, 1982.
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