NMR investigation of the nature of water in disposable incontinence pads containing superabsorbent polymers and fluffed wood pulp

The use of nuclear magnetic resonance to make quantitative measurements of the absorption processes occurring in the constituent materials of disposable incontinence pads was investigated. Modern incontinence pads often have a layered structure with the absorbent layer principally consisting of superabsorbent polymer (SAP) grains distributed in fluffed wood pulp. T ₂-weighted and magnetisation transfer contrast (MTC) measurements were made using samples of SAP, fluffed wood pulp, and a mixture of 50% SAP and 50% fluffed wood pulp following the addition of water and NaCl solutions. The T ₂-weighted signal always decreased on mixing the liquid and material samples, with SAP showing the smallest magnitude of signal decline. The MTC change was minimal for SAP, but increased for the other samples. The small changes in the T ₂-weighted signals and the MTC observed when adding water to SAP indicate that water is not principally being tightly bound by the polymer. The absorption time-course was further evaluated by acquiring proton-density-weighted 1D profiles after the addition of water to the samples. In SAP, the fitted time constant was comparable to the activation energy measured by other methods. The relaxation times both declined on mixing the liquids with samples of SAP and fluffed wood pulp. The reduction in spin–spin relaxation is sufficient to account for the T ₂-weighted signal changes.