August, 1962
THERMAL DEHYDRATION OF PORQUS GLASS
1517
CHANGES I N LENGTH AND INFHARED TRANSMITTANCE DURING TI-IERXfAL DEHYDRATIOK OF POROUS GLASS AT TEIkIPERATURES UP TO 1200~ BY THOXAS H. ELMER, IAN D. CHAPMAN, AND MARTIN E. KORDBERO Research d? Development Division,Corning Glass Worlcs, Corning, New York Received March 3.4, I963
Infrared spectra and length changes were measured as a plate of orous glass was heated in vacuo fromroom temperature to 1200'. Shrinkage is shown to be due to rearrangement of the si?ica network brought about by reaction between silanol groups on the surface to form strong Si-0-Si bridges and water, The elimination of water on heating is shown by infrared measurements to occur at temperatures up to that corresponding to complete consolidation. Changes in intensity and assignment of the infrared bands observed during degassing and rewetting of the porous glass are discussed.
Introduction The numerous publications dealing with the surface chemistry of silica gel and porous glass show thai there exists considerable interest in these materials Investigations have shown that these materials contain appreciable amounts of silanol (hydroxyl) groups on the surfaces. I n recent studies porous glass has been used successfully as adsorbent for studying changes in infrared spectra of physically adsorbed molecules. Folman and Yates' also included water in their adsorption studies. The thermal dehydration of porous glass up to 4001' has been investigated by Kiselev and Lygin.2 Little and Mathieu3 also studied the thermal dehydration of porous glass. It was felt valuable I O use infrared spectroscopy to provide information during the course of dehydration, together with length change measurements up to consolidation of the porous glass to see if correlation exists between length change and loss of water from the porous glass. Experimental Samples.--The porous glass used in this study was in the form of thin plates, 5.5 cm. X 1.5 cm. X 0.25 mm. Porous glass is an intermediate glass obtained by heat treating and leaching a special soft alkali borosilicate One phase, whkh is rich in boric oxide and alkali, is leached out to leave a porods high-silica structure. The composition of porous glass on the basis of ignited weight is 96% SiOz, 3% %Oa, 0.5% R203 ROZ (chiefly A1203 and ZrOz), and traces of NasO and As203. Infrared Measurements.-The porous glass plate W ~ B inserted in a cell containing a fused auartz window. The cell was connected to a mecKanica1 punip which could evacuate the system to a pressure of 0.2 mm. The cell was placed inside a Vycor Brand Code 7913 glass tube which protruded from an elec1,ric furnace capable of attaining 1200". Temperatures were measured MTith a Chromel-Alumel tlrermocouple. All infrared spectra were recorded at room temperature with a Perkin-Elmer Model 21 infrared spectrophotometer with sodium chloride optics. Length Measurements.-The length changes obscrved during heating were obtained by measuring with a travelling Imcroscope the distance between two index marks, 3 cm. apart, on the surface of the plate in the cell under vacuum. All measurements were made after the porous glass had cooled t o room temperature. The length measurement for the wet sample involved placing the plate in a Petri dish partially filled with distilled water to obtain a "wet" reading. Procedure.--The cell containing the porous glass plate
+
(1) M. Folman and D. J. C. Yates, Trans. Faraday Soc., 54, Part 11, 1884 (1958). (2) A. V. Kiselev and V. I. Lygin, Proc. Second Intern. Congr. Surface Aclzvity, 2 , 204 (1957). ( 3 ) L. H. Little and M. V. Mathieu, Actea Congr. Intern Catalyse, b", Paris, 1, 771 (l9GO). (4) M. E. Noidberg, J . Am. Ceram. SOC.,27,299 (1944).
was placed in the furnace a t the predetermined temperature and was pumped for 1 hr. (longer in some cases). The cell then was sealed by means of a glass stopcock, removed from the furnace and vacuum attachment, and allowed to cool to room temperature. After making both infrared and length change measurements the cell again was placed in the furnace and pumped at the next higher temperature. The procedure was repeated until the porous glass was completely consolidated.
Results and Discussion Length Change Measurements.-The distance between two points on the plate was measured after it first had been immersed in water, after both air drying and pumping a t room temperature, and after pumping while being heated at 100' intervals from 100 to 1200'. The results are shown graphically in Fig. 1. The 0.4% decrease in length between AB is a phenomenon commonly observed during drying of porous materials, particularly in the ceramic field. It has been studied recently by Amberg and McIntosh5 and Folman and Yates.l The former two investigators6 point out that contraction occurs on desorbing water from a porous glass rod initia,lly in an 87% relative humidity atmosphere due first to a deepening of the menisci formed in the pores and the decreasing spreading force of the adsorbed film in filled pores, the increasing value of the negative pressure as reversible desorption takes place, and finally the increase in surface free energy as desorption takes place from the walls of the pores. The destruction of concave menisci results in a loss of capillary forces and causes a slight expansion which is observed by an increase iii length of the porous glass rod, but the iiet process is a contraction which Amberg and ktcIntosh report to be about 0.2%. It is riot clcarly known whet her all the physically adsorbed u atcr is removed even at 200' (see Infrared Measuremclits) and it, may well be that the small contractions observed up to 200' are due to the filial stages of thc above process as the last traces of physically adsorbed water are removed. Above 200' other factors must cause the small but steady contraction that occurs up t o about 900'. Younge has found that above ea. 180' silanol groups on a silica gel start to condense to form silica-oxygen bonds and water. Wolf and Beyer7 point out that DeBoer and c o - ~ o r k e r s , ~ ~ ~ ( 5 ) C. H. Amberg and
P.. McIntosh, Can. J . Chem., 30, 1012 (1952). (6) G. J. Young, J . Colloid See., 13,67 (1958). (7) F. Wolf and H. Beyer, 2.anorg. allgem. Chem., 300,33 (1959). (8) J. H. DeBoer, Aneew. Chem., 70,389 (1958). (9) J. H. DeBoer, M. E. H. Hermana, and J. M. Vleeskens, Proe. Koninkl. Ned. Akad. Wetensohap., B60,45,54 (1957).
surface of the porous glass. Wolf and Beyer' point out that at very high temperatures the loss in water fvom silica gel is not large enough to explain the rapid decrease in internal surface area and pore volume and suggest the following reaction scheme to explain this.
'
'
'l'he fact that we have observed littlc change in the Lcwater absorption bands in relatively thick porous
glass measured both after 900' heating and after plunging into a furnace above 1100' to rapidly Fig. 1.--Length of porous glass at room temperature as a consolidate the glass leads one to conclude that the hirrction of successive heating in vacua at temperatures sintering reaction under these conditions probably f r o m 25 to 1200". Warpage of the sample above 1100" is due chiefly to joining of the silica surfaces as caused point C to be low in value. It normally would indicated by reaction 111, described by Wolf and be expected to fall on the dotted line. Beyer. Infrared Measurements.-Infrared spectra mere measured between 1.2 and 5.0 p. The sharp absorption band observed at around 2.7 p is due to stretching vibrations of relatively unassociated OH groups. Overtones of the fundamental 0-H vibration band also were observed in the region of 1.4 and 2.2 p. Absorption bands at 3.64 and 3.95 p are due to boric oxide in the structure of the glass. The main band of interest is the band between 2.7 and 3.0 p. The broadness of this band is a common phenomenon in OH systems that are strongly associated by hydrogen bonding. The asymmetric decrease in the width of the band as conwlidation proceeds is one of the significant observat ions in this work. This phenomenon generally is thought to be due to condensation reactions between silanol groiips or remoral of physically adsorbed water. P'ig 2 'I'hm-rial dehydration rcJactions. Youngb points out that all of t h e truly physical \ v t m + M ork T\ ith silica gr4 agrws with that of adsorbed water is removed from the silica siirfarr liohlschiit ter and Kampf,lo showed that the ther- hy mwuatioii at room temperaturr. BPnesi and ~ ' workcd with silica gel, agree with mal dehydration of silanol surfaces can be repre- J ~ u e s , mho scnted by two different rractions as illustrated Young and state that adsorbed water is completely schematically in Fig. 2 . According to reaction I removed by evacuation at room temperature within 1wo surface Si-OH groups 011 a particle can react the limits of their means of detection. Brnesi and with each other to form an Si-OSi bridge and Jones base their conclusion upon the appearance water. In this reaction both the pore volume and and disappearance of a strong absorption band at, internal surface area remain constant, but the sur- about 6.1 p due to a v2 fundamental for water when face has changed chemically, becoming more an evacuated film of grade 950 silica gel was exhydrophobic. Reaction I1 (Fig. 2) involves the posed to n-ater vapor and then vacuum pumped. dehydration between two Si-OH groups on surfaces It was not possible with our cell and with the thickbelonging to different particles. It results in a ness of the plate used to follow the infrared transdecrease in internal surface area (sintering). De mittance curve beyond 4.8 p and hence this obserBoer and eo-workers showed thal, the siloxane vation could not be confirmed for porous glass in groups formed during dehydration reaction I can this work. However, Little and mat hie^,^ who be hydrated with water to silanol groups, but those used a thinner sample and CaFz optics, note the presence of a band a t 6.1 p which decreases sharply produced by reaction 11cannot be broken. bbove MOO' the shrinkage process is greatly in intensity up to 200'. Furthermore, from studies accelerated. At these high temperatures the in this and other laboratories, activation temperasilanol concentration in t'he porous plate also de- tures of about 180' are needed to obtain maximum creases markedly as will be shown later, and this water vapor adsorption on porous glass at low relaconfirms that shrinkage can be accounted for by tire pressures. It is difficult to see why this should rearrangement of the silica structure due t o con- be so if all the physically adsorbed water has been delisation reactions between silanol groups on the driven off at room temperature and no condensation reaction is postulated below 180'. Also, lhe (10) 11. \T'. ~ < O l I l h d r u t t ? l alld c. Kaillpf, z.QnO1U. a k p C'I CnL., 2.5;
L
200
I
I
I
I
400 600 TEMPERATURE
1
800
1
I
1000
I
I 1200
OC.
ilk
292, 298 (1957)
(11) 13.-4.B m m and 4.V. Jones, J . Phfls. Chem., 63, 170 (10.59).
August, 1962
THERMaL
bands in the region around 2 8 p are decreaaing in width below 200' and above room temperature, indicating a loss of water from one source or another from within the porous glass. Therefme, one can conclude that physically- adsorbed water is present in porous glass at temperatures up to 200'. Due to scattering, which seemed to be unavoidable in the case of the silica gel used by Benesi and Jones, satisfactory observations in the region of 6.1 p w r e not possible, and as Miller1* points out, due to lhp great difference in absorbancy index for the stretching and bending vibrzltiow in water moled e s , amounts of water too small to shorn a noticeable band at about 6.1 p will cause an appreciable ubsorpiion a i 2.8-2.9 p . The condwisation reaction between adjacent silanol groups on the surface will occur first betwrcn closest neighbors which hence are also the most hydrogeu-bonded species. Hence, as the lemperature is raised, the band decreases asymmetrically in width as association decreases. This is clearly shown in the spectra taken following heat treatments up to 400' (Fig. 3A). Above this temperature both the peak height and the width of the band decrease, the latier in a fashion not explained on the basis of the decrease in peak height. 'The peak also is displaced from 2.7 l o 2.64 fi as the plate is heated from 300 to 1000" (Fig. 3B). The band disspprars completely at above 1000' leaving only the background caused by the fused silica cell. I n the case of additional plates which were heated rapidly ta consolidation temperatures mithout thorough degassing the residual peaks observed after consolidation were all a t 2.73 p. The displacement of the peak to higher wave lengths obscrved in the final consolidated glass probably is chic to modification of the stretching vibrations of OH groups by the closer packing of oxygeiis in the glass striicture. It would, of course, have been predicted from the study by Rundle and Parasol13 of 0-H stretching vibrations and bond distances. The 2.73 F band also is present in fused quarte14 and fused silica. The effect of heating at 800" followed by the admitting of water vapor at p = 4.6 mm. a t room (miperuture is shown in Fig. 4. The pPak at 2.67 p mows to a higher vave length and eventually becomes lost as water is physically absorbed and complex hydrogen bonding occnrs. The diminution in the intensity of the so-called OHfreeband on adsorption of water reported by Sidorov'6 for porous glass heated at 300, 540, and 650' and reacted with water vapor al p = 16 mm. at room temperature was not observed. Instead it was found that the intensity of the OH peak increased as the sample was exposed to water, as sholyn in Fig. 4. This increase can he explained by the breaking- of ovpgeti bonds to form sjlariol groups as per reactioii I it) li'ig. 2. The effect of prolonged hratiiig of allotller plat(. of porous glass. also 0.25 mm. thick. indicated (12) J.
1559
DBIIYDRATION OF POROUS GLASS
G,MiLler J Phys Chem. 65, 800 (19611
(13) R E. Rundle and XI Parasol J C h m Phus 20, 1487 (1982). dlld R . 'c\. Dougla.; J S u r (:loas T e c h , 45, 117 T
(14) R . \ . -\dams ( 1I 1 3 1 ) .
(15) G..J. Paikoi, C o ~ n i n gUlssb \ V o i I q plkvatc ouniiuuiiieattoa. (16) A. N. Sidoroir, Opt Spect? VI11 [ G I , 424 (1960).
901
WAVELENGTH
p.
Fig. 3.-Infrared spectra at room temperature of porous glass affer successive heating in occcuo at temperatures from 25 to 1200": 1, empty cell; 2, water saturated; 3, 25'; 4, 100"; 5, 200'; G , 300"; 7, 400'; 8, 500"; 9. GOO"; 10, 700"; 11, 800"; 12, 900"; 13, 1000O; 14, 1100 and 1200".
- 0
I
Fig. 4.-Iiifrared spc 1 a t room tempc~returch c t w w n ~ n t :3l fi of porous glass Jrcwtwl to 800' a t i d tLxposed l o e r vapor tit pressure 4.6 n i t 1 1 for t h r times iridicated
that there is not a great deal of difference between a I-, 8-. and 24-hr. bake in the amount of water driven off. This indicates that there is a finite amount of a1er \vhich l n r c i i ~ o i dat :t givrii teinpcruturc aiid that a dy stair is achicycd ill less than 1 hr.
