Rheological Control of Microsphere Expansion and Cellular Stabilization in Styrene–Butadiene–Styrene Foams

Lightweight elastomeric foams for footwear, sealing and damping applications require substantial density reduction without loss of cellular stability, yet the role of matrix rheology in expandable-microsphere foaming of oil-extended thermoplastic elastomers remains insufficiently understood. Herein, styrene–butadiene–styrene (SBS) foams were prepared with 1–3 phr thermally expandable microspheres, 10–30 phr additional processing oil and three nominal densities (0.80, 0.65 and 0.45 g cm−3). Increasing oil content reduced matrix elastic and flow resistance by approximately 2–3-fold and increased the mean pore size from 25.2 to 49.6 µm under the intermediate-density condition. Under the lowest-density condition, however, excessive softening reduced cellular stability, raising the pore-size coefficient of variation to 49.7% for LD-30-3. Combined optical microscopy and dielectric measurements showed that larger pores at comparable density did not necessarily correspond to a higher bulk void fraction, while the permittivity response was consistent with a predominantly discrete closed-cell structure. Greater pore expansion did not coincide with maximum mechanical performance: the highest mean toughness (11.50 MJ m−3) was obtained for HD-10-3. Cyclic testing showed that increasing oil content reduced load-bearing capacity but nearly doubled conditioned damping (from approximately 3–4% to 6–7%). Overall, matrix rheological resistance can be used to balance microsphere expansion, cellular retention and function-specific performance in lightweight SBS foams.