THE EXTENT OF ERRORS ASSOCIATED WITH CONTACT ANGLES .2. FACTORS AFFECTING DATA OBTAINED USING A WILHELMY PLATE TECHNIQUE FOR POWDERS

The reasons for error, and the magnitude of error that may be expected have been investigated for Wilhelmy plate contact angle measurements on compacted plates. A range of different materials have been consider ed, including a homologous series of powders (alkyl p-hydroxybenzoates ), and alpha-lactose monohydrate (from three different suppliers). The effect of powder and plate storage conditions, the compaction force a nd dwell time used to prepare the plate, and the approach to defining the buoyancy slope of the Wilhelmy plate experiment have all been cons idered. The effect of compaction seems to be material dependent, some materials are unaffected by large changes in applied pressure, whilst others are particular sensitive to changes in the compaction process. Propyl p-hydroxybenzoate was especially sensitive to changes in compac tion, such that even when trying to produce identical compacts, variab ility was noted for different days of plate preparation. Variability i n measured contact angle was not associated with humidity equilibratio n, or time after the plate was made for the materials studied. The maj or cause of variability in the contact angle seemed to be the plate ro ughness. There is some concern that the plate roughness will not only affect the result by normal hysteresis effects, but also because the m easured perimeter (external dimensions) will underestimate the true pe rimeter of a rough plate. Having noted many causes for concern with th is technique, it was observed that standard deviations for the contact angles were smaller than for other contact angle techniques for powde red samples, however, accuracy and precision should not be confused.