| Agar gels are environmentally benign materials used as aqueous binders for powder consolidation processes such as injection molding, due to their potential for easy binder removal. During cooling from the sol state, agar forms a three-dimensional open cellular structure consisting of spatially aggregated double helices.; Dynamic rheology was utilized to elucidate the relation between the thermal history of agar sols and their gelation behavior. The apparent onset of gelation, Tonset, increased with increasing agar concentration, but decreased with increasing cooling rate. The asymptotic equilibrium value of the storage modulus, G′ e, that was reached after gelation was complete, increased by approximately an order of magnitude with increasing agar concentration (1–3 wt%), but decreased with increasing cooling rate.; The mechanical spectra at 30°C of 1–3 wt% agar gels, were approximated by the dynamic response of a generalized Maxwell model consisting of four elastic and four viscous elements. The gel's relaxation spectrum, H(τ), at each agar concentration, was found to cover a wide range of relaxation times. It is theorized that relaxation processes within the aggregates of double helices occur at short relaxation times. Relaxation processes occurring at the junctions correspond to longer relaxation times.; A fundamental structural/rheological model was developed to describe the dynamic rheological behavior of agar sols as a function of agar concentration and cooling rate. The model includes time-temperature dependent contributions to the dynamic moduli from non-associated agar molecules and from a three-dimensional network. The model was successfully fitted to a number of gelation curves covering 1–3 wt% agar concentration and cooling rates from 0.5 to 20°C min−1. The theoretical gelation temperature, was found to increase with agar concentration and decrease with increasing cooling rate.; A study compared the characteristic microstructural features of agar gels at three different concentrations and prepared under slow cooling or quenching conditions. Results indicated that the distributions of the suprafiber lengths or suprafiber diameters shift to lower and higher values respectively with increasing agar concentration. Quenched agar sols formed a homogeneous gel network with smaller pore size compared to sols cooled slowly. |
