Role of fiber volume fraction in the rheology of pulp fiber foams

This study investigates the rheological behavior of aqueous foams containing 12–18 wt% of pulp fibers and having air contents of 63–80 %. The flow behavior of these foams is well described by the Herschel–Bulkley model, exhibiting a yield stress and power-law behavior at high shear rates. Amplitude sweep and frequency sweep measurements reveal a predominantly elastic response at small deformations, with the storage modulus (G′) dominating over the loss modulus (G″). The shear stiffness of the studied fiber foams is shown to be largely dominated by the presence of a continuous, three-dimensional fiber network within the foam. We introduce fiber volume fraction as a unified structural parameter integrating fiber concentration and air content, enabling direct correlation between fiber foam composition and rheology. Within this framework, the storage modulus exhibits a power-law dependence on fiber volume fraction. This dependence, including the value of the power-law exponent, is in agreement with the predictions from the tube model. These findings provide a foundation for optimizing foam forming processes, with implications for the energy-efficient manufacturing of fiber-based products.