Untersuchungen zur - OPUS - Universität Würzburg

116
SUMMARY
shaker-mixer. In the course of m ixing the numb er of flow regulator adsorbates increases
whereas th eir s ize decreases. Two m ixtures co nsisting of the sam e ingredients but m ixed
differently can be com pared by counting the numb er of gue st particles on the host particles’
surface.
The connection of the surface coverage of the host particles and the flow properties of the
powders was proven by tensile strength m easurements. W ith increa sing m ixing tim e the
tensile strength drops until an optimum in f low properties is achieved. Thus, it is possible to
reduce the mixing time needed to reach the m inimum achievable tensile strength. Powerful
glidants like AEROSIL ® R812 are able to develop their flow regulating potency after a short
mixing ti me even at gentle m ixing conditions. Flow regulators that tend to build stronger
agglomerates require higher shear forces or longer m ixing tim es to achieve a maxim um
decrease of tensile strength. However, all m ixing conditions lead to an identical m inimum
tensile strength after longer m ixing tim es. Thus, an improvem ent of the param eter
“effectiveness” by means of a higher energy input during mixing was not observed.
When extending the mixing time up to three days , after passing a platea u phase, a re-increase
of tensile strength was observed. The higher the energy input during m
the more distinctive a re-increase of tensile strength occurs.
ixing, the sooner and
For a better understanding of th e re-increase of tensile stre ngth further investigation was
performed. Powder samples of which the nanostr uctured material had already lost its function
as flow aid due to in tense mechanical stress were once m ore mixed with pure co rn starch. It
was assumed that during mixing the agglomerates of the flow regulator will distribute evenly
on the corn starch surf ace of both com ponents. The presence of free host particle surface
could break up a complete nanom aterial film on the surf ace of the par ticles of the
“overmixed” powder which eventually should result in lower tensile strength values.
The results of the tensile streng th measurements demonstrated that the addition of pure host
particles to “overm ixed” powders does not lead to an im provement in flowability.
Consequently, the loss of the flow regulating function of nanostructured materials after longer
mixing times is not caused by the form ation of a coating due to an excessive coverage of the
host particles, as proposed by Meyer [ 4]. One can rather draw the conclusion th at the guest
particle adsorbates flatten during mixing and are now too small to act as surface roughnesses.
Eber [ 6] postulated that hydrophobic nanostructured materials do not lose their flow
regulating function after longer mixing times. This hypothesis was disproved investigating the
chemically modified silica AEROSIL ® R812. It was demonstrated that only a com bination of