DRYING METHODS PRESERVING THE TEXTURAL PROPERTIES OF GELS

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DRYING METHODS PRESERVING THE TEXTURAL PROPERTIES OF GELS

G. Pajonk

To cite this version:

G. Pajonk. DRYING METHODS PRESERVING THE TEXTURAL PROPERTIES OF GELS. Jour-

nal de Physique Colloques, 1989, 50 (C4), pp.C4-13-C4-22. �10.1051/jphyscol:1989403�. �jpa-00229478�

REVUE DE PHYSIQUE APPLIQUÉE

Colloque C4, Supplément au n°4, Tome 24, Avril 1989 C4-13

DRYING METHODS PRESERVING THE TEXTURAL PROPERTIES OF GELS

G.M. PAJONK

Université Claude Bernard Lyon I, Laboratoire de Thermodynamique et Cinétique Chimiques, CNRS UR-231, 43 bd du 11 novembre 1918, F-69622 Villeurbanne Cedex, France

Résumé - Il est possible de préserver plus ou moins la texture des gels au cours du séchage grâce à trois procédés principaux qui diffèrent en gros par le domaine des températures et des pressions qui les caractérise. Dans l'ordre croissant des températures et des pressions on peut distinguer : la cryodessication qui permet d'obtenir des cryogels (basses températures et pressions) - le séchage classique conduisant aux xérogels (températures et pressions habituelles ou faibles) et enfin le séchage dans les conditions supercritiques qui donne les aérogels (températures et pressions élevées).

Abstract - To preserve more or less the gels textural properties during their drying step three main methods can be applied which differ roughly by the temperature and pressure range in which they are conducted. In increasing temperatures and pressures ranking one may distinguish : i) the freeze drying process giving "cryogels" (low pressures and temperatures), ii) the ordinary drying method leading to xerogels (ordinary or low pressures and temperatures) and finally, iii) the supercritical drying method producing aerogels (high pressures and temperatures).

1 - INTRODUCTION

The sol-gel technique has become a widely used method in order to- obtain highly divided solids (powders, monoliths, microspheres, etc..) having textural properties very well developed (surface area, porous volume, etc...). The dried gel is generally a precursor such as in the fabrication of glasses, catalysts, ceramics, adsorbents, composites whose properties are essential with respect to the final desired product /l, 2, 3, 4/.

As a gel is usually regarded as a semi-solid having immobilized the solvent (mostly water) in a network of fine capillaries the drying conditions (nature of solvent, rate of heating or method of evacuation of the liquid part, duration, final temperature, etc..) are important factors that influence the texture of the dried solid (generally named xerogel after the greek word for dried).

Figure 1 depicts the sol-gel transformation leading to a hydrogel for instance /l, 3/.

Fig. 1 - a) sol, b) gel.

Now in order to preserve the porous texture of the wet gel surface tension forces developed by the liquid-vapour interface menisci in the fine capillaries (or pores) must be counterbalanced if shrinkage is to be avoided. This shrinkage process is at the heart of the drying problem and the following methods described below give instructions how to "escape"

pore collapse and surface area decrease phenomena taking into account that in this paper hydrothermal treatments modifying for instance surface areas (or particle size) will not be described.

A general picture of the drying step of a wet gel into a dried one is given on figures 2 for a ceramic body and 3 for clay as examples /5/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989403

Crltlcal molsture content

I

I

^0.O6

15

~ r y ware

0 10 20 30 40 50

Moisture content (%)

F i g . 2

-

R a t e of d r y i n g and d r y i n g s h r i n k a g e f o r a c e r a m i c body. At a c r i t i c a l m o i s t u r e c o n t e n t which c o r r e s p o n d s t o t h e s o l i d p a r t i c l e s coming i n t o c o n t a c t , t h e r a t e of d r y i n g b e g i n s t o d e c r e a s e and t h e s h r i n k a g e s t o p s .

I t c a n b e s e e n h e r e t h a t t h e s h r i n k a g e i s a l m o s t completed i n t h e c o n s t a n t r a t e d r y i n g p e r i o d ( f i g u r e s 2 and 3 ) . A t t h e c r i t i c a l p o i n t t h e r a t e b e g i n s t o d e c r e a s e c o n t i n u o u s l y . The f i r s t r a t e p e r i o d c o r r e s p o n d s t o t h e r a t e of e v a p o r a t i o n of " f r e e " w a t e r f i l m s w h i l e t h e d e c r e a s i n g r a t e p e r i o d b e l o n g s t o w a t e r b e i n g removed from t h e i n t e r i o r of t h e body t o t h e s u r f a c e . A more d e t a i l e d comment. i s g i v e n below when t h e c a s e of s i l i c a g e l i s c o n s i d e r e d ( s e e f i g u r e 6 ) .

F i g . 3

-

Drying p r o c e s s f o r a c l a y body showing ( a ) wet body, ( b ) c r i t i c a l p o i n t and ( c ) d r y ware.

A good r e p r e s e n t a t i o n of how t h e s u r f a c e t e n s i o n f o r c e s a r e a c t i n g on t h e p o r e w a l l s i s g i v e n i n F i g u r e 4

111.

Fig. 4

-

The f o r c e s t h a t c a u s e s i l i c a g e l t o s h r i n k d u r i n g d r y i n g r e s e m b l e t h o s e t h a t c a u s e wet p l a t e s of g l a s s t o draw t o g e t h e r .

F o r i n s t a n c e i f t h e p o r z w i d t h i s of t h e o r d e r of 1.5 nm t h e c a p i l l a r y compression e x e r t e d w i l l be e q u i v a l e n t t o 10 p s i and h i g h e r f o r s m a l l e r p o r e s .

F i n a l l y a s s i l i c a g e l s a r e one of t h e most f r e q u e n t g e l s F i g u r e 5 d e s c r i b e s t h e hydrogel-xerogel t r a n s f o r m a t i o n i n d e t a i l s / I / .

Fig. 5

-

Evaporating f i l m of s i l i c a s o l t o g e l and drying-schematic c r o s s s e c t i o n . ( a ) Sol ; ( b ) c o n c e n t r a t e d sol-beginning of a g g r e g a t i o n ; ( c ) g e l compressed by s u r f a c e t e n s i o n ; (d) f r a c t u r i n g of g e l by shrinkage ; ( e ) d r i e d l o o s e g e l fragments. W = water s u r f a c e , S = s o l i d s u b s t r a t e .

A complete s e t of d a t a concerning t h e d r y i n g of a hydrogel i s r e p r e s e n t e d i n F i g u r e 6 f o r s i l i c a and can be g e n e r a l i z e d f o r o t h e r wet g e l s a s w e l l / 6 / . I n t h i s f i g u r e s t e p I ( c o n s t a n t r a t e p e r i o d ) i s r e l a t e d t o t h e d e c r e a s e of volume of t h e " s t i l l " hydrogel while s t e p I1 shows t h e e f f e c t of c a p i l l a r y compressive f o r c e s counterbalanced by t h e r e s i s t a n c e of t h e a c t u a l framework of t h e g e l . Then s t e p 111 corresponding t o t h e d e c r e a s i n g d r y i n g r a t e occurs now without a p p r e c i a b l e volume change and i s accompanied by t h e h y s t e r e s i s e x h i b i t e d by t h e a d s o r p t i o n - d e s o r p t i o n isotherm of water by t h e g e l . T h i s means t h a t water i s removed from t h e pores, t h e l a r g e r ones being emptied f i r s t . The s m a l l e r t h e pores t h e slower t h e d r y i n g r a t e . F i n a l l y s t e p I V r e p r e s e n t s e v a p o r a t i o n of water from c o n t a c t p o i n t s between t h e s i l i c a p a r t i c l e s .

decrease

Op

wahr r ~ t e n t

-

Fig. 6

-

Drying s t e p s and t h e formation of t h e framework of x e r o g e l s , dW/dt = d r y i n g r a t e , A l l l o = r e l a t i v e deformation, P / P ~ = r e l a t i v e vapor p r e s s u r e , F = c o n t r a c t i o n f o r c e .

C4-16 REVUE DE PHYSIQUE APPLIQUEE

The pore volume and pore r a d i u s m o d i f i c a t i o n s of a g e l d u r i n g t h e d r y i n g s t e p a r e summarized on f i g u r e 7 / 3 / .

Fig. 7

-

V : porous volume, S = s u r f a c e a r e a , R = 2 V / S = 2 t g a = p o r e r a d i i

~ ~ c o r r e s ~ o n d s t o t h e h y d r o g e l , B c o r r e s p o n d s ?o t h e d r i e d g e l .

A s t h e most encountered way of d r y i n g g i v e s x e r o g e l , t h e o b t e n t i o n of x e r o g e l s w i l l be t r e a t e d f i r s t , followed by a e r o g e l s and f i n a l l y by c r y o g e l s .

2

-

^XEROGELS

According t o Barby 1 7 1 t h e d r y i n g method of g e l s c a n have f i v e major i n f l u e n c e s .

F i r s t when w a t e r i s p r e s e n t t h e hydrogel can be t o some e x t e n t h y d r o t h e r m a l l y t r e a t e d (steaming) and i t c a n l o o s e s u r f a c e a r e a and i t i n c r e a s e s i t s pore volume a s p i c t u r e d i n f i g u r e 8 f o r s i l i c a .

8 0 0 '

Thus i t i s p o s s i b l e now t o d i s t i n g u i s h between "hot" and "cold" d r y i n g under t h e s e c o n d i t i o n s . For i n s t a n c e l o w e r i n g t h e d r y i n g t e m p e r a t u r e impedes g e l breakage, t h i s r e s u l t c a n be a l s o o b t a i n e d by i n c r e a s i n g t h e r e l a t i v e humidity o r r e d u c i n g t h e a i r - f l o w a c r o s s t h e g e l / 2 / . Ternan e t a 1 / 8 / p r e p a r e d h i g h l y porous alumina x e r o g e l s u s i n g t h e low t e m p e r a t u r e d r y i n g method w h i l e when d r y i n g a t h i g h e r t e m p e r a t u r e s (T

>

333 K) p o r e c o l l a p s e occured due t o p r e s s u r e and t e m p e r a t u r e e f f e c t s . They concluded t h a t t h e low d r y i n g t e m p e r a t u r e i n f l u e n c e i s t o remove w a t e r from t h e l a r g e pores.

700

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Steaming o c c u r s when t h e w a t e r v a p o r i s n o t removed from t h e d r y i n g system a s f a s t a s i t i s g e n e r a t e d . I t h a s been observed t h a t i f t h i s p r o c e s s b u i l d s up i n t h e c o n s t a n t r a t e s t e p t h e n t h e s i l i c a g e l e x h i b i t s l a r g e s u r f a c e a r e a and pore volume. Now i f steaming proceeds d u r i n g t h e d e c r e a s i n g r a t e s t e p t h e n t h e s u r f a c e a r e a a s w e l l a s t h e pore volume d e c r e a s e w h i l e c r a c k s developed i n t h e g e l /2/.

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393 K r e s u l t . i n a d e c r e a s e of p o r e volume w i t h d e c r e a s i n g p r e d r y i n g t e m p e r a t u r e s but does n o t v a r y t h e s u r f a c e a r e a and t h e hydroxyl c o n t e n t

191.

R e s u l t s a r e shown on t a b l e 1 and f i g u r e 9.

F i g . 8

-

Drying h y d r o g e l s 30020 40 60 80 100

Woter 1%)

pH of f o r m a t i o n P r e d r y i n g temp. 9ET P o r e volume OH c o n t e n t i n X

i n "C 3

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FCg. 9

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N 2 a d s o r p t i o n i s o t h e r m s .

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I. S i l i c a p r e p a r e d a t pH = 3.2 p r e d r i e d a t 20°C 11. S i l i c a p r e p a r e d a t pH = 3.2 n o p r e d r y i n g 111. S i l i c a p r e p a r e d a t pH = 7 p r e d r i e d a t 20°C I V . S i l i c a p r e p a r e d a t pH = 7 no p r e d r y i n g .

S e c o n d l y t h e r a t e of d r y i n g i s a n i m p o r t a n t p a r a m e t e r b e c a u s e i t can a f f e c t t h e p a r t i c l e s i z e t o a g r e a t e x t e n t 121. I n p a r t i c u l a r i n o r d e r t o p r e p a r e m o n o l i t h s c o n s i d e r a b l e d r y i n g t i m e s a r e n e c e s s a r y 1101. D r y i n g t i m e s may v a r y from h o u r s t o weeks a s i n t h e c a s e of l e a d - t i t a n a t e m o n o l i t h 1 1 1 1 a t 307 K o r months a s f o r T i 0 m o n o l i t h 1121 a t room t e m p e r a t u r e .

2

I t i s w e l l known a t l e a s t f o r s i l i c a g e l s t h a t s t e p I of f i g u r e 6 d e t e r m i n e s t h e t o t a l p o r o s i t y o f t h e x e r o g e l w h i l e s t e p I1 imposes t h e p o r e s i z e d i s t r i b u t i o n . The s u r f a c e a r e a i s a f u n c t i o n o f s t e p I11 1131. Thus by a r a p i d d r y i n g of t h e g e l d u r i n g s t e p I and a s m a l l e r one d u r i n g s t e p I1 i t i s p o s s i b l e t o p r e p a r e bimodal p o r o u s s i l i c a g e l s .

I n g e n e r a l a r a p i d d r y i n g i n c r e a s e s b o t h p o r e volume and p o r e s i z e d i s t r i b u t i o n ( p o r e s i z e d i s t r i b u t i o n s a r e broadened). T h i s i s a l s o c o n f i r m e d f o r s i l i c a g e l s made from a l k o x i d e s 1121-

T h i r d l y i t h a s been o b s e r v e d t h a t i f t h e d r y i n g r a t e i s s u c h t h a t w a t e r i s e v a p o r a t e d more r a p i d l y t h a n i t c a n be removed t h e n t h e r e i s a d e c r e a s e of g e l s k r i n k a g e 1 2 , 71.

F o u r t h l y i t i s w e l l a g r e e d t h a t t h e d e g r e e of s h r i n k a g e i s a f u n c t i o n o f t h e s u r f a c e t e n s i o n p r o p e r t i e s of t h e s o l v e n t . The s m a l l e r t h e l i q u i d s u r f a c e t e n s i o n t h e l o w e r t h e d e g r e e of s h r i n k a g e 1 2 , 71. T h i s remark o p e n s t h e f i e l d o f nonaqueous d r y i n g . Water may be r e p l a c e d by a n o r g a n i c l i q u i d o f lower s u r f a c e t e n s i o n o r by u s i n g t h e same o r g a n i c s o l v e n t i n s t e a d of w a t e r f o r t h e g e l a t i o n s t e p . A l c o h o l s a r e commonly u s e d i n t h i s a p p l i c a t i o n .

A z e o l i t i c t e x t u r e t y p e of alumina was p r e p a r e d t h r o u g h t h e s o l - g e l method u s i n g methanol a s s o l v e n t and t h e a l c o g e l was d r i e d a t room t e m p e r a t u r e and under vacuum / 1 4 / . T h i s t y p e of alumina e x h i b i t e d a uniquf m i c r o p o r o u s t e x t u r e (Langmuir N 2 i s o t h e r m f o r i n s t a n c e ) and a v e r y h i g h s u r f a c e a r e a & 600 m / g ) .

C4-18 REVUE DE PHYSIQUE APPLIQUEE

An i n t e r e s t i n g d r y i n g p r c e s s which c a n be r e l a t e d t o non aqueous d r y i n g i n v o l v i n g DCCA ( d r y i n g c o n t r o l c h e m i c a l a d d i t i v e ) i s d e s c r i b e d i n t h e p a p e r of Wang and Hench / 1 5 / . A d d i t i o n of formamide i n t h e Na 0-Si02 s o l p r e p a r a t i o n p r e v e n t s f r a c t u r e of t h e x e r o g e l . I t i s b e l i e v e d t h a t t h e amine g r o u p s b u i l d up hydrogen bonds between t h e h y d r o x i d e g r o u p s o f t h e 2 s i l i c a p a r t i c l e s u r f a c e t h u s improving t h e s t r e n g t h of t h e g e l . The main r e s u l t i s t h a t w i t h DCCA t h e c r i t i c a l s h r i n k a g e r a t e ( l e a d i n g t o c r a k s when overcome) i s s u b s t a n t i a l l y h i g h e r t h a n w i t h o u t t h u s a l l o w i n g h i g h e r d r y i n g r a t e s and s u b s e q u e n t l y s h o r t e r d r y i n g t i m e s w i t h o u t c r a c k i n g e f f e c t s .

U r a n i a s p h e r e s U02 were a l s o p r e p a r e d by i n t e r n a l g e l a t i o n and d r i e d a c c o r d i n g two ways : s t e a m o r a i r d r y i n g . The r e s u l t i n g x e r o g e l s p h e r e s showed d e n s i t i e s d e p e n d i n g upon t h e d r y i n g c o n d i t i o n s / 1 6 / . F o r ejxample s t e a m d r y i n g a t 373 K gave m i c r o s p h e r e s w i t h d e n s i t i e s v a r y i n g between 9.7 t o 1 g/cm w h i l e a i r d r y i n g a t 293 K l e d t o d e n s i t i e s comprised between 1.3 t o 1.6 g/cm

.

3

-

^AEROGELS

To p r e s e r v e a t most t h e o r i g i n a l t e x t u r e of a wet g e l t h e s u r f a c e t e n s i o n f o r c e s which d e v e l o p i n t h e p o r e s when t h e s o l v e n t i s e v a c u a t e d g i v i n g a l i q u i d - v a p o r meniscus must be c o m p l e t e l y e l i m i n a t e d d u r i n g t h e d r y i n g s t a g e . T h i s i s a c h i e v e d by s u p e r c r i t i c a l d r y i n g which g i v e s " a e r o g e l s " i n s t e a d of common x e r o g e l s . D e t a i l s c a n be found i n r e f e r e n c e / 1 7 / . Water a s s o l v e n t must be d i s c a r d e d b e c a u s e due t o i t s h i g h c r i t i c a l c o n s t a n t s (T = 647 K , P = 218 b a r s ) t h e g e l s a r e a l w a y s r e c r i s t a l l i z e d showing poor t e x t u r a l p r o p e r t i e s f Water i s u&d o n l y a s a r e a c t a n t p a r t n e r i n t h e s o l s t e p . G e n e r a l l y a l c o h o l s a r e p r e f e r e d a s s o l v e n t s o r CO / 1 8 / (T = 304 K , P = 73 b a r s ) . The s u p e r c r i t i c a l d r y i n g method i s t h e b e s t way up t o now t o 2 p r e p a r e C m o n o l i t h s w t t h o u t c r a c k s and of l a r g e dimensions. Again h e r e t h e h e a t i n g r a t e of t h e a u t o c l a v e and t h e v e n t i n g r a t e a r e i m p o r t a n t f a c t o r s c o n t r o l l i n g t h e q u a l i t y of t h e a e r o g e l s / 1 9 / . A t y p i c a l s u p e r c r i t i c a l p r o c e d u r e i s d e s c r i b e d on f i g u r e 10.

timelhours

b I

phase 1 2 3 L

F i g . 10

-

A u t o c l a v e c y c l e w i t h t h e f o u r p h a s e s : 1 = h e a t i n g , 2 = e q u i l i b r i u m , 3 = v a p o u r o u t l e t , 4 = c o o l i n g .

Improvment of t h e a e r o g e l d r y i n g method have been c l a i m e d by Mulder and van L i e r o p who a p p l i e d a p r e p r e s s u r e of 80 b a r s of N 2 p r i o r t o t h e h e a t i n g s t e p of t h e a u t o c l a v e / 2 0 / i n o r d e r t o a v o i d s h r i n k a g e . T a b l e 2 shows t h e i n f l u e n c e of t h e p r e s s u r e upon t h e s h r i n k a g e . Moreover t h i s l a s t method d o e s no more r e q u i r e t o i n t r o d u c e e x t r a a l c o h o l i n t h e a u t o c l a v e t o m a i n t a i n t h e s a t u r a t i o n p r e s s u r e o f t h e l i q u i d ( h e r e e t h a n o l f o r i n s t a n c e ) and i t s h o r t e n s t h e t i m e of t h e whole p r o c e d u r e a s i t c a n be s e e n on f i g u r e 11.

N p r e s s u r e S p e c i f i c d c j n s i t y

( b a r ) (g/cm )

S h r i n k a g e ( X )

73 ( c r a c k e d ) 52

7 0

T a b l e 2

-

I n i t i a l N P r e s s u r e and C o r r e s p o n d i n g D e n s i t y and S h r i n k a g e of a Gel 2

pressure

100

20 1w 200 2W 3W0C temperature

Fig. 1 1

-

(a) Pressure of ethanol from room temperature to 300°C.

(b) Temperature and total pressure during a drying process, starting with 80 bar N and a heating rate of 50e/h. After reaching 300°C decompression follows an3 finally the vessel is cooled down.

Other fluids can be used in supercritical drying such as the Freons 13, 23 or 116 but they are more expensive than CH O H , C H OH or C02.

3 2 5

The supercritical drying method gives large size monoliths and very large pore volume aerogels under the form of monoliths, powders or granules and the general rules given for the xerogels also apply for aerogels.

4

-

CRYOGELS (OR FREEZE-DRIED GELS)

The same reason of choosing the supercritical drying method can be involved in this process which consists to freeze the solvent (mainly water) and then to sublimate it under low pressure. Again the direct solid-vapour transformation can be achieved without the formation of a meniscus in the pores. The term cryogel is proposed by the author to distinguish the so dried gels from aerogels, though the resulting solids should be a priori comparable.

Figure 12 shows the pressure-temperature diagram for water. In this picture AB represents the curve for subliming ice. (Note that C is the criatical point for water). Now when a salt is dissolved in water the pressure temperature diagram for the system is represented on figure 13. The freezing point of the system is depressed as is the water vapour pressure. At saturation an invariant point Q exists where four phases are in equilibrium : ice, anhydrous salt, saturated salt solution and water pressure. Q is often called the cryohydric point

/21/. At pressures below this point water can be removed by sublimation until the anhydrous

salt remains in a freeze-dryer as shown on figure 14 schematically.

I

C

Steam

I

Temperature. ' C

Fig. 12

-

Pressure-temperature diagram for water.

-12 0

TfMFERATURE P C )

Fig. 13

-

Temperature-pressure relations in a salt-water system. The dotted lines pertain to pure water (or solvent).

C4-20 REVUE DE PHYSIQUE APPLIQUEE

The main drawback of t h i s d r y i n g method i s t h e time r e q u i r e d t o s u b l i m a t e t h e i c e ( o r t h e f r o z e n s o l v e n t ) . So h e a t i s s u p p l i e d from an e x t e r n a l s o u r c e t o a c c e l e r a t e ( w i t h o u t m e l t i n g ) t h e s u b l i m a t i o n s t e p , f o r i n s t a n c e v i a an i n f r a r e d lamp 1221. I t was shown t h a t t h e time r e q u i r e d t o sublime 100 g of i c e was reduced from 40 h t o 4 h when h e a t was s u p p l i e d by a i-r lamp, a l l o t h e r c o n d i t i o n s being t h e same /23/.

I t has been checked t h a t c r y o g e l s such a s L i doped NiS04 / 2 2 / , aluminium s u l f a t e / 2 4 / , ammonium p a r a t u n g s t a t e 1251 a r e more developed from t h e t e x t u r a l p o i n t of view t h a n t h e i r c o r r e s p o n d i n g x e r o g e l s .

VACUUM

E

^c=3

REFRlGERANl c=:>

CONDENSER-

OR REFRIGERANT +-,

H E A T C3 F i g . 14

-

Schematic diagram of a f r e e z e dryer.

The main p a r a m e t e r s c o n t r o l l i n g t h e t e x t u r a l p r o p e r t i e s of t h e f i n a l c r y o g e l seem t o be t h e r a t e of f r e e z i n g , t h e t e m p e r a t u r e of f r e e z i n g and t h e n a t u r e of t h e s o l v e n t (which a l s o d e t e r m i n e s t h e r a t e of s u b l i m a t i o n ) .

F o r i n s t a n c e i f a s i l i c a g e l i s f r o z e n v e r y q u i c k l y an e x t r e m e l y porous honeycomb m a t e r i a l c a n be o b t a i n e d , i f t h e f r e e z i n g r a t e of t h e g e l i s lowered chunks of porous s i l i c a c r y o g e l a r e p r e p a r e d a s d e s c r i b e d by Mahler and Chowdhry /26/.

The t e m p e r a t u r e s of f r e e z i n g a s w e l l a s t h e d r y i n g t e m p e r a t u r e s seemed t o e x e r t an i n f l u e n c e on t h e BET a r e a of samples of pulpwood f i b r e s a c c o r d i n g Swanson /27/. T a b l e 3 , shows t h a t f o r a c o n s t a n t d r y i n g t e m p e r a t u r e d e c r e a s i n g t h e f r e e z i n g t e m p e r a t u r e r e s u l t s i n i n c r e a s i n g t h e s u r f a c e a r e a , t h e same e f f e c t a p p l i e s a l s o when f o r a c o n s t a n t f r e e z i n g t e m p e r a t u r e t h e d r y i n g t e m p e r a t u r e i s decreased.

F r e e z i n g t e m p e r a t u r e "C Drying t e m p e r a t u r e "C BET a r e a (m2 g-l)

T a b l e 3

-

S u r f a c e a r e a s of f i b r e s d r i e d by s u b l i m a t i o n of water

F i n a l l y Beyer e t a l . 1241 observed t h a t t h e r a t e s of subliming i c e i n aluminium s u l f a t e - i c e system gave d i f f e r e n t r e s u l t s . F o r i n s t a n c e u s i n g t h e l o n g d u r a t i o n s u b l i m a t i o n (70 up t o 122 h) l e d t o a m i x t u r e of amorphous and c r i s t a l l i z e d phases w h i l e u s i n g a n i n f r a - r e d lamp t o a c c e l e r a t e t h e s u b l i m a t i o n (33 up t o 49 h) of t h e same m i x t u r e gave a q u i t e complete amorphous c r y o g e l . Due t o p r o c e s s e s of o v u l a t i o n o r s p h e r o i d i z a t i o n o c c u r i n g when w a t e r s e p a r a t e s i n t o c h a n n e l s 1241 a m a c r o p o r o s i t y developed i n b o t h t y p e s of c r y o g e l s d u r i n g s u b l i m a t i o n . A d i f f e r e n c e was however observed : t h e high r a t e sublimed c r y o g e l showed s m a l l e r a v e r a g e pore d i a w t e r , p a r t i c l e s i z e and s u r f a c e a r e a t h a n t h e o t h e r one ( w i t h o u t h e a t s u p p l i e d by t h e i-r lamp).

A comparison of d r y i n g a mixed o x i d e NiO-A1 0 e i t h e r by t h e s u p e r c r i t i c a l p r o c e s s g i v i n g a n .2 3

a e r o g e l o r by f r e e z e - d r y i n g i s d i s c u s s e d I n paper no 23 i n t h i s Symposium 1281. The main d i f f e r e n c e between t h e two g e l s i s i n r e l a t i o n w i t h t h e i r r e s p e c t i v e macropore volumes.

Two o t h e r d r y i n g modes a r e worthy t o be mentionned here.

Spray d r y i n g i s widely used t o o b t a i n microspheres of x e r o g e l . The p a r t i c l e s i z e depends upon t h e d r y i n g r a t e among o t h e r parameters. Spray d r y i n g i s c o n s i d e r e d a s a f a s t d r y i n g method.

Freeze-thawing 1261 i s a n i n t e r e s t i n g d r y i n g p r o c e s s l e a d i n g t o f i b r i l l a r g e l s d e v e l o p i n g p o r o s i t y and h i g h s u r f a c e a r e a s s u c h a s s i l i c a and z i r c o n i a f i b e r s . T h i s morphology seems t o be due t o t h e i c e s t r u c t u r e p l a y i n g t h e r o l e of a mould. As t h e s o l v e n t i s n o t sublimed h e r e s u r f a c e t e n s i o n f o r c e s b u i l d up d u r i n g t h e thawing s t e p , t h e r e f o r e i t i s advantageous t o r e p l a c e w a t e r by a lower s u r f a c e t e n s i o n l i q u i d such a s a c e t o n e i n o r d e r t o develop t h e t o t a l p o r o s i t y of t h e f i n a l g e l . F i g u r e 15 d e m o n s t r a t e s t h e e f f e c t of s u b s t i t u t i n g a c e t o n e t o w a t e r on t h e t o t a l p o r o s i t y a s w e l l a s on t h e p o r e s i z e d i s t r i b u t i o n .

Fig.

0.035

-

0 839-

- 6

8.825-

-

J

2

^0.028-

15

-

E f f e c t of washing medium on p o r e s i z e d i s t r i b u t i o n of freeze-thawed s i l i c a . 6

-

CONCLUSIONS

-

-

' L " l " " l ' . "

(bl

To summarize t h e many d r y i n g methods surveyed i n t h i s paper i t i s convenient t o keep i n mind p r i m a r i l y what i s t o be avoided i n t h e f i n a l g e l r a t h e r t h a n what p r o p e r t i e s a r e expected because t h e s e c h a r a c t e r i s t i c s depend on t h e t h r e e i m p o r t a n t p r e v i o u s s t a g e s of t h e s o l - g e l , p r o c e s s p r i n c i p a l l y : h y d r o l y s i s , c o n d e n s a t i o n and ageing.

- -

-

F

e . e , G

3

0 >

e.O'Oj

.

I n o t h e r words c a r e f u l d r y i n g p r o c e s s e s a r e t o be used i n o r d e r t o p r e s e r v e t h e g e n e r a l t e x t u r e of t h e wet g e l . I n p a r t i c u l a r i f c r a c k s a n d / o r s h r i n k a g e a r e n o t wanted a g e n e r a l r u l e seems t o a p p l y whatever t h e p r e c i s e d r y i n g method t o be f i n a l l y choosen : if w a t e r i s p r e s e n t a s a s o l v e n t i t may be i m p o r t a n t t o r e p l a c e i t by a 1,ower s u r f a c e t e n s i o n medium.

Moreover combination of w a t e r (even a s a r e a c t a n t i f i n e x c e s s ) and h i g h t e m p e r a t u r e s a r e q u i t e always d e t r i m e n t a l towards t h e t e x t u r a l p r o p e r t i e s of t h e d r i e d g e l w i t h r e s p e c t t o t h o s e c h a r a c t e r i z i n g t h e s t i l l soaked one, a t l e a s t f o r s i l i c a 1291.

-

" ' . l ' . L . l " . '

g ^r -

-

.

^{" ' 1}

-

' . ' .

Acetone

Wash

-

,I,# ULDO. 288. 388. 4BB.

' " .

sea. 4e'a

. ' * . I

F

AIT-

PORE D I A E T E R ( A)

C4-22 REVUE DE PHYSIQUE APPLIQUEE

I n 1972 R.B. Keey w r o t e i n h i s book / 51 "The word d r y i n g i s commonly u s e d t o d e s c r i b e any p r o c e s s i n which w a t e r ( o r s o l v e n t ) i s removed from a s u b s t a n c e " and adds he : "Drying i s a commonly p r a c t i s e d a r t but a n e g l e c t e d science".

The s o l - g e l p r o c e s s which i s now i n c r e a s i n g l y developed t o f a b r i c a t e h i g h t e c h n o l o g y m a t e r i a l s h a s been a n i n c e n t i v e t o l o o k a t t h e d r y i n g s t e p i n more d e t a i l s s i n c e though not c o n s i d e r e d a s a c e n t r a l o r key s t e p i t may w e l l d e t e r m i n e t h e f i n a l p r o p e r t i e s e x h i b i t e d by t h e end m a t e r i a l .

REFERENCES

/ I / I l e r , R.K. i n "The Chemistry of S i l i c a " , J o h n Wiley, New-York 1979.

/ 2 / W i n y a l l , M.E. i n "Applied I n d u s t r i a l C a t a l y s i s " Bruce E. Leach Ed., Academic P r e s s , New-York, Vol. 3 , 1984.

/ 3 / Le P a g e , J.F. i n " C a t a l y s e de c o n t a c t " Ed. Technip P a r i s , 1978.

/ 4 / " U l t r a s t r u c t u r e P r o c e s s i n g o f Ceramics G l a s s e s and Composites" L.L. Hench and D.R. U l r i c h Eds, J o h n Wiley, New-York, 1984.

/ 5 / Kingery, W.D. i n " I n t r o d u c t i o n t o Ceramics" J o h n Wiley, New-York, 1960 and Keey R.B. i n "Drying, P r i n c i p l e s and P r a c t i c e " Pergamon P r e s s , New-York 1975.

/ 6 / Boreskov, G.K. i n " P r e p a r a t i o n of C a t a l y s t s " , B. Delmon, P.A. J a c o b s and G. P o n c e l e t Eds., E l s e v i e r , Amsterdam, 1976.

/ 7 / Barby, D. i n " C h a r a c t e r i z a t i o n of Powder S u r f a c e " , G.D. P a r f i t t and K.S.W. S i n g E d s , Academic P r e s s , New-York, 1976.

Ternan, M., Packwood, R.H., Buchanan, R.M. and P a r s o n s , B.I., Can. J. Chem. Eng., (1982) 33.

191 Okkerse, C. and De Boer, J.H., J . Chimie P h y s i q u e ,

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/11/ Gurkovich, S.R. and Blum, J . B . i n op c i t , r e f . 4 , p. 152.

/12/ P r a s s a s , M. and Hench, L.L. i n op c i t r e f . 4 , p. 100.

1131 Fenelonov, V.A., G a v r i l o v , V. Yu and Simonova, L.G. i n " P r e p a r a t i o n of c a t a l y s t s III", G. P o n c e l e t , P. Grange and B. Delmon Eds., E l s e v i e r , Amsterdam, 1983, 665.

1141 T o u r n i e r , G., L a c r o i x - R e p e l l i n , M., Pajonk, G.M. and T e i c h n e r , S .J. i n " P r e p a r a t i o n of c a t a l y s t s I V " , B. Delmon, P. Grange and G. P o n c e l e t Eds., E l s e v i e r , Amsterdam 1987, 333.

1151 Wang, S.H. and Hench, L.L. i n Mat. Res. Soc. Symp. P r o c e e d , v o l . 32, 71, North H o l l a n d , New-York, 1984.

1161 Haas, P.H., Begovich, J . M . , Ryon, A.D. and Vavruska, J.S., Ind. Eng. Chem. Prod. Res.

Dev.,

2

(1980) 461.

1171 T e i c h n e r , S.J. i n "Aerogels", p. 22, J. F r i c k e Ed., S p r i n g e r V e r l a g , B e r l i n , 1986.

1181 Tewari, P.H., Hunt, A.J. and L o f f t u s , K.D. i n op c i t r e f . 17, p. 31.

/19/ Henning, S. i n o p c i t r e f . 1 7 , p. 38.

/20/ Mulder, C.A.M. and Van L i e r o p , J.G. i n o p c i t r e f . 1 7 , p. 68.

1211 S c h n e t t l e r , F.J., Monforte, F.R. and Rhodes, W.W. i n "Science of Ceramics" Vol. 4. The B r i t i s h Ceramic S o c i e t y , S t o k e on T r e n t , 1968, p. 79.

1221 K e l l y , J., H i b b e r t , D.B. and Tseung, A.C.C., J . Mat. S c i . , (1978) 1053.

1231 H i b b e r t , D.B. and Tseung, C.C., J. Mat. S c i . ,

16

(1979) 2665.

/24/ Beyer, L.A., K a l n a s , C.E., Roy, D. and Lloyd, I.K., Am. Ceram. Soc. B u l l . ,

6

⁽¹⁹⁸⁷⁾

1647.

1251 S a l e , F.R. i n "Fine P a r t i c l e s " , W.E. Kuhn and J. Ehretsmann Eds. The E l e c t r o c h e m i c a l S o c i e t y , P r i n c e t o n , 1974, p. 283.

/26/ Mahler, W. and Chowdhry, U. i n op c i t r e f . 4 , p. 207.

/27/ Swanson, J.W. i n " C h a r a c t e r i z a t i o n of porous s o l i d s " , S.J. Gregg, K.S.W. S i n g and H.F.

S t o e c k l i Eds, S o c i e t y of Chemical I n d u s t r y London, 1979, p. 339.

1281 Klvana, D . , Chaouki, J., R e p e l l i n - L a c r o i x , M. and P a j o n k , G.M., 2nd I n t e r n a t i o n a l Symposium on A e r o g e l s ISA 2

-

M o n t p e l l i e r 21st-23rd s e p t . 1988.

1291 G a v r i l o v , V. Yu, Karnaukhov, A.P. and Fenelonov, V.B., Kinet. i K a t a l . ,

19

(1978) 1549, ( e n g l i s h t r a n s l a t i o n ) .