Currently, poly(alpha-methylstyrene) mandrel precursors are suspended in a
salt-containing water bath during curing. Matching the density of the precu
rsors to their bath almost, but not quite, eliminates shell sagging caused
by gravity. This sagging is opposed by the interfacial tension between the
plastic containing oil solution and the water bath, but the tension is bare
ly adequate to give satisfactory sphericity. We found that adding a small a
mount (<0.1 wt%) of high-molecular-weight poly(acrylic acid) to the water b
ath substantially increased the interfacial tension. Combining that change
with a gentler shell curing process enabled consistent production of more s
pherical mandrels.