Viscosity-Temperature Properties of Alkyl Silicates

Abstract

N ORDER to determine the influence of the structure of the I alkyl and siloxane chain on the viscosity-temperature properties, the viscosity indices and relative viscosity-temperature numbers were measured for both alkyl orthosilicates and alkyl polysilicates. ALKYL ORTHOSILICATESA variety of alkyl orthosilicates has been synthesized in these laboratories for examination as potential lubricants, and heat and power transfer liquids. Of practical importance throughout the wide liquid range of these materials are the viscosity-temperature characteristics and the effects of structure upon the rate of change of viscosity with temperature. This paper presents an evaluation based upon viscosity-temperature number (V.T.N.) and relative viscosity-temperature number (R.V.T.N.) as described by Sanderson (2,s) in a study of the viscosity-temperature span occurring between two arbitrarily chosen viscosity limits.Thus, in application of this method a liquid of higher viscositytemperature number has a flatter viscosity-temperature curve, where liquids of similar viscosities are being examined. The relative viscosity-temperature number classifies the shape and position of the curve with respect to temperature, and is adequate for comparison of liquids of different viscosities. By means of relative viscosity-temperature number each liquid is compared with the member, of corresponding viscosity a t a chosen temperature, of a homologous series of hypothetical standards. CLASSIFICATION O F ALKYL ORTHOSILICATESThe viscosity-temperature charts of the American Society for Testing Materials were used as a means of determining viscositytemperature numbers, choosing 5 and 100 centistokes as viscosity limits. The alkyl silicates were found generally to give straight line plots of viscosity versus temperature over wide ranges of temperature on the ASTM charts.At the start of this examination an assignment was made of alkyl orthosilicates to definite homologous series dependent upon alkyl group structure. This was accomplished by a plot of viscosity-temperature number against the log of kinematic viscosity a t a fixed temperature. These data are presented in Figure 1. The linear correspondence observed for the homologous series of n-alkyl orthosilicates (curve A ) appears also to include the 2-methylpropyl, 3-methylbutyl, and 2-ethylbutyl derivatives. As a result the entire group may be described as symmetrical, primary alkyl orthosilicates. Curves B and C have been designated as unsymmetrical, primary alkyl, and secondary alkyl orthosilicates, respectively.

Silicate Esters and Related Compounds

“…As the backbone becomes larger, its influence on the slope overshadows the effect of the alkyl groups, as can be seen in Table IV. These results are in agreement with, and extend, the findings of Morgan and Olds (8).…”

“…They are also indebted to the U. S. Air Force, Wright Air Development Division, for financial assistance under Contract AF 33(038)-9831 with part of the work. Tetra (1-hexoxy) (8) 25 Tetra(l-octoxy) (8) 30 Tetra(2-methyl-l-butoxy) (9) 88 Tetra(2-ethyl-l-butoxy) (9) 75 T etra (2-ethyl-1 -hexoxy) Tri(fert-butoxy)silanol (14) >300…”

Preparation and Comparison of the Physical Properties of Alkyl and Alkforyl Silicates.

“…The melting points of the first two homologs were determined using the magnetic plunger method described by Stock (4,6). The melting point for the trimer determined by this method was found to be slightly lower than the value first reported by Seel and Langer (5); however, the capillary tube method gave a slightly higher value than that reported by the same investigators.…”

“…From Figures 1 and 2, it can be seen that all lines are parallel and flat, reflecting the low viscosity temperature n210°F. coefficient (VTC) of organosiliccn fluids VTC-1----nlOO F. (5,12). For liquids of the same viscosity at 77 °F.…”