Assessing rheometry for measuring the viscosity-average degree of polymerisation of cellulose in paper degradation studies

Abstract

In paper degradation studies, the viscosity-average degree of polymerisation (DPv) is often used as a key indicator of the extent of degradation of cellulosic paper. DPv can be deduced from the viscosity of dilute paper solutions, as typically measured through glass capillary viscometry. The current study proposes an efficient, alternative method to evaluate DPv of cellulosic paper, which is based on rotational rheometry. The proposed methodology relies on the application of a shear flow in a thin film of cellulose solution to measure its dynamic viscosity, from which DPv can be subsequently derived in a straightforward fashion. Rheometry allows to measure the viscosity for a range of shear rates, which results in multiple DPv evaluations per sample, and thus in statistically representative data from an individual test. Further, rheometry typically requires considerably less paper mass per test than glass capillary viscometry, which makes the method attractive for paper degradation studies with limited sample availability. Also, rheometry measurements are less work-intensive than glass capillary viscometry measurements. The rheometry method has been applied to 4 hygrothermally aged cellulose paper samples and the unaged counterpart. The measurement results regarding the age-dependency of DPv and the number of cellulose chain scissions are compared to those obtained by glass capillary viscometry, showing a very good agreement. At a longer ageing time, both experimental methods reveal a non-linear decrease in time of DPv, and a non-linear increase in time of the number of cellulose chain scissions, which indicate that the cellulose ageing process is realistically captured. The agreement in measurement results further demonstrates that rheometry is an easy-to-use, accurate and efficient alternative for DPv measurements by glass capillary viscometry.