Liquid resistivity of pharmaceutical propellants using novel resistivity cell

Abstract

Metered-dose inhalers employ propellants to produce pharmaceutical aerosols for treating respiratory conditions like asthma. In the liquid phase, the DC volume resistivity of pharmaceutical propellants, including R134a, R152a, and R227ea, was studied at saturation pressures and room temperature (not vapour phase). These measurements are essential for industries like refrigerants. Aerosols from metered dose inhalers (MDIs) with these propellants become electrically charged, affecting medicament deposition in lung. The resistivity was measured using a novel concentric cylinder-type capacitance cell designed in-house. The resistivity for the propellants (R134a, R152a, and R227ea) was found to be 3.02 × 10¹⁰ Ωm, 2.37 × 10⁹ Ωm and 1.31 × 10¹⁰ Ωm, respectively. The electrical resistivity data obtained was found to be at least two orders of magnitude higher than the limited data available in the literature. Challenges in the resistivity cell's development and performance are discussed, with a focus on various propellants and their mixtures with ethanol and moisture concentrations. The resistivity of propellant mixtures containing moisture concentrations ranging from 5 to 500 ppm and ethanol concentrations ranging between 1000 and 125,000 ppm was determined. The resistivity was tested across 10-min and 1-h periods and was performed in accordance with the contemporary IEC 60247 standard.

Figure 1

CAD drawings (2D) of resistivity cell components: (A) cross section of the complete resistivity cell and (B) internal components of the resistivity cell. COMSOL simulations of the electrical field strength across the bottom (C), and at the top (D) of the anode. The plots show 2D visualisations of the field strength in V/m with a 10⁵-scaling factor for clarity. The electric field is homogenous (parallel field lines) within the measuring area but distorts (curved lines) near the top and bottom edges.

Figure 2

(A) The experimental setup of the resistivity measurement system and associated equipment, (B) Resistivity cell and manometer, and (C) Protective cage enclosing the resistivity cell.

Figure 3

Graphs showing the change in current over time for each of the propellants for each test that is completed, including (A) R134a, (B) R227ea, and (C) R152a. The average room temperature for these measurements was about 20 °C. (D) Bar graph showing the mean resistivity values for each of the pure propellants measured at 10-min and 1-h intervals.

Figure 4

Spatial configuration of R152a, also known as 1,1-Difluoroethane.

Figure 5

Graphs showing the resistivity values of 134a and 152a propellant mixtures with different concentrations of water: (A) 134a + water, (B) line graph of 134a + water, (C) 152a + water, and (D) line graph of 152a + water. The average room temperature for the set of measurements with R134a was 23.6 °C, and the average room humidity was 39.7%. For the measurements taken with 152a, the average room temperature was 27.6 °C, and the average room humidity was 42%.

Figure 6

Graphs showing the resistivity values of 134a and 152a propellant mixtures with different concentrations of ethanol: (A) 134a + ethanol, (B) line graph of 134a + ethanol, (C) 152a + ethanol, and (D) line graph of 152a + ethanol. The average room temperature for the set of measurements with R134a was 24.5 °C, and the average room humidity was 40.3%. For the measurements taken with 152a, the average room temperature was 25.4 °C and the average room humidity was 47%.

Figure 7

Graphs showing the change in resistivity for two different propellant mixtures with (A) different concentrations of water and (B) ethanol.