Physical-Chemical Variables in Processing Na 2 O - B 2 O 3 - SiO 2 Gel Monoliths
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PHYSICAL-CHEMICAL VARIABLES IN PROCESSING Na2 0 - B2 03 - SiO 2 GEL MONOLITHS Gerard Orcel and Larry L. Hench Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 USA ABSTRACT The addition of a dryinq control chemical additive to the solvent allows short drying times for (DCCA) The variables involved in the use monolithic gel samples. of a DCCA in preparing gels from the Na2 0-SiO2 -B 20 3 system Due to its higher reactivity, a DCCA are described. influences the chemical behavior of the material at each step of the processing. The effects of formamide as a DCCA on the qelation process, surface charge, and surface area are presented. The variations of the physical properties of the gel, due to the addition of the DCCA, are related to the Although the DCCA reactions of the formamide with the sol. helps in producing monolithic dried gels, the problem of densification is not solved as yet. INTRODUCTION It has been recently described that the addition of a Drying Control Chemical Additive (DCCA) to the solvent allows short drying procedures during the preparation of gels in different chemical systems such as: HowSi0 2, Si0 2 - Li2O, Si0 2 - Na2 0, Si0 2 - B2 03 - Na2O and A12 0 3 [1-41. ever, fully dried monolithic gels are more difficult to obtain when the In addition, the mechanisms number of constituents of the gel increases. responsible for the general influence of a DCCA is not fully understood as Our purpose in this pape(r is to increase the understanding of the yet. DCCA approach by studying the effect of formamide as a drying control chemical additive on the chemical and physical properties of a gel in the ternary system Si0
2
- B 2 0 3 - Na2 0.
EXPERIMENTAL PROCEDURE The composition studied was: 42 mole % SiO 2 - 30 mole % B2 03 - 28 mole % Na2 0. The procedure followed for the manufacture of the qel is shown schematically in Fig. 1. The total amount of water added corresponds to The gels the stoichiometric quantity for the hydrolysis of the alkoxides. were cast in Pyrex@ glass vials, and aged at 60°C for 24 hours in closed The drying process involved a combination of heating treatcontainers. ments under regular air atmosphere and in approximately I Torr of vacuum. RESULTS AND DISCUSSION The chemical reactions written as follows:
which occur during the gelation process can be
Si(OC 2 H5 ) 4 + 4 H2 0 + Si(OH)x(OC 2 H5 )y + xC2 H5 0H + y H2 0
(1)
B(OCH 3 ) 3 + 3 H2 0
+ B(OH)u(OCH 3 )v + u CH3 0H + v H2 0
(2)
NaOCH 3 + H2 0
+ Na+ + OH- + CH3 0H
(3)
Hat. Res. Soc.
Syp.
Proc.
Vol.
32 (1984)
Published by Elsevier Science Publishing Co.,
Hydrolysis
Inc.
80
=-Si-OR'
+ R"O-Si=- +
-Si-O-Si- + R'OR"
(4)
-Si-OR'
+ R"'O-B=
+
-Si-O-B= + R'OR"'
(5) Polycondensation
=B-OR"'
+ R""O-B=
+
=B-O-B= + R" 'OR"...
(6)
with x + y =4 and u + v = 3
The rate of these reactions (1-6), and subsequently the structural characteristics of the resulting gel, are influenced by the experimental conditions such as: temperature, pH, nature and concentration of the reactants [6-91
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