Decreasing Out-of-Round in Poly(α-Methylstyrene) Mandrels by Increasing Interfacial Tension

Currently, poly(α-methylstyrene) mandrel precursors are suspended in a salt-containing water bath during curing. Matching the density of the precursors 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 barely adequate to give satisfactory sphericity. We found that adding a small amount (<0.1 wt%) of high-molecular-weight poly(acrylic acid) to the water bath substantially increased the interfacial tension. Combining that change with a gentler shell curing process enabled consistent production of more spherical mandrels.