Effect of wheat flour pre-cooking on the composite modulus of wheat flour and carboxylated styrene–butadiene latex

Commercial wheat flours with two different concentrations of insoluble protein were used as fillers to reinforce styrene–butadiene latex composites and their viscoelastic properties were examined. Both wheat flours were also cooked at 55, 70, or 95 °C for 1 h in an aqueous dispersion prior to mixing with latex in order to swell the starch present in the flour and increase its surface area. The aqueous wheat flour dispersions were then mixed with the styrene–butadiene latex to form rubber composites using freeze-drying and compression molding methods. Viscosity measurements indicated that the surface area of the starch increased with the cook temperature, and increased surface area was directly proportional to composite reinforcement; a 40% wheat flour-filled composite pre-cooked at 95 °C increased the oscillatory storage modulus by a factor of ∼200 over the unfilled latex. The protein/starch ratio was also directly proportional to composite reinforcement; for all samples the high-protein data exhibited larger storage moduli (G′) than the corresponding low-protein data. The difference in G′ was enhanced directly with the concentration of filler except at a cook temperature of 95 °C, where the difference in G′ became independent of filler concentration. Instant recovery experiments showed that wheat flour-filled composites recovered over 90% of their initial storage modulus until cook temperature reached 95 °C, where there was a significant drop. Long term recovery experiments also showed a drop for the 95 °C cook temperature, and this is most likely due to the increasing connectivity of rigid filler domains relative to the more elastic rubber domains as starch swelling was increased. For cook temperatures below 95 °C, the wheat flour composites all recovered beyond their initial moduli (G′/G′0 > 1), suggesting that although these composites were homogeneous, they were not at equilibrium and after perturbation were able to rearrange to form stronger, more equilibrated filler–filler and filler–polymer interactions.