Lam, Kwok Kai; (1992) Assessment of the adhesion of powders to a metal surface. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A centrifuge method has been employed to study, statistically, the adhesion of individual particles of some pharmaceutical powder excipients to stainless steel. The surface of the steel substrate was characterized by profilometry, which was defined by the various surface roughness parameters. By altering the relative position of the steel disk in a specially designed rotor tube, the centrifugal force can act both as a compression force as well as a detachment force. Preliminary pressing of powder particles on to the steel surface was found to increase the strength of particle adhesion. The relative adhesiveness of different solid excipient materials was compared quantitatively by means of the adhesion ratio derived in this study, which allows a more objective means of assessment. In general, adhesion to stainless steel was stronger for soft materials than hard ones, on the basis that plastic deformation has occurred. Moreover, the strengths of adhesion bonds were appreciably affected by the duration that the adhering particles were initially under a compression force before separation. Particulate materials having an inherent weak solid structure was particularly sensitive to such effect. Variation of particle adhesion forces to stainless steel was shown to be due partly to the non-uniformity of particle size in a powder sieve fraction. Though bigger particles exhibited greater forces of adhesion, due to their larger masses, they were more easily dislodged than the smaller particles by centrifugation. Investigations into two powder-steel adhesive systems indicated a considerable role played by the molecular van der Waals forces in the particle adhesion. Adhesive interactions were examined in terms of surface energetics, from which an apparent energy of adhesion was calculated for either system, which may provide an alternative means of evaluating particles-substrate adhesion. A form of Arrhenius relationship was found between temperature and the median adhesion force or the amount of median adhesion force established per unit of the preliminary press-on force, from which the apparent activation energy for an adhesion process was obtained. With less energy of activation was required for a soft material, as found, an implication of this is that, adhesion bond formation would be energetically more favourable for a soft solid.

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