Rheological determination of the swollen gel strength of superabsorbent polymer hydrogels

To measure the swollen gel strength of hydrogels, single isolated samples with a certain shape, e.g. spherical or cylindrical, are often tested. However, the commercial superabsobent polymer (SAP) hydrogels are mostly sugar-like particles with irregular shapes. Therefore, the usual testing of the strength of the swollen state of the gels is impossible. In this paper, an experimental method is investigated for the measuring the swollen gel strength of SAPs. Firstly, we determined absorbency under load (AUL) of a typical SAP sample and then the mechanical strength of the swollen sample was measured by a rheological method. The characterization was conducted by a controlled strain rheometer at 25 °C. Dependency of the rheological properties of the sample on strain and frequency was investigated. To determine the linear viscoelastic (LVE) zone, dynamic storage modulus (G′) and loss modulus (G″), measurements were carried out at constant frequency and in a range of strains on the SAP sample. Furthermore, G′ was obtained at constant strain over a range of frequencies. The storage modulus of the swollen SAP gel (already absorbed saline under 0.3-0.9 psi) was measured to be above 1000 Pa at 25 °C.