By Bob McGregor, General Manager, Brookfield Engineering
Patients are quick to complain when a medical ointment stings after application. But there may be another reason that they might be dissatisfied with the way an ointment behaves when it is too thick. Not only is it hard to get out of the tube or jar, but it also has a rough feeling to the skin as you try to lather it on.
The ointment may well have been tested for viscosity, resistance to flow, by the manufacturer and passed the QC check. But the test method may not have correctly simulated how the customer will rub the ointment onto the skin during the application process. Single point viscosity checks, a standard staple of the QC world, may not be sufficient to qualify ointments. This article will explain the importance of choosing appropriate shear rates to measure viscosity when testing ointments and the analytical technique for coming up with those values.
When you squeeze ointment out of a tube, you are shearing the material and the rate of shear is related to how hard you press down on the tube. The harder you squeeze, the faster the material comes out. A slow steady squeeze produces a constant stream of ointment and gives you the best possibility to control the amount that is expelled from the tube. The shearing action on the ointment is quantified mathematically by the volume flow rate coming out of the tube in cm³/sec and the diameter of the tube opening in cm. A simplified calculation for shear rate is to determine the average velocity for the ointment as it squirts out and divide that number by the radius of the tube opening.