(414f) Using a High Temperature, High Pressure Couette Viscometer to Assess Krytox® Oils As a Deepwater Viscosity Standard

Perfluorinated polypropylene oxide oils are particularly attractive HTHPVS candidates because they are commercially available, clear, non-flammable, non-volatile, thermally stable, inert liquids.  DuPont manufactures the Krytox® series of polydisperse perfluorpolyether oils, which have the chemical formula CF₃CF₂CF₂O-(CFCF₃CF₂O)_n-CF₂CF₃, 9<n<60.  The series, in order of increasing molecular weight, includes Krytox® 101, 102, 104, and 107.  Viscosity measurements of these liquids were conducted with a new Chandler Couette viscometer rated to 600 ^oF and 40,000 psia, and can be used for fluids with viscosity values of ~3 – 300 cp.  The Krytox® fluids were tested at temperatures up to 260 ^oC and pressures up to 40,000 psia, over an rpm range of 50-600.  Each oil exhibited shear-thinning characteristics in the 50 – 300 rpm range, but the viscosity value remained relatively constant in the 300 – 600 rpm range.  Krytox® 101 exhibited a viscosity of ~17 cp at 260 ^oC and 241 MPa (510 – 1021 shear (1/s)), while the Krytox® 102 exhibited a viscosity of ~25 cp at 260 ^oC and 241 MPa (510 – 1021 shear (1/s)).  Krytox® density was also measured in a high temperature, high pressure, and windowed, variable volume densimeter.

It is anticipated that blends of Krytox 101 and 102 will be capable of meeting the desired viscosity value of 20 cp. If a more monodisperse viscosity standard is desired, either the 101 or 102 Krytox® oil can be fractionated with supercritical CO₂.  This will result in a series of fractions that cover a wide range of viscosity values greater than and less than the parent oil viscosity.  The fractionation conditions of the parent oil could possibly be tailored to yield a fraction that exhibits a viscosity of 20 cp at 260 ^oC and 241 MPa.