Comparison of the Dimensional Stability of Kel-F 81 and Neoflon CTFE M400H Polychlorotrifluoroethylenes Used in Valve Seat Applications

Abstract

The dimensional stability of polychlorotrifluoroethylene (PCTFE) valve seats used in gas cylinder regulator applications was evaluated by thermomechanical analysis (TMA). Testing focused on two commercial grades of PCTFE, Kel-F® 81 and Neoflon® CTFE M400H, and on actual PCTFE valve seats obtained from different manufacturers. The effects of resin grade, percent crystallinity, and process history on TMA deflection were evaluated. TMA results frequently showed low-temperature dimensional instability between 40 and 70 °C. The corresponding permanent height change resulting from brief cyclic heating of as-received material to 150 °C ranged from to +3.9 to -8.5 percent. Complementary differential scanning calorimetry data indicated the origin of the dimensional instability to be anomalous relaxation at or close to the glass transition. The data are discussed in the context of several proposed valve seat failure mechanisms: contaminant promotion, flow resonance, and flow friction. The combined data show significant property variations within the PCTFE resin family. Such property variations could have important implications for the use of PCTFE in valve seat applications.

Effect of Molding and Machining on Neoflon CTFE M400H Polychlorotrifluoroethylene Rod Stock and Valve Seat Properties

“…Previously published data [1] shows considerable variation in the permanent height change as measured by TMA caused by thermal cycling of several varieties of PCTFE, including valve seats taken from oxygen and corrosive gas service, as well as molded bar stock and sheet. These materials exhibited permanent expansions as high as +3.9 percent and permanent contractions as low as -8.5 percent following repetitive thermal cycling between -20 °C (-4 °F) and 150 °C (300 °F).…”

“…were sectioned. Other details about specimen preparation appear elsewhere [1]. Dimensional stability was evaluated by cycling each specimen three times between -20 and 150 °C, followed by measuring the permanent height change at 0 °C.…”

“…However, gas purity analysis showed that uncombusted, residual oxygen was within specification. Infrared and energy dispersive spectroscopy further showed that no contaminants or organic compounds were present in the remaining, uncombusted valve seat material or on seat plug surfaces [1]. Therefore, contaminant-promoted ignition did not appear to be responsible for the failures.…”

“…Observations of extruded material along the outer edge of the coined or loaded seat area [1] produced by cylinder overuse or poppet overload led to concerns that accelerated gas flow across a deformed seat surface could generate enough localized heating to ignite the polymeric seat. Low molecular weight or highly amorphous quick-quenched PCTFE grades might be expected to be especially prone to this type of deformation.…”

“…Subsequent revelation of low-temperature dimensional instability by thermomechanical analysis (TMA) in a variety of PCTFE sheet and rod stock samples [1] led to new concerns that PCTFE valve seats could undergo excessive expansion or contraction during service. During expansion, additional extrusion and accompanying "flow friction" could occur.…”