Rheological characterization of microfibrillated cellulose suspensions after freezing

This work aims to investigate the rheological properties of microfibrillated cellulose (MFC) suspensions. The effect of some experimental parameters, such as cellulose concentration, temperature, ionic strength and pH has been studied. For that purpose, suspensions of microfibrillated cellulose have been prepared by strong mechanical treatments of a purified sugar-beet pulp cellulose-based residue in aqueous medium. Cellulose suspensions at different concentrations (from 0.25 to 3 w/w%) have been found to display a viscoelastic solid-like behaviour, even for the lowest concentration tested. The storage modulus at 0.1 rad s−1 increased strongly upon increasing concentration from 0.25 to 3 w/w%, following a power law with an exponent of 2.58. All suspensions exhibited a shear-thinning behaviour. It was also found that viscoelastic properties of the suspensions of cellulose are not affected by temperature or by varying pH from 4.5 to 9 while the G′ and G″ moduli increased as salt concentration of the suspensions increased. This reinforcement of the viscoelastic properties by increasing ionic strength can be related to a screening of the electrostatic repulsions between the microfibrils, due to the presence of uronic acid groups, enhancing the fiber–fiber interactions. Mechanical treatment did not affect cellulose crystallinity. The effect of freezing was investigated as an alternative way to the most conventional cellulose preparation that consists to freeze–dry the suspensions for their conservation. It was shown that freezing preserved the rheological properties of the suspensions, contrary to freeze drying.