Mechanical and Microstructural Properties of Two-Step Acid-Base Catalyzed Silica Gels
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EXPERIMENTAL PROCEDURE The precursors used in preparing the sols were Si(OC 2H 5 )4 , C2H 5 OH, and deionized H2 0, in the molar ratio 1:2:2. Concentrated HC1 and NH 3 solutions were used to provide the catalysts for the hydrolysis and polycondensation reactions. The concentrations of the catalysts are reported herein as relative molar amounts. A standard sol mixing procedure was used throughout this investigation. First, the Si(OC 2 H 5 )4 and ethanol were mixed thoroughly. Half of the required amount of water was mixed with HC1 and added to the first solution. After the desired time of hydrolysis, the remaining water was mixed with NH 3 and added to the sol, and mixing was continued for ten minutes. Sol batch size varied depending on the parameters of the experiment. For experiments in which aging time and hydrolysis time were varied for a given composition, a large sol batch was prepared (1250 g). When the catalyst amounts were varied, smaller (250 g) sols were prepared for each catalyst composition. Sols were cast into polymethylpentene cylinders for gelation. Initial gel dimensions were 23.0 mm in diameter and about 125 mm in length. All mixing and gelation were conducted at room temperature (20 °C), except as noted. Gels were aged at room temperature for two days, then heated over 24 hours to 70 OC and held at 70 OC for up to 30 days. All aging times are reported as the time from the point of gelation. A standard three point bend test method [10] was used to measure the modulus of rupture (MOR) and shear modulus (G) of the wet gels. The gels were immersed in ethanol during the measurements. After aging, the diameter of the gels varied between 20.5 and 22.0 mm. The distance between the test supports was 76.2 mm, thus the span/diameter ratio was between 3.5 and 3.7. Since the smallest ratio recommended by the ASTM standard is 4.4 [10], the data for G presented here can be used to compare different specimens, but may not represent the true shear moduli of the gels. Deflection vs. load data were taken at a constant crosshead speed of 1 mm/min, using an Instron® testing machine with a 100 N load cell. Each data point for MOR or G represents an arithmetical average of 2 - 5 test specimens. For microstructural analysis, wet gels were dried to crack-free monolithic xerogels by pinhole drying in an oven at 70 - 78 OC. The specific (BET) surface area, pore size distribution, and pore volume of the xerogels were determined by standard nitrogen adsorption-desorption techniques at the boiling point of nitrogen [11]. RESULTS The modulus of rupture was calculated from the beam bending data using the equation [10]: MOR
8Lrw 3
(1)
-d
where Lr is the load at rupture (N), w is the distance between the supports (mm), and d is the diameter of the rod (mm). Since the fluid in the pores prevents any volume change in the rod during deformation, the gel is in a state of pure shear [ 12]. Thus the shear modulus rather than the elastic modulus is found from the data: 3
G
-
4Lw 4 91ryd
(2)
Here, L is the load (N) at a selected
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